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| -rw-r--r-- | Documentation/security/IMA-templates.rst | 94 | ||||
| -rw-r--r-- | Documentation/security/LSM-sctp.rst | 175 | ||||
| -rw-r--r-- | Documentation/security/LSM.rst | 14 | ||||
| -rw-r--r-- | Documentation/security/SELinux-sctp.rst | 158 | ||||
| -rw-r--r-- | Documentation/security/credentials.rst | 561 | ||||
| -rw-r--r-- | Documentation/security/index.rst | 15 | ||||
| -rw-r--r-- | Documentation/security/keys/core.rst | 1546 | ||||
| -rw-r--r-- | Documentation/security/keys/ecryptfs.rst | 73 | ||||
| -rw-r--r-- | Documentation/security/keys/index.rst | 11 | ||||
| -rw-r--r-- | Documentation/security/keys/request-key.rst | 199 | ||||
| -rw-r--r-- | Documentation/security/keys/trusted-encrypted.rst | 175 | ||||
| -rw-r--r-- | Documentation/security/self-protection.rst | 317 | ||||
| -rw-r--r-- | Documentation/security/tpm/index.rst | 7 | ||||
| -rw-r--r-- | Documentation/security/tpm/tpm_vtpm_proxy.rst | 50 | ||||
| -rw-r--r-- | Documentation/security/tpm/xen-tpmfront.txt | 113 |
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diff --git a/Documentation/security/IMA-templates.rst b/Documentation/security/IMA-templates.rst new file mode 100644 index 000000000..2cd0e273c --- /dev/null +++ b/Documentation/security/IMA-templates.rst @@ -0,0 +1,94 @@ +================================= +IMA Template Management Mechanism +================================= + + +Introduction +============ + +The original ``ima`` template is fixed length, containing the filedata hash +and pathname. The filedata hash is limited to 20 bytes (md5/sha1). +The pathname is a null terminated string, limited to 255 characters. +To overcome these limitations and to add additional file metadata, it is +necessary to extend the current version of IMA by defining additional +templates. For example, information that could be possibly reported are +the inode UID/GID or the LSM labels either of the inode and of the process +that is accessing it. + +However, the main problem to introduce this feature is that, each time +a new template is defined, the functions that generate and display +the measurements list would include the code for handling a new format +and, thus, would significantly grow over the time. + +The proposed solution solves this problem by separating the template +management from the remaining IMA code. The core of this solution is the +definition of two new data structures: a template descriptor, to determine +which information should be included in the measurement list; a template +field, to generate and display data of a given type. + +Managing templates with these structures is very simple. To support +a new data type, developers define the field identifier and implement +two functions, init() and show(), respectively to generate and display +measurement entries. Defining a new template descriptor requires +specifying the template format (a string of field identifiers separated +by the ``|`` character) through the ``ima_template_fmt`` kernel command line +parameter. At boot time, IMA initializes the chosen template descriptor +by translating the format into an array of template fields structures taken +from the set of the supported ones. + +After the initialization step, IMA will call ``ima_alloc_init_template()`` +(new function defined within the patches for the new template management +mechanism) to generate a new measurement entry by using the template +descriptor chosen through the kernel configuration or through the newly +introduced ``ima_template`` and ``ima_template_fmt`` kernel command line parameters. +It is during this phase that the advantages of the new architecture are +clearly shown: the latter function will not contain specific code to handle +a given template but, instead, it simply calls the ``init()`` method of the template +fields associated to the chosen template descriptor and store the result +(pointer to allocated data and data length) in the measurement entry structure. + +The same mechanism is employed to display measurements entries. +The functions ``ima[_ascii]_measurements_show()`` retrieve, for each entry, +the template descriptor used to produce that entry and call the show() +method for each item of the array of template fields structures. + + + +Supported Template Fields and Descriptors +========================================= + +In the following, there is the list of supported template fields +``('<identifier>': description)``, that can be used to define new template +descriptors by adding their identifier to the format string +(support for more data types will be added later): + + - 'd': the digest of the event (i.e. the digest of a measured file), + calculated with the SHA1 or MD5 hash algorithm; + - 'n': the name of the event (i.e. the file name), with size up to 255 bytes; + - 'd-ng': the digest of the event, calculated with an arbitrary hash + algorithm (field format: [<hash algo>:]digest, where the digest + prefix is shown only if the hash algorithm is not SHA1 or MD5); + - 'n-ng': the name of the event, without size limitations; + - 'sig': the file signature. + + +Below, there is the list of defined template descriptors: + + - "ima": its format is ``d|n``; + - "ima-ng" (default): its format is ``d-ng|n-ng``; + - "ima-sig": its format is ``d-ng|n-ng|sig``. + + + +Use +=== + +To specify the template descriptor to be used to generate measurement entries, +currently the following methods are supported: + + - select a template descriptor among those supported in the kernel + configuration (``ima-ng`` is the default choice); + - specify a template descriptor name from the kernel command line through + the ``ima_template=`` parameter; + - register a new template descriptor with custom format through the kernel + command line parameter ``ima_template_fmt=``. diff --git a/Documentation/security/LSM-sctp.rst b/Documentation/security/LSM-sctp.rst new file mode 100644 index 000000000..6e5a3925a --- /dev/null +++ b/Documentation/security/LSM-sctp.rst @@ -0,0 +1,175 @@ +SCTP LSM Support +================ + +For security module support, three SCTP specific hooks have been implemented:: + + security_sctp_assoc_request() + security_sctp_bind_connect() + security_sctp_sk_clone() + +Also the following security hook has been utilised:: + + security_inet_conn_established() + +The usage of these hooks are described below with the SELinux implementation +described in ``Documentation/security/SELinux-sctp.rst`` + + +security_sctp_assoc_request() +----------------------------- +Passes the ``@ep`` and ``@chunk->skb`` of the association INIT packet to the +security module. Returns 0 on success, error on failure. +:: + + @ep - pointer to sctp endpoint structure. + @skb - pointer to skbuff of association packet. + + +security_sctp_bind_connect() +----------------------------- +Passes one or more ipv4/ipv6 addresses to the security module for validation +based on the ``@optname`` that will result in either a bind or connect +service as shown in the permission check tables below. +Returns 0 on success, error on failure. +:: + + @sk - Pointer to sock structure. + @optname - Name of the option to validate. + @address - One or more ipv4 / ipv6 addresses. + @addrlen - The total length of address(s). This is calculated on each + ipv4 or ipv6 address using sizeof(struct sockaddr_in) or + sizeof(struct sockaddr_in6). + + ------------------------------------------------------------------ + | BIND Type Checks | + | @optname | @address contains | + |----------------------------|-----------------------------------| + | SCTP_SOCKOPT_BINDX_ADD | One or more ipv4 / ipv6 addresses | + | SCTP_PRIMARY_ADDR | Single ipv4 or ipv6 address | + | SCTP_SET_PEER_PRIMARY_ADDR | Single ipv4 or ipv6 address | + ------------------------------------------------------------------ + + ------------------------------------------------------------------ + | CONNECT Type Checks | + | @optname | @address contains | + |----------------------------|-----------------------------------| + | SCTP_SOCKOPT_CONNECTX | One or more ipv4 / ipv6 addresses | + | SCTP_PARAM_ADD_IP | One or more ipv4 / ipv6 addresses | + | SCTP_SENDMSG_CONNECT | Single ipv4 or ipv6 address | + | SCTP_PARAM_SET_PRIMARY | Single ipv4 or ipv6 address | + ------------------------------------------------------------------ + +A summary of the ``@optname`` entries is as follows:: + + SCTP_SOCKOPT_BINDX_ADD - Allows additional bind addresses to be + associated after (optionally) calling + bind(3). + sctp_bindx(3) adds a set of bind + addresses on a socket. + + SCTP_SOCKOPT_CONNECTX - Allows the allocation of multiple + addresses for reaching a peer + (multi-homed). + sctp_connectx(3) initiates a connection + on an SCTP socket using multiple + destination addresses. + + SCTP_SENDMSG_CONNECT - Initiate a connection that is generated by a + sendmsg(2) or sctp_sendmsg(3) on a new asociation. + + SCTP_PRIMARY_ADDR - Set local primary address. + + SCTP_SET_PEER_PRIMARY_ADDR - Request peer sets address as + association primary. + + SCTP_PARAM_ADD_IP - These are used when Dynamic Address + SCTP_PARAM_SET_PRIMARY - Reconfiguration is enabled as explained below. + + +To support Dynamic Address Reconfiguration the following parameters must be +enabled on both endpoints (or use the appropriate **setsockopt**\(2)):: + + /proc/sys/net/sctp/addip_enable + /proc/sys/net/sctp/addip_noauth_enable + +then the following *_PARAM_*'s are sent to the peer in an +ASCONF chunk when the corresponding ``@optname``'s are present:: + + @optname ASCONF Parameter + ---------- ------------------ + SCTP_SOCKOPT_BINDX_ADD -> SCTP_PARAM_ADD_IP + SCTP_SET_PEER_PRIMARY_ADDR -> SCTP_PARAM_SET_PRIMARY + + +security_sctp_sk_clone() +------------------------- +Called whenever a new socket is created by **accept**\(2) +(i.e. a TCP style socket) or when a socket is 'peeled off' e.g userspace +calls **sctp_peeloff**\(3). +:: + + @ep - pointer to current sctp endpoint structure. + @sk - pointer to current sock structure. + @sk - pointer to new sock structure. + + +security_inet_conn_established() +--------------------------------- +Called when a COOKIE ACK is received:: + + @sk - pointer to sock structure. + @skb - pointer to skbuff of the COOKIE ACK packet. + + +Security Hooks used for Association Establishment +================================================= +The following diagram shows the use of ``security_sctp_bind_connect()``, +``security_sctp_assoc_request()``, ``security_inet_conn_established()`` when +establishing an association. +:: + + SCTP endpoint "A" SCTP endpoint "Z" + ================= ================= + sctp_sf_do_prm_asoc() + Association setup can be initiated + by a connect(2), sctp_connectx(3), + sendmsg(2) or sctp_sendmsg(3). + These will result in a call to + security_sctp_bind_connect() to + initiate an association to + SCTP peer endpoint "Z". + INIT ---------------------------------------------> + sctp_sf_do_5_1B_init() + Respond to an INIT chunk. + SCTP peer endpoint "A" is + asking for an association. Call + security_sctp_assoc_request() + to set the peer label if first + association. + If not first association, check + whether allowed, IF so send: + <----------------------------------------------- INIT ACK + | ELSE audit event and silently + | discard the packet. + | + COOKIE ECHO ------------------------------------------> + | + | + | + <------------------------------------------- COOKIE ACK + | | + sctp_sf_do_5_1E_ca | + Call security_inet_conn_established() | + to set the peer label. | + | | + | If SCTP_SOCKET_TCP or peeled off + | socket security_sctp_sk_clone() is + | called to clone the new socket. + | | + ESTABLISHED ESTABLISHED + | | + ------------------------------------------------------------------ + | Association Established | + ------------------------------------------------------------------ + + diff --git a/Documentation/security/LSM.rst b/Documentation/security/LSM.rst new file mode 100644 index 000000000..98522e0e1 --- /dev/null +++ b/Documentation/security/LSM.rst @@ -0,0 +1,14 @@ +================================= +Linux Security Module Development +================================= + +Based on https://lkml.org/lkml/2007/10/26/215, +a new LSM is accepted into the kernel when its intent (a description of +what it tries to protect against and in what cases one would expect to +use it) has been appropriately documented in ``Documentation/security/LSM.rst``. +This allows an LSM's code to be easily compared to its goals, and so +that end users and distros can make a more informed decision about which +LSMs suit their requirements. + +For extensive documentation on the available LSM hook interfaces, please +see ``include/linux/lsm_hooks.h``. diff --git a/Documentation/security/SELinux-sctp.rst b/Documentation/security/SELinux-sctp.rst new file mode 100644 index 000000000..a332cb1c5 --- /dev/null +++ b/Documentation/security/SELinux-sctp.rst @@ -0,0 +1,158 @@ +SCTP SELinux Support +===================== + +Security Hooks +=============== + +``Documentation/security/LSM-sctp.rst`` describes the following SCTP security +hooks with the SELinux specifics expanded below:: + + security_sctp_assoc_request() + security_sctp_bind_connect() + security_sctp_sk_clone() + security_inet_conn_established() + + +security_sctp_assoc_request() +----------------------------- +Passes the ``@ep`` and ``@chunk->skb`` of the association INIT packet to the +security module. Returns 0 on success, error on failure. +:: + + @ep - pointer to sctp endpoint structure. + @skb - pointer to skbuff of association packet. + +The security module performs the following operations: + IF this is the first association on ``@ep->base.sk``, then set the peer + sid to that in ``@skb``. This will ensure there is only one peer sid + assigned to ``@ep->base.sk`` that may support multiple associations. + + ELSE validate the ``@ep->base.sk peer_sid`` against the ``@skb peer sid`` + to determine whether the association should be allowed or denied. + + Set the sctp ``@ep sid`` to socket's sid (from ``ep->base.sk``) with + MLS portion taken from ``@skb peer sid``. This will be used by SCTP + TCP style sockets and peeled off connections as they cause a new socket + to be generated. + + If IP security options are configured (CIPSO/CALIPSO), then the ip + options are set on the socket. + + +security_sctp_bind_connect() +----------------------------- +Checks permissions required for ipv4/ipv6 addresses based on the ``@optname`` +as follows:: + + ------------------------------------------------------------------ + | BIND Permission Checks | + | @optname | @address contains | + |----------------------------|-----------------------------------| + | SCTP_SOCKOPT_BINDX_ADD | One or more ipv4 / ipv6 addresses | + | SCTP_PRIMARY_ADDR | Single ipv4 or ipv6 address | + | SCTP_SET_PEER_PRIMARY_ADDR | Single ipv4 or ipv6 address | + ------------------------------------------------------------------ + + ------------------------------------------------------------------ + | CONNECT Permission Checks | + | @optname | @address contains | + |----------------------------|-----------------------------------| + | SCTP_SOCKOPT_CONNECTX | One or more ipv4 / ipv6 addresses | + | SCTP_PARAM_ADD_IP | One or more ipv4 / ipv6 addresses | + | SCTP_SENDMSG_CONNECT | Single ipv4 or ipv6 address | + | SCTP_PARAM_SET_PRIMARY | Single ipv4 or ipv6 address | + ------------------------------------------------------------------ + + +``Documentation/security/LSM-sctp.rst`` gives a summary of the ``@optname`` +entries and also describes ASCONF chunk processing when Dynamic Address +Reconfiguration is enabled. + + +security_sctp_sk_clone() +------------------------- +Called whenever a new socket is created by **accept**\(2) (i.e. a TCP style +socket) or when a socket is 'peeled off' e.g userspace calls +**sctp_peeloff**\(3). ``security_sctp_sk_clone()`` will set the new +sockets sid and peer sid to that contained in the ``@ep sid`` and +``@ep peer sid`` respectively. +:: + + @ep - pointer to current sctp endpoint structure. + @sk - pointer to current sock structure. + @sk - pointer to new sock structure. + + +security_inet_conn_established() +--------------------------------- +Called when a COOKIE ACK is received where it sets the connection's peer sid +to that in ``@skb``:: + + @sk - pointer to sock structure. + @skb - pointer to skbuff of the COOKIE ACK packet. + + +Policy Statements +================== +The following class and permissions to support SCTP are available within the +kernel:: + + class sctp_socket inherits socket { node_bind } + +whenever the following policy capability is enabled:: + + policycap extended_socket_class; + +SELinux SCTP support adds the ``name_connect`` permission for connecting +to a specific port type and the ``association`` permission that is explained +in the section below. + +If userspace tools have been updated, SCTP will support the ``portcon`` +statement as shown in the following example:: + + portcon sctp 1024-1036 system_u:object_r:sctp_ports_t:s0 + + +SCTP Peer Labeling +=================== +An SCTP socket will only have one peer label assigned to it. This will be +assigned during the establishment of the first association. Any further +associations on this socket will have their packet peer label compared to +the sockets peer label, and only if they are different will the +``association`` permission be validated. This is validated by checking the +socket peer sid against the received packets peer sid to determine whether +the association should be allowed or denied. + +NOTES: + 1) If peer labeling is not enabled, then the peer context will always be + ``SECINITSID_UNLABELED`` (``unlabeled_t`` in Reference Policy). + + 2) As SCTP can support more than one transport address per endpoint + (multi-homing) on a single socket, it is possible to configure policy + and NetLabel to provide different peer labels for each of these. As the + socket peer label is determined by the first associations transport + address, it is recommended that all peer labels are consistent. + + 3) **getpeercon**\(3) may be used by userspace to retrieve the sockets peer + context. + + 4) While not SCTP specific, be aware when using NetLabel that if a label + is assigned to a specific interface, and that interface 'goes down', + then the NetLabel service will remove the entry. Therefore ensure that + the network startup scripts call **netlabelctl**\(8) to set the required + label (see **netlabel-config**\(8) helper script for details). + + 5) The NetLabel SCTP peer labeling rules apply as discussed in the following + set of posts tagged "netlabel" at: http://www.paul-moore.com/blog/t. + + 6) CIPSO is only supported for IPv4 addressing: ``socket(AF_INET, ...)`` + CALIPSO is only supported for IPv6 addressing: ``socket(AF_INET6, ...)`` + + Note the following when testing CIPSO/CALIPSO: + a) CIPSO will send an ICMP packet if an SCTP packet cannot be + delivered because of an invalid label. + b) CALIPSO does not send an ICMP packet, just silently discards it. + + 7) IPSEC is not supported as RFC 3554 - sctp/ipsec support has not been + implemented in userspace (**racoon**\(8) or **ipsec_pluto**\(8)), + although the kernel supports SCTP/IPSEC. diff --git a/Documentation/security/credentials.rst b/Documentation/security/credentials.rst new file mode 100644 index 000000000..5bb7125fa --- /dev/null +++ b/Documentation/security/credentials.rst @@ -0,0 +1,561 @@ +==================== +Credentials in Linux +==================== + +By: David Howells <dhowells@redhat.com> + +.. contents:: :local: + +Overview +======== + +There are several parts to the security check performed by Linux when one +object acts upon another: + + 1. Objects. + + Objects are things in the system that may be acted upon directly by + userspace programs. Linux has a variety of actionable objects, including: + + - Tasks + - Files/inodes + - Sockets + - Message queues + - Shared memory segments + - Semaphores + - Keys + + As a part of the description of all these objects there is a set of + credentials. What's in the set depends on the type of object. + + 2. Object ownership. + + Amongst the credentials of most objects, there will be a subset that + indicates the ownership of that object. This is used for resource + accounting and limitation (disk quotas and task rlimits for example). + + In a standard UNIX filesystem, for instance, this will be defined by the + UID marked on the inode. + + 3. The objective context. + + Also amongst the credentials of those objects, there will be a subset that + indicates the 'objective context' of that object. This may or may not be + the same set as in (2) - in standard UNIX files, for instance, this is the + defined by the UID and the GID marked on the inode. + + The objective context is used as part of the security calculation that is + carried out when an object is acted upon. + + 4. Subjects. + + A subject is an object that is acting upon another object. + + Most of the objects in the system are inactive: they don't act on other + objects within the system. Processes/tasks are the obvious exception: + they do stuff; they access and manipulate things. + + Objects other than tasks may under some circumstances also be subjects. + For instance an open file may send SIGIO to a task using the UID and EUID + given to it by a task that called ``fcntl(F_SETOWN)`` upon it. In this case, + the file struct will have a subjective context too. + + 5. The subjective context. + + A subject has an additional interpretation of its credentials. A subset + of its credentials forms the 'subjective context'. The subjective context + is used as part of the security calculation that is carried out when a + subject acts. + + A Linux task, for example, has the FSUID, FSGID and the supplementary + group list for when it is acting upon a file - which are quite separate + from the real UID and GID that normally form the objective context of the + task. + + 6. Actions. + + Linux has a number of actions available that a subject may perform upon an + object. The set of actions available depends on the nature of the subject + and the object. + + Actions include reading, writing, creating and deleting files; forking or + signalling and tracing tasks. + + 7. Rules, access control lists and security calculations. + + When a subject acts upon an object, a security calculation is made. This + involves taking the subjective context, the objective context and the + action, and searching one or more sets of rules to see whether the subject + is granted or denied permission to act in the desired manner on the + object, given those contexts. + + There are two main sources of rules: + + a. Discretionary access control (DAC): + + Sometimes the object will include sets of rules as part of its + description. This is an 'Access Control List' or 'ACL'. A Linux + file may supply more than one ACL. + + A traditional UNIX file, for example, includes a permissions mask that + is an abbreviated ACL with three fixed classes of subject ('user', + 'group' and 'other'), each of which may be granted certain privileges + ('read', 'write' and 'execute' - whatever those map to for the object + in question). UNIX file permissions do not allow the arbitrary + specification of subjects, however, and so are of limited use. + + A Linux file might also sport a POSIX ACL. This is a list of rules + that grants various permissions to arbitrary subjects. + + b. Mandatory access control (MAC): + + The system as a whole may have one or more sets of rules that get + applied to all subjects and objects, regardless of their source. + SELinux and Smack are examples of this. + + In the case of SELinux and Smack, each object is given a label as part + of its credentials. When an action is requested, they take the + subject label, the object label and the action and look for a rule + that says that this action is either granted or denied. + + +Types of Credentials +==================== + +The Linux kernel supports the following types of credentials: + + 1. Traditional UNIX credentials. + + - Real User ID + - Real Group ID + + The UID and GID are carried by most, if not all, Linux objects, even if in + some cases it has to be invented (FAT or CIFS files for example, which are + derived from Windows). These (mostly) define the objective context of + that object, with tasks being slightly different in some cases. + + - Effective, Saved and FS User ID + - Effective, Saved and FS Group ID + - Supplementary groups + + These are additional credentials used by tasks only. Usually, an + EUID/EGID/GROUPS will be used as the subjective context, and real UID/GID + will be used as the objective. For tasks, it should be noted that this is + not always true. + + 2. Capabilities. + + - Set of permitted capabilities + - Set of inheritable capabilities + - Set of effective capabilities + - Capability bounding set + + These are only carried by tasks. They indicate superior capabilities + granted piecemeal to a task that an ordinary task wouldn't otherwise have. + These are manipulated implicitly by changes to the traditional UNIX + credentials, but can also be manipulated directly by the ``capset()`` + system call. + + The permitted capabilities are those caps that the process might grant + itself to its effective or permitted sets through ``capset()``. This + inheritable set might also be so constrained. + + The effective capabilities are the ones that a task is actually allowed to + make use of itself. + + The inheritable capabilities are the ones that may get passed across + ``execve()``. + + The bounding set limits the capabilities that may be inherited across + ``execve()``, especially when a binary is executed that will execute as + UID 0. + + 3. Secure management flags (securebits). + + These are only carried by tasks. These govern the way the above + credentials are manipulated and inherited over certain operations such as + execve(). They aren't used directly as objective or subjective + credentials. + + 4. Keys and keyrings. + + These are only carried by tasks. They carry and cache security tokens + that don't fit into the other standard UNIX credentials. They are for + making such things as network filesystem keys available to the file + accesses performed by processes, without the necessity of ordinary + programs having to know about security details involved. + + Keyrings are a special type of key. They carry sets of other keys and can + be searched for the desired key. Each process may subscribe to a number + of keyrings: + + Per-thread keying + Per-process keyring + Per-session keyring + + When a process accesses a key, if not already present, it will normally be + cached on one of these keyrings for future accesses to find. + + For more information on using keys, see ``Documentation/security/keys/*``. + + 5. LSM + + The Linux Security Module allows extra controls to be placed over the + operations that a task may do. Currently Linux supports several LSM + options. + + Some work by labelling the objects in a system and then applying sets of + rules (policies) that say what operations a task with one label may do to + an object with another label. + + 6. AF_KEY + + This is a socket-based approach to credential management for networking + stacks [RFC 2367]. It isn't discussed by this document as it doesn't + interact directly with task and file credentials; rather it keeps system + level credentials. + + +When a file is opened, part of the opening task's subjective context is +recorded in the file struct created. This allows operations using that file +struct to use those credentials instead of the subjective context of the task +that issued the operation. An example of this would be a file opened on a +network filesystem where the credentials of the opened file should be presented +to the server, regardless of who is actually doing a read or a write upon it. + + +File Markings +============= + +Files on disk or obtained over the network may have annotations that form the +objective security context of that file. Depending on the type of filesystem, +this may include one or more of the following: + + * UNIX UID, GID, mode; + * Windows user ID; + * Access control list; + * LSM security label; + * UNIX exec privilege escalation bits (SUID/SGID); + * File capabilities exec privilege escalation bits. + +These are compared to the task's subjective security context, and certain +operations allowed or disallowed as a result. In the case of execve(), the +privilege escalation bits come into play, and may allow the resulting process +extra privileges, based on the annotations on the executable file. + + +Task Credentials +================ + +In Linux, all of a task's credentials are held in (uid, gid) or through +(groups, keys, LSM security) a refcounted structure of type 'struct cred'. +Each task points to its credentials by a pointer called 'cred' in its +task_struct. + +Once a set of credentials has been prepared and committed, it may not be +changed, barring the following exceptions: + + 1. its reference count may be changed; + + 2. the reference count on the group_info struct it points to may be changed; + + 3. the reference count on the security data it points to may be changed; + + 4. the reference count on any keyrings it points to may be changed; + + 5. any keyrings it points to may be revoked, expired or have their security + attributes changed; and + + 6. the contents of any keyrings to which it points may be changed (the whole + point of keyrings being a shared set of credentials, modifiable by anyone + with appropriate access). + +To alter anything in the cred struct, the copy-and-replace principle must be +adhered to. First take a copy, then alter the copy and then use RCU to change +the task pointer to make it point to the new copy. There are wrappers to aid +with this (see below). + +A task may only alter its _own_ credentials; it is no longer permitted for a +task to alter another's credentials. This means the ``capset()`` system call +is no longer permitted to take any PID other than the one of the current +process. Also ``keyctl_instantiate()`` and ``keyctl_negate()`` functions no +longer permit attachment to process-specific keyrings in the requesting +process as the instantiating process may need to create them. + + +Immutable Credentials +--------------------- + +Once a set of credentials has been made public (by calling ``commit_creds()`` +for example), it must be considered immutable, barring two exceptions: + + 1. The reference count may be altered. + + 2. Whilst the keyring subscriptions of a set of credentials may not be + changed, the keyrings subscribed to may have their contents altered. + +To catch accidental credential alteration at compile time, struct task_struct +has _const_ pointers to its credential sets, as does struct file. Furthermore, +certain functions such as ``get_cred()`` and ``put_cred()`` operate on const +pointers, thus rendering casts unnecessary, but require to temporarily ditch +the const qualification to be able to alter the reference count. + + +Accessing Task Credentials +-------------------------- + +A task being able to alter only its own credentials permits the current process +to read or replace its own credentials without the need for any form of locking +-- which simplifies things greatly. It can just call:: + + const struct cred *current_cred() + +to get a pointer to its credentials structure, and it doesn't have to release +it afterwards. + +There are convenience wrappers for retrieving specific aspects of a task's +credentials (the value is simply returned in each case):: + + uid_t current_uid(void) Current's real UID + gid_t current_gid(void) Current's real GID + uid_t current_euid(void) Current's effective UID + gid_t current_egid(void) Current's effective GID + uid_t current_fsuid(void) Current's file access UID + gid_t current_fsgid(void) Current's file access GID + kernel_cap_t current_cap(void) Current's effective capabilities + void *current_security(void) Current's LSM security pointer + struct user_struct *current_user(void) Current's user account + +There are also convenience wrappers for retrieving specific associated pairs of +a task's credentials:: + + void current_uid_gid(uid_t *, gid_t *); + void current_euid_egid(uid_t *, gid_t *); + void current_fsuid_fsgid(uid_t *, gid_t *); + +which return these pairs of values through their arguments after retrieving +them from the current task's credentials. + + +In addition, there is a function for obtaining a reference on the current +process's current set of credentials:: + + const struct cred *get_current_cred(void); + +and functions for getting references to one of the credentials that don't +actually live in struct cred:: + + struct user_struct *get_current_user(void); + struct group_info *get_current_groups(void); + +which get references to the current process's user accounting structure and +supplementary groups list respectively. + +Once a reference has been obtained, it must be released with ``put_cred()``, +``free_uid()`` or ``put_group_info()`` as appropriate. + + +Accessing Another Task's Credentials +------------------------------------ + +Whilst a task may access its own credentials without the need for locking, the +same is not true of a task wanting to access another task's credentials. It +must use the RCU read lock and ``rcu_dereference()``. + +The ``rcu_dereference()`` is wrapped by:: + + const struct cred *__task_cred(struct task_struct *task); + +This should be used inside the RCU read lock, as in the following example:: + + void foo(struct task_struct *t, struct foo_data *f) + { + const struct cred *tcred; + ... + rcu_read_lock(); + tcred = __task_cred(t); + f->uid = tcred->uid; + f->gid = tcred->gid; + f->groups = get_group_info(tcred->groups); + rcu_read_unlock(); + ... + } + +Should it be necessary to hold another task's credentials for a long period of +time, and possibly to sleep whilst doing so, then the caller should get a +reference on them using:: + + const struct cred *get_task_cred(struct task_struct *task); + +This does all the RCU magic inside of it. The caller must call put_cred() on +the credentials so obtained when they're finished with. + +.. note:: + The result of ``__task_cred()`` should not be passed directly to + ``get_cred()`` as this may race with ``commit_cred()``. + +There are a couple of convenience functions to access bits of another task's +credentials, hiding the RCU magic from the caller:: + + uid_t task_uid(task) Task's real UID + uid_t task_euid(task) Task's effective UID + +If the caller is holding the RCU read lock at the time anyway, then:: + + __task_cred(task)->uid + __task_cred(task)->euid + +should be used instead. Similarly, if multiple aspects of a task's credentials +need to be accessed, RCU read lock should be used, ``__task_cred()`` called, +the result stored in a temporary pointer and then the credential aspects called +from that before dropping the lock. This prevents the potentially expensive +RCU magic from being invoked multiple times. + +Should some other single aspect of another task's credentials need to be +accessed, then this can be used:: + + task_cred_xxx(task, member) + +where 'member' is a non-pointer member of the cred struct. For instance:: + + uid_t task_cred_xxx(task, suid); + +will retrieve 'struct cred::suid' from the task, doing the appropriate RCU +magic. This may not be used for pointer members as what they point to may +disappear the moment the RCU read lock is dropped. + + +Altering Credentials +-------------------- + +As previously mentioned, a task may only alter its own credentials, and may not +alter those of another task. This means that it doesn't need to use any +locking to alter its own credentials. + +To alter the current process's credentials, a function should first prepare a +new set of credentials by calling:: + + struct cred *prepare_creds(void); + +this locks current->cred_replace_mutex and then allocates and constructs a +duplicate of the current process's credentials, returning with the mutex still +held if successful. It returns NULL if not successful (out of memory). + +The mutex prevents ``ptrace()`` from altering the ptrace state of a process +whilst security checks on credentials construction and changing is taking place +as the ptrace state may alter the outcome, particularly in the case of +``execve()``. + +The new credentials set should be altered appropriately, and any security +checks and hooks done. Both the current and the proposed sets of credentials +are available for this purpose as current_cred() will return the current set +still at this point. + +When replacing the group list, the new list must be sorted before it +is added to the credential, as a binary search is used to test for +membership. In practice, this means :c:func:`groups_sort` should be +called before :c:func:`set_groups` or :c:func:`set_current_groups`. +:c:func:`groups_sort)` must not be called on a ``struct group_list`` which +is shared as it may permute elements as part of the sorting process +even if the array is already sorted. + +When the credential set is ready, it should be committed to the current process +by calling:: + + int commit_creds(struct cred *new); + +This will alter various aspects of the credentials and the process, giving the +LSM a chance to do likewise, then it will use ``rcu_assign_pointer()`` to +actually commit the new credentials to ``current->cred``, it will release +``current->cred_replace_mutex`` to allow ``ptrace()`` to take place, and it +will notify the scheduler and others of the changes. + +This function is guaranteed to return 0, so that it can be tail-called at the +end of such functions as ``sys_setresuid()``. + +Note that this function consumes the caller's reference to the new credentials. +The caller should _not_ call ``put_cred()`` on the new credentials afterwards. + +Furthermore, once this function has been called on a new set of credentials, +those credentials may _not_ be changed further. + + +Should the security checks fail or some other error occur after +``prepare_creds()`` has been called, then the following function should be +invoked:: + + void abort_creds(struct cred *new); + +This releases the lock on ``current->cred_replace_mutex`` that +``prepare_creds()`` got and then releases the new credentials. + + +A typical credentials alteration function would look something like this:: + + int alter_suid(uid_t suid) + { + struct cred *new; + int ret; + + new = prepare_creds(); + if (!new) + return -ENOMEM; + + new->suid = suid; + ret = security_alter_suid(new); + if (ret < 0) { + abort_creds(new); + return ret; + } + + return commit_creds(new); + } + + +Managing Credentials +-------------------- + +There are some functions to help manage credentials: + + - ``void put_cred(const struct cred *cred);`` + + This releases a reference to the given set of credentials. If the + reference count reaches zero, the credentials will be scheduled for + destruction by the RCU system. + + - ``const struct cred *get_cred(const struct cred *cred);`` + + This gets a reference on a live set of credentials, returning a pointer to + that set of credentials. + + - ``struct cred *get_new_cred(struct cred *cred);`` + + This gets a reference on a set of credentials that is under construction + and is thus still mutable, returning a pointer to that set of credentials. + + +Open File Credentials +===================== + +When a new file is opened, a reference is obtained on the opening task's +credentials and this is attached to the file struct as ``f_cred`` in place of +``f_uid`` and ``f_gid``. Code that used to access ``file->f_uid`` and +``file->f_gid`` should now access ``file->f_cred->fsuid`` and +``file->f_cred->fsgid``. + +It is safe to access ``f_cred`` without the use of RCU or locking because the +pointer will not change over the lifetime of the file struct, and nor will the +contents of the cred struct pointed to, barring the exceptions listed above +(see the Task Credentials section). + + +Overriding the VFS's Use of Credentials +======================================= + +Under some circumstances it is desirable to override the credentials used by +the VFS, and that can be done by calling into such as ``vfs_mkdir()`` with a +different set of credentials. This is done in the following places: + + * ``sys_faccessat()``. + * ``do_coredump()``. + * nfs4recover.c. diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst new file mode 100644 index 000000000..85492bfca --- /dev/null +++ b/Documentation/security/index.rst @@ -0,0 +1,15 @@ +====================== +Security Documentation +====================== + +.. toctree:: + :maxdepth: 1 + + credentials + IMA-templates + keys/index + LSM + LSM-sctp + SELinux-sctp + self-protection + tpm/index diff --git a/Documentation/security/keys/core.rst b/Documentation/security/keys/core.rst new file mode 100644 index 000000000..9ce7256c6 --- /dev/null +++ b/Documentation/security/keys/core.rst @@ -0,0 +1,1546 @@ +============================ +Kernel Key Retention Service +============================ + +This service allows cryptographic keys, authentication tokens, cross-domain +user mappings, and similar to be cached in the kernel for the use of +filesystems and other kernel services. + +Keyrings are permitted; these are a special type of key that can hold links to +other keys. Processes each have three standard keyring subscriptions that a +kernel service can search for relevant keys. + +The key service can be configured on by enabling: + + "Security options"/"Enable access key retention support" (CONFIG_KEYS) + +This document has the following sections: + +.. contents:: :local: + + +Key Overview +============ + +In this context, keys represent units of cryptographic data, authentication +tokens, keyrings, etc.. These are represented in the kernel by struct key. + +Each key has a number of attributes: + + - A serial number. + - A type. + - A description (for matching a key in a search). + - Access control information. + - An expiry time. + - A payload. + - State. + + + * Each key is issued a serial number of type key_serial_t that is unique for + the lifetime of that key. All serial numbers are positive non-zero 32-bit + integers. + + Userspace programs can use a key's serial numbers as a way to gain access + to it, subject to permission checking. + + * Each key is of a defined "type". Types must be registered inside the + kernel by a kernel service (such as a filesystem) before keys of that type + can be added or used. Userspace programs cannot define new types directly. + + Key types are represented in the kernel by struct key_type. This defines a + number of operations that can be performed on a key of that type. + + Should a type be removed from the system, all the keys of that type will + be invalidated. + + * Each key has a description. This should be a printable string. The key + type provides an operation to perform a match between the description on a + key and a criterion string. + + * Each key has an owner user ID, a group ID and a permissions mask. These + are used to control what a process may do to a key from userspace, and + whether a kernel service will be able to find the key. + + * Each key can be set to expire at a specific time by the key type's + instantiation function. Keys can also be immortal. + + * Each key can have a payload. This is a quantity of data that represent the + actual "key". In the case of a keyring, this is a list of keys to which + the keyring links; in the case of a user-defined key, it's an arbitrary + blob of data. + + Having a payload is not required; and the payload can, in fact, just be a + value stored in the struct key itself. + + When a key is instantiated, the key type's instantiation function is + called with a blob of data, and that then creates the key's payload in + some way. + + Similarly, when userspace wants to read back the contents of the key, if + permitted, another key type operation will be called to convert the key's + attached payload back into a blob of data. + + * Each key can be in one of a number of basic states: + + * Uninstantiated. The key exists, but does not have any data attached. + Keys being requested from userspace will be in this state. + + * Instantiated. This is the normal state. The key is fully formed, and + has data attached. + + * Negative. This is a relatively short-lived state. The key acts as a + note saying that a previous call out to userspace failed, and acts as + a throttle on key lookups. A negative key can be updated to a normal + state. + + * Expired. Keys can have lifetimes set. If their lifetime is exceeded, + they traverse to this state. An expired key can be updated back to a + normal state. + + * Revoked. A key is put in this state by userspace action. It can't be + found or operated upon (apart from by unlinking it). + + * Dead. The key's type was unregistered, and so the key is now useless. + +Keys in the last three states are subject to garbage collection. See the +section on "Garbage collection". + + +Key Service Overview +==================== + +The key service provides a number of features besides keys: + + * The key service defines three special key types: + + (+) "keyring" + + Keyrings are special keys that contain a list of other keys. Keyring + lists can be modified using various system calls. Keyrings should not + be given a payload when created. + + (+) "user" + + A key of this type has a description and a payload that are arbitrary + blobs of data. These can be created, updated and read by userspace, + and aren't intended for use by kernel services. + + (+) "logon" + + Like a "user" key, a "logon" key has a payload that is an arbitrary + blob of data. It is intended as a place to store secrets which are + accessible to the kernel but not to userspace programs. + + The description can be arbitrary, but must be prefixed with a non-zero + length string that describes the key "subclass". The subclass is + separated from the rest of the description by a ':'. "logon" keys can + be created and updated from userspace, but the payload is only + readable from kernel space. + + * Each process subscribes to three keyrings: a thread-specific keyring, a + process-specific keyring, and a session-specific keyring. + + The thread-specific keyring is discarded from the child when any sort of + clone, fork, vfork or execve occurs. A new keyring is created only when + required. + + The process-specific keyring is replaced with an empty one in the child on + clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is + shared. execve also discards the process's process keyring and creates a + new one. + + The session-specific keyring is persistent across clone, fork, vfork and + execve, even when the latter executes a set-UID or set-GID binary. A + process can, however, replace its current session keyring with a new one + by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous + new one, or to attempt to create or join one of a specific name. + + The ownership of the thread keyring changes when the real UID and GID of + the thread changes. + + * Each user ID resident in the system holds two special keyrings: a user + specific keyring and a default user session keyring. The default session + keyring is initialised with a link to the user-specific keyring. + + When a process changes its real UID, if it used to have no session key, it + will be subscribed to the default session key for the new UID. + + If a process attempts to access its session key when it doesn't have one, + it will be subscribed to the default for its current UID. + + * Each user has two quotas against which the keys they own are tracked. One + limits the total number of keys and keyrings, the other limits the total + amount of description and payload space that can be consumed. + + The user can view information on this and other statistics through procfs + files. The root user may also alter the quota limits through sysctl files + (see the section "New procfs files"). + + Process-specific and thread-specific keyrings are not counted towards a + user's quota. + + If a system call that modifies a key or keyring in some way would put the + user over quota, the operation is refused and error EDQUOT is returned. + + * There's a system call interface by which userspace programs can create and + manipulate keys and keyrings. + + * There's a kernel interface by which services can register types and search + for keys. + + * There's a way for the a search done from the kernel to call back to + userspace to request a key that can't be found in a process's keyrings. + + * An optional filesystem is available through which the key database can be + viewed and manipulated. + + +Key Access Permissions +====================== + +Keys have an owner user ID, a group access ID, and a permissions mask. The mask +has up to eight bits each for possessor, user, group and other access. Only +six of each set of eight bits are defined. These permissions granted are: + + * View + + This permits a key or keyring's attributes to be viewed - including key + type and description. + + * Read + + This permits a key's payload to be viewed or a keyring's list of linked + keys. + + * Write + + This permits a key's payload to be instantiated or updated, or it allows a + link to be added to or removed from a keyring. + + * Search + + This permits keyrings to be searched and keys to be found. Searches can + only recurse into nested keyrings that have search permission set. + + * Link + + This permits a key or keyring to be linked to. To create a link from a + keyring to a key, a process must have Write permission on the keyring and + Link permission on the key. + + * Set Attribute + + This permits a key's UID, GID and permissions mask to be changed. + +For changing the ownership, group ID or permissions mask, being the owner of +the key or having the sysadmin capability is sufficient. + + +SELinux Support +=============== + +The security class "key" has been added to SELinux so that mandatory access +controls can be applied to keys created within various contexts. This support +is preliminary, and is likely to change quite significantly in the near future. +Currently, all of the basic permissions explained above are provided in SELinux +as well; SELinux is simply invoked after all basic permission checks have been +performed. + +The value of the file /proc/self/attr/keycreate influences the labeling of +newly-created keys. If the contents of that file correspond to an SELinux +security context, then the key will be assigned that context. Otherwise, the +key will be assigned the current context of the task that invoked the key +creation request. Tasks must be granted explicit permission to assign a +particular context to newly-created keys, using the "create" permission in the +key security class. + +The default keyrings associated with users will be labeled with the default +context of the user if and only if the login programs have been instrumented to +properly initialize keycreate during the login process. Otherwise, they will +be labeled with the context of the login program itself. + +Note, however, that the default keyrings associated with the root user are +labeled with the default kernel context, since they are created early in the +boot process, before root has a chance to log in. + +The keyrings associated with new threads are each labeled with the context of +their associated thread, and both session and process keyrings are handled +similarly. + + +New ProcFS Files +================ + +Two files have been added to procfs by which an administrator can find out +about the status of the key service: + + * /proc/keys + + This lists the keys that are currently viewable by the task reading the + file, giving information about their type, description and permissions. + It is not possible to view the payload of the key this way, though some + information about it may be given. + + The only keys included in the list are those that grant View permission to + the reading process whether or not it possesses them. Note that LSM + security checks are still performed, and may further filter out keys that + the current process is not authorised to view. + + The contents of the file look like this:: + + SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY + 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4 + 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty + 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty + 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty + 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4 + 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty + 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0 + 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0 + 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0 + + The flags are:: + + I Instantiated + R Revoked + D Dead + Q Contributes to user's quota + U Under construction by callback to userspace + N Negative key + + + * /proc/key-users + + This file lists the tracking data for each user that has at least one key + on the system. Such data includes quota information and statistics:: + + [root@andromeda root]# cat /proc/key-users + 0: 46 45/45 1/100 13/10000 + 29: 2 2/2 2/100 40/10000 + 32: 2 2/2 2/100 40/10000 + 38: 2 2/2 2/100 40/10000 + + The format of each line is:: + + <UID>: User ID to which this applies + <usage> Structure refcount + <inst>/<keys> Total number of keys and number instantiated + <keys>/<max> Key count quota + <bytes>/<max> Key size quota + + +Four new sysctl files have been added also for the purpose of controlling the +quota limits on keys: + + * /proc/sys/kernel/keys/root_maxkeys + /proc/sys/kernel/keys/root_maxbytes + + These files hold the maximum number of keys that root may have and the + maximum total number of bytes of data that root may have stored in those + keys. + + * /proc/sys/kernel/keys/maxkeys + /proc/sys/kernel/keys/maxbytes + + These files hold the maximum number of keys that each non-root user may + have and the maximum total number of bytes of data that each of those + users may have stored in their keys. + +Root may alter these by writing each new limit as a decimal number string to +the appropriate file. + + +Userspace System Call Interface +=============================== + +Userspace can manipulate keys directly through three new syscalls: add_key, +request_key and keyctl. The latter provides a number of functions for +manipulating keys. + +When referring to a key directly, userspace programs should use the key's +serial number (a positive 32-bit integer). However, there are some special +values available for referring to special keys and keyrings that relate to the +process making the call:: + + CONSTANT VALUE KEY REFERENCED + ============================== ====== =========================== + KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring + KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring + KEY_SPEC_SESSION_KEYRING -3 session-specific keyring + KEY_SPEC_USER_KEYRING -4 UID-specific keyring + KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring + KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring + KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key() + authorisation key + + +The main syscalls are: + + * Create a new key of given type, description and payload and add it to the + nominated keyring:: + + key_serial_t add_key(const char *type, const char *desc, + const void *payload, size_t plen, + key_serial_t keyring); + + If a key of the same type and description as that proposed already exists + in the keyring, this will try to update it with the given payload, or it + will return error EEXIST if that function is not supported by the key + type. The process must also have permission to write to the key to be able + to update it. The new key will have all user permissions granted and no + group or third party permissions. + + Otherwise, this will attempt to create a new key of the specified type and + description, and to instantiate it with the supplied payload and attach it + to the keyring. In this case, an error will be generated if the process + does not have permission to write to the keyring. + + If the key type supports it, if the description is NULL or an empty + string, the key type will try and generate a description from the content + of the payload. + + The payload is optional, and the pointer can be NULL if not required by + the type. The payload is plen in size, and plen can be zero for an empty + payload. + + A new keyring can be generated by setting type "keyring", the keyring name + as the description (or NULL) and setting the payload to NULL. + + User defined keys can be created by specifying type "user". It is + recommended that a user defined key's description by prefixed with a type + ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting + ticket. + + Any other type must have been registered with the kernel in advance by a + kernel service such as a filesystem. + + The ID of the new or updated key is returned if successful. + + + * Search the process's keyrings for a key, potentially calling out to + userspace to create it:: + + key_serial_t request_key(const char *type, const char *description, + const char *callout_info, + key_serial_t dest_keyring); + + This function searches all the process's keyrings in the order thread, + process, session for a matching key. This works very much like + KEYCTL_SEARCH, including the optional attachment of the discovered key to + a keyring. + + If a key cannot be found, and if callout_info is not NULL, then + /sbin/request-key will be invoked in an attempt to obtain a key. The + callout_info string will be passed as an argument to the program. + + See also Documentation/security/keys/request-key.rst. + + +The keyctl syscall functions are: + + * Map a special key ID to a real key ID for this process:: + + key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id, + int create); + + The special key specified by "id" is looked up (with the key being created + if necessary) and the ID of the key or keyring thus found is returned if + it exists. + + If the key does not yet exist, the key will be created if "create" is + non-zero; and the error ENOKEY will be returned if "create" is zero. + + + * Replace the session keyring this process subscribes to with a new one:: + + key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name); + + If name is NULL, an anonymous keyring is created attached to the process + as its session keyring, displacing the old session keyring. + + If name is not NULL, if a keyring of that name exists, the process + attempts to attach it as the session keyring, returning an error if that + is not permitted; otherwise a new keyring of that name is created and + attached as the session keyring. + + To attach to a named keyring, the keyring must have search permission for + the process's ownership. + + The ID of the new session keyring is returned if successful. + + + * Update the specified key:: + + long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload, + size_t plen); + + This will try to update the specified key with the given payload, or it + will return error EOPNOTSUPP if that function is not supported by the key + type. The process must also have permission to write to the key to be able + to update it. + + The payload is of length plen, and may be absent or empty as for + add_key(). + + + * Revoke a key:: + + long keyctl(KEYCTL_REVOKE, key_serial_t key); + + This makes a key unavailable for further operations. Further attempts to + use the key will be met with error EKEYREVOKED, and the key will no longer + be findable. + + + * Change the ownership of a key:: + + long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid); + + This function permits a key's owner and group ID to be changed. Either one + of uid or gid can be set to -1 to suppress that change. + + Only the superuser can change a key's owner to something other than the + key's current owner. Similarly, only the superuser can change a key's + group ID to something other than the calling process's group ID or one of + its group list members. + + + * Change the permissions mask on a key:: + + long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm); + + This function permits the owner of a key or the superuser to change the + permissions mask on a key. + + Only bits the available bits are permitted; if any other bits are set, + error EINVAL will be returned. + + + * Describe a key:: + + long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer, + size_t buflen); + + This function returns a summary of the key's attributes (but not its + payload data) as a string in the buffer provided. + + Unless there's an error, it always returns the amount of data it could + produce, even if that's too big for the buffer, but it won't copy more + than requested to userspace. If the buffer pointer is NULL then no copy + will take place. + + A process must have view permission on the key for this function to be + successful. + + If successful, a string is placed in the buffer in the following format:: + + <type>;<uid>;<gid>;<perm>;<description> + + Where type and description are strings, uid and gid are decimal, and perm + is hexadecimal. A NUL character is included at the end of the string if + the buffer is sufficiently big. + + This can be parsed with:: + + sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc); + + + * Clear out a keyring:: + + long keyctl(KEYCTL_CLEAR, key_serial_t keyring); + + This function clears the list of keys attached to a keyring. The calling + process must have write permission on the keyring, and it must be a + keyring (or else error ENOTDIR will result). + + This function can also be used to clear special kernel keyrings if they + are appropriately marked if the user has CAP_SYS_ADMIN capability. The + DNS resolver cache keyring is an example of this. + + + * Link a key into a keyring:: + + long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key); + + This function creates a link from the keyring to the key. The process must + have write permission on the keyring and must have link permission on the + key. + + Should the keyring not be a keyring, error ENOTDIR will result; and if the + keyring is full, error ENFILE will result. + + The link procedure checks the nesting of the keyrings, returning ELOOP if + it appears too deep or EDEADLK if the link would introduce a cycle. + + Any links within the keyring to keys that match the new key in terms of + type and description will be discarded from the keyring as the new one is + added. + + + * Unlink a key or keyring from another keyring:: + + long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key); + + This function looks through the keyring for the first link to the + specified key, and removes it if found. Subsequent links to that key are + ignored. The process must have write permission on the keyring. + + If the keyring is not a keyring, error ENOTDIR will result; and if the key + is not present, error ENOENT will be the result. + + + * Search a keyring tree for a key:: + + key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring, + const char *type, const char *description, + key_serial_t dest_keyring); + + This searches the keyring tree headed by the specified keyring until a key + is found that matches the type and description criteria. Each keyring is + checked for keys before recursion into its children occurs. + + The process must have search permission on the top level keyring, or else + error EACCES will result. Only keyrings that the process has search + permission on will be recursed into, and only keys and keyrings for which + a process has search permission can be matched. If the specified keyring + is not a keyring, ENOTDIR will result. + + If the search succeeds, the function will attempt to link the found key + into the destination keyring if one is supplied (non-zero ID). All the + constraints applicable to KEYCTL_LINK apply in this case too. + + Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search + fails. On success, the resulting key ID will be returned. + + + * Read the payload data from a key:: + + long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer, + size_t buflen); + + This function attempts to read the payload data from the specified key + into the buffer. The process must have read permission on the key to + succeed. + + The returned data will be processed for presentation by the key type. For + instance, a keyring will return an array of key_serial_t entries + representing the IDs of all the keys to which it is subscribed. The user + defined key type will return its data as is. If a key type does not + implement this function, error EOPNOTSUPP will result. + + If the specified buffer is too small, then the size of the buffer required + will be returned. Note that in this case, the contents of the buffer may + have been overwritten in some undefined way. + + Otherwise, on success, the function will return the amount of data copied + into the buffer. + + * Instantiate a partially constructed key:: + + long keyctl(KEYCTL_INSTANTIATE, key_serial_t key, + const void *payload, size_t plen, + key_serial_t keyring); + long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key, + const struct iovec *payload_iov, unsigned ioc, + key_serial_t keyring); + + If the kernel calls back to userspace to complete the instantiation of a + key, userspace should use this call to supply data for the key before the + invoked process returns, or else the key will be marked negative + automatically. + + The process must have write access on the key to be able to instantiate + it, and the key must be uninstantiated. + + If a keyring is specified (non-zero), the key will also be linked into + that keyring, however all the constraints applying in KEYCTL_LINK apply in + this case too. + + The payload and plen arguments describe the payload data as for add_key(). + + The payload_iov and ioc arguments describe the payload data in an iovec + array instead of a single buffer. + + + * Negatively instantiate a partially constructed key:: + + long keyctl(KEYCTL_NEGATE, key_serial_t key, + unsigned timeout, key_serial_t keyring); + long keyctl(KEYCTL_REJECT, key_serial_t key, + unsigned timeout, unsigned error, key_serial_t keyring); + + If the kernel calls back to userspace to complete the instantiation of a + key, userspace should use this call mark the key as negative before the + invoked process returns if it is unable to fulfill the request. + + The process must have write access on the key to be able to instantiate + it, and the key must be uninstantiated. + + If a keyring is specified (non-zero), the key will also be linked into + that keyring, however all the constraints applying in KEYCTL_LINK apply in + this case too. + + If the key is rejected, future searches for it will return the specified + error code until the rejected key expires. Negating the key is the same + as rejecting the key with ENOKEY as the error code. + + + * Set the default request-key destination keyring:: + + long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl); + + This sets the default keyring to which implicitly requested keys will be + attached for this thread. reqkey_defl should be one of these constants:: + + CONSTANT VALUE NEW DEFAULT KEYRING + ====================================== ====== ======================= + KEY_REQKEY_DEFL_NO_CHANGE -1 No change + KEY_REQKEY_DEFL_DEFAULT 0 Default[1] + KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring + KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring + KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring + KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring + KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring + KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring + + The old default will be returned if successful and error EINVAL will be + returned if reqkey_defl is not one of the above values. + + The default keyring can be overridden by the keyring indicated to the + request_key() system call. + + Note that this setting is inherited across fork/exec. + + [1] The default is: the thread keyring if there is one, otherwise + the process keyring if there is one, otherwise the session keyring if + there is one, otherwise the user default session keyring. + + + * Set the timeout on a key:: + + long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout); + + This sets or clears the timeout on a key. The timeout can be 0 to clear + the timeout or a number of seconds to set the expiry time that far into + the future. + + The process must have attribute modification access on a key to set its + timeout. Timeouts may not be set with this function on negative, revoked + or expired keys. + + + * Assume the authority granted to instantiate a key:: + + long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key); + + This assumes or divests the authority required to instantiate the + specified key. Authority can only be assumed if the thread has the + authorisation key associated with the specified key in its keyrings + somewhere. + + Once authority is assumed, searches for keys will also search the + requester's keyrings using the requester's security label, UID, GID and + groups. + + If the requested authority is unavailable, error EPERM will be returned, + likewise if the authority has been revoked because the target key is + already instantiated. + + If the specified key is 0, then any assumed authority will be divested. + + The assumed authoritative key is inherited across fork and exec. + + + * Get the LSM security context attached to a key:: + + long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer, + size_t buflen) + + This function returns a string that represents the LSM security context + attached to a key in the buffer provided. + + Unless there's an error, it always returns the amount of data it could + produce, even if that's too big for the buffer, but it won't copy more + than requested to userspace. If the buffer pointer is NULL then no copy + will take place. + + A NUL character is included at the end of the string if the buffer is + sufficiently big. This is included in the returned count. If no LSM is + in force then an empty string will be returned. + + A process must have view permission on the key for this function to be + successful. + + + * Install the calling process's session keyring on its parent:: + + long keyctl(KEYCTL_SESSION_TO_PARENT); + + This functions attempts to install the calling process's session keyring + on to the calling process's parent, replacing the parent's current session + keyring. + + The calling process must have the same ownership as its parent, the + keyring must have the same ownership as the calling process, the calling + process must have LINK permission on the keyring and the active LSM module + mustn't deny permission, otherwise error EPERM will be returned. + + Error ENOMEM will be returned if there was insufficient memory to complete + the operation, otherwise 0 will be returned to indicate success. + + The keyring will be replaced next time the parent process leaves the + kernel and resumes executing userspace. + + + * Invalidate a key:: + + long keyctl(KEYCTL_INVALIDATE, key_serial_t key); + + This function marks a key as being invalidated and then wakes up the + garbage collector. The garbage collector immediately removes invalidated + keys from all keyrings and deletes the key when its reference count + reaches zero. + + Keys that are marked invalidated become invisible to normal key operations + immediately, though they are still visible in /proc/keys until deleted + (they're marked with an 'i' flag). + + A process must have search permission on the key for this function to be + successful. + + * Compute a Diffie-Hellman shared secret or public key:: + + long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params, + char *buffer, size_t buflen, struct keyctl_kdf_params *kdf); + + The params struct contains serial numbers for three keys:: + + - The prime, p, known to both parties + - The local private key + - The base integer, which is either a shared generator or the + remote public key + + The value computed is:: + + result = base ^ private (mod prime) + + If the base is the shared generator, the result is the local + public key. If the base is the remote public key, the result is + the shared secret. + + If the parameter kdf is NULL, the following applies: + + - The buffer length must be at least the length of the prime, or zero. + + - If the buffer length is nonzero, the length of the result is + returned when it is successfully calculated and copied in to the + buffer. When the buffer length is zero, the minimum required + buffer length is returned. + + The kdf parameter allows the caller to apply a key derivation function + (KDF) on the Diffie-Hellman computation where only the result + of the KDF is returned to the caller. The KDF is characterized with + struct keyctl_kdf_params as follows: + + - ``char *hashname`` specifies the NUL terminated string identifying + the hash used from the kernel crypto API and applied for the KDF + operation. The KDF implemenation complies with SP800-56A as well + as with SP800-108 (the counter KDF). + + - ``char *otherinfo`` specifies the OtherInfo data as documented in + SP800-56A section 5.8.1.2. The length of the buffer is given with + otherinfolen. The format of OtherInfo is defined by the caller. + The otherinfo pointer may be NULL if no OtherInfo shall be used. + + This function will return error EOPNOTSUPP if the key type is not + supported, error ENOKEY if the key could not be found, or error + EACCES if the key is not readable by the caller. In addition, the + function will return EMSGSIZE when the parameter kdf is non-NULL + and either the buffer length or the OtherInfo length exceeds the + allowed length. + + * Restrict keyring linkage:: + + long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring, + const char *type, const char *restriction); + + An existing keyring can restrict linkage of additional keys by evaluating + the contents of the key according to a restriction scheme. + + "keyring" is the key ID for an existing keyring to apply a restriction + to. It may be empty or may already have keys linked. Existing linked keys + will remain in the keyring even if the new restriction would reject them. + + "type" is a registered key type. + + "restriction" is a string describing how key linkage is to be restricted. + The format varies depending on the key type, and the string is passed to + the lookup_restriction() function for the requested type. It may specify + a method and relevant data for the restriction such as signature + verification or constraints on key payload. If the requested key type is + later unregistered, no keys may be added to the keyring after the key type + is removed. + + To apply a keyring restriction the process must have Set Attribute + permission and the keyring must not be previously restricted. + + One application of restricted keyrings is to verify X.509 certificate + chains or individual certificate signatures using the asymmetric key type. + See Documentation/crypto/asymmetric-keys.txt for specific restrictions + applicable to the asymmetric key type. + + +Kernel Services +=============== + +The kernel services for key management are fairly simple to deal with. They can +be broken down into two areas: keys and key types. + +Dealing with keys is fairly straightforward. Firstly, the kernel service +registers its type, then it searches for a key of that type. It should retain +the key as long as it has need of it, and then it should release it. For a +filesystem or device file, a search would probably be performed during the open +call, and the key released upon close. How to deal with conflicting keys due to +two different users opening the same file is left to the filesystem author to +solve. + +To access the key manager, the following header must be #included:: + + <linux/key.h> + +Specific key types should have a header file under include/keys/ that should be +used to access that type. For keys of type "user", for example, that would be:: + + <keys/user-type.h> + +Note that there are two different types of pointers to keys that may be +encountered: + + * struct key * + + This simply points to the key structure itself. Key structures will be at + least four-byte aligned. + + * key_ref_t + + This is equivalent to a ``struct key *``, but the least significant bit is set + if the caller "possesses" the key. By "possession" it is meant that the + calling processes has a searchable link to the key from one of its + keyrings. There are three functions for dealing with these:: + + key_ref_t make_key_ref(const struct key *key, bool possession); + + struct key *key_ref_to_ptr(const key_ref_t key_ref); + + bool is_key_possessed(const key_ref_t key_ref); + + The first function constructs a key reference from a key pointer and + possession information (which must be true or false). + + The second function retrieves the key pointer from a reference and the + third retrieves the possession flag. + +When accessing a key's payload contents, certain precautions must be taken to +prevent access vs modification races. See the section "Notes on accessing +payload contents" for more information. + + * To search for a key, call:: + + struct key *request_key(const struct key_type *type, + const char *description, + const char *callout_info); + + This is used to request a key or keyring with a description that matches + the description specified according to the key type's match_preparse() + method. This permits approximate matching to occur. If callout_string is + not NULL, then /sbin/request-key will be invoked in an attempt to obtain + the key from userspace. In that case, callout_string will be passed as an + argument to the program. + + Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be + returned. + + If successful, the key will have been attached to the default keyring for + implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING. + + See also Documentation/security/keys/request-key.rst. + + + * To search for a key, passing auxiliary data to the upcaller, call:: + + struct key *request_key_with_auxdata(const struct key_type *type, + const char *description, + const void *callout_info, + size_t callout_len, + void *aux); + + This is identical to request_key(), except that the auxiliary data is + passed to the key_type->request_key() op if it exists, and the callout_info + is a blob of length callout_len, if given (the length may be 0). + + + * A key can be requested asynchronously by calling one of:: + + struct key *request_key_async(const struct key_type *type, + const char *description, + const void *callout_info, + size_t callout_len); + + or:: + + struct key *request_key_async_with_auxdata(const struct key_type *type, + const char *description, + const char *callout_info, + size_t callout_len, + void *aux); + + which are asynchronous equivalents of request_key() and + request_key_with_auxdata() respectively. + + These two functions return with the key potentially still under + construction. To wait for construction completion, the following should be + called:: + + int wait_for_key_construction(struct key *key, bool intr); + + The function will wait for the key to finish being constructed and then + invokes key_validate() to return an appropriate value to indicate the state + of the key (0 indicates the key is usable). + + If intr is true, then the wait can be interrupted by a signal, in which + case error ERESTARTSYS will be returned. + + + * When it is no longer required, the key should be released using:: + + void key_put(struct key *key); + + Or:: + + void key_ref_put(key_ref_t key_ref); + + These can be called from interrupt context. If CONFIG_KEYS is not set then + the argument will not be parsed. + + + * Extra references can be made to a key by calling one of the following + functions:: + + struct key *__key_get(struct key *key); + struct key *key_get(struct key *key); + + Keys so references will need to be disposed of by calling key_put() when + they've been finished with. The key pointer passed in will be returned. + + In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set + then the key will not be dereferenced and no increment will take place. + + + * A key's serial number can be obtained by calling:: + + key_serial_t key_serial(struct key *key); + + If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the + latter case without parsing the argument). + + + * If a keyring was found in the search, this can be further searched by:: + + key_ref_t keyring_search(key_ref_t keyring_ref, + const struct key_type *type, + const char *description) + + This searches the keyring tree specified for a matching key. Error ENOKEY + is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful, + the returned key will need to be released. + + The possession attribute from the keyring reference is used to control + access through the permissions mask and is propagated to the returned key + reference pointer if successful. + + + * A keyring can be created by:: + + struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid, + const struct cred *cred, + key_perm_t perm, + struct key_restriction *restrict_link, + unsigned long flags, + struct key *dest); + + This creates a keyring with the given attributes and returns it. If dest + is not NULL, the new keyring will be linked into the keyring to which it + points. No permission checks are made upon the destination keyring. + + Error EDQUOT can be returned if the keyring would overload the quota (pass + KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted + towards the user's quota). Error ENOMEM can also be returned. + + If restrict_link is not NULL, it should point to a structure that contains + the function that will be called each time an attempt is made to link a + key into the new keyring. The structure may also contain a key pointer + and an associated key type. The function is called to check whether a key + may be added into the keyring or not. The key type is used by the garbage + collector to clean up function or data pointers in this structure if the + given key type is unregistered. Callers of key_create_or_update() within + the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check. + An example of using this is to manage rings of cryptographic keys that are + set up when the kernel boots where userspace is also permitted to add keys + - provided they can be verified by a key the kernel already has. + + When called, the restriction function will be passed the keyring being + added to, the key type, the payload of the key being added, and data to be + used in the restriction check. Note that when a new key is being created, + this is called between payload preparsing and actual key creation. The + function should return 0 to allow the link or an error to reject it. + + A convenience function, restrict_link_reject, exists to always return + -EPERM to in this case. + + + * To check the validity of a key, this function can be called:: + + int validate_key(struct key *key); + + This checks that the key in question hasn't expired or and hasn't been + revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will + be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be + returned (in the latter case without parsing the argument). + + + * To register a key type, the following function should be called:: + + int register_key_type(struct key_type *type); + + This will return error EEXIST if a type of the same name is already + present. + + + * To unregister a key type, call:: + + void unregister_key_type(struct key_type *type); + + +Under some circumstances, it may be desirable to deal with a bundle of keys. +The facility provides access to the keyring type for managing such a bundle:: + + struct key_type key_type_keyring; + +This can be used with a function such as request_key() to find a specific +keyring in a process's keyrings. A keyring thus found can then be searched +with keyring_search(). Note that it is not possible to use request_key() to +search a specific keyring, so using keyrings in this way is of limited utility. + + +Notes On Accessing Payload Contents +=================================== + +The simplest payload is just data stored in key->payload directly. In this +case, there's no need to indulge in RCU or locking when accessing the payload. + +More complex payload contents must be allocated and pointers to them set in the +key->payload.data[] array. One of the following ways must be selected to +access the data: + + 1) Unmodifiable key type. + + If the key type does not have a modify method, then the key's payload can + be accessed without any form of locking, provided that it's known to be + instantiated (uninstantiated keys cannot be "found"). + + 2) The key's semaphore. + + The semaphore could be used to govern access to the payload and to control + the payload pointer. It must be write-locked for modifications and would + have to be read-locked for general access. The disadvantage of doing this + is that the accessor may be required to sleep. + + 3) RCU. + + RCU must be used when the semaphore isn't already held; if the semaphore + is held then the contents can't change under you unexpectedly as the + semaphore must still be used to serialise modifications to the key. The + key management code takes care of this for the key type. + + However, this means using:: + + rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock() + + to read the pointer, and:: + + rcu_dereference() ... rcu_assign_pointer() ... call_rcu() + + to set the pointer and dispose of the old contents after a grace period. + Note that only the key type should ever modify a key's payload. + + Furthermore, an RCU controlled payload must hold a struct rcu_head for the + use of call_rcu() and, if the payload is of variable size, the length of + the payload. key->datalen cannot be relied upon to be consistent with the + payload just dereferenced if the key's semaphore is not held. + + Note that key->payload.data[0] has a shadow that is marked for __rcu + usage. This is called key->payload.rcu_data0. The following accessors + wrap the RCU calls to this element: + + a) Set or change the first payload pointer:: + + rcu_assign_keypointer(struct key *key, void *data); + + b) Read the first payload pointer with the key semaphore held:: + + [const] void *dereference_key_locked([const] struct key *key); + + Note that the return value will inherit its constness from the key + parameter. Static analysis will give an error if it things the lock + isn't held. + + c) Read the first payload pointer with the RCU read lock held:: + + const void *dereference_key_rcu(const struct key *key); + + +Defining a Key Type +=================== + +A kernel service may want to define its own key type. For instance, an AFS +filesystem might want to define a Kerberos 5 ticket key type. To do this, it +author fills in a key_type struct and registers it with the system. + +Source files that implement key types should include the following header file:: + + <linux/key-type.h> + +The structure has a number of fields, some of which are mandatory: + + * ``const char *name`` + + The name of the key type. This is used to translate a key type name + supplied by userspace into a pointer to the structure. + + + * ``size_t def_datalen`` + + This is optional - it supplies the default payload data length as + contributed to the quota. If the key type's payload is always or almost + always the same size, then this is a more efficient way to do things. + + The data length (and quota) on a particular key can always be changed + during instantiation or update by calling:: + + int key_payload_reserve(struct key *key, size_t datalen); + + With the revised data length. Error EDQUOT will be returned if this is not + viable. + + + * ``int (*vet_description)(const char *description);`` + + This optional method is called to vet a key description. If the key type + doesn't approve of the key description, it may return an error, otherwise + it should return 0. + + + * ``int (*preparse)(struct key_preparsed_payload *prep);`` + + This optional method permits the key type to attempt to parse payload + before a key is created (add key) or the key semaphore is taken (update or + instantiate key). The structure pointed to by prep looks like:: + + struct key_preparsed_payload { + char *description; + union key_payload payload; + const void *data; + size_t datalen; + size_t quotalen; + time_t expiry; + }; + + Before calling the method, the caller will fill in data and datalen with + the payload blob parameters; quotalen will be filled in with the default + quota size from the key type; expiry will be set to TIME_T_MAX and the + rest will be cleared. + + If a description can be proposed from the payload contents, that should be + attached as a string to the description field. This will be used for the + key description if the caller of add_key() passes NULL or "". + + The method can attach anything it likes to payload. This is merely passed + along to the instantiate() or update() operations. If set, the expiry + time will be applied to the key if it is instantiated from this data. + + The method should return 0 if successful or a negative error code + otherwise. + + + * ``void (*free_preparse)(struct key_preparsed_payload *prep);`` + + This method is only required if the preparse() method is provided, + otherwise it is unused. It cleans up anything attached to the description + and payload fields of the key_preparsed_payload struct as filled in by the + preparse() method. It will always be called after preparse() returns + successfully, even if instantiate() or update() succeed. + + + * ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);`` + + This method is called to attach a payload to a key during construction. + The payload attached need not bear any relation to the data passed to this + function. + + The prep->data and prep->datalen fields will define the original payload + blob. If preparse() was supplied then other fields may be filled in also. + + If the amount of data attached to the key differs from the size in + keytype->def_datalen, then key_payload_reserve() should be called. + + This method does not have to lock the key in order to attach a payload. + The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents + anything else from gaining access to the key. + + It is safe to sleep in this method. + + generic_key_instantiate() is provided to simply copy the data from + prep->payload.data[] to key->payload.data[], with RCU-safe assignment on + the first element. It will then clear prep->payload.data[] so that the + free_preparse method doesn't release the data. + + + * ``int (*update)(struct key *key, const void *data, size_t datalen);`` + + If this type of key can be updated, then this method should be provided. + It is called to update a key's payload from the blob of data provided. + + The prep->data and prep->datalen fields will define the original payload + blob. If preparse() was supplied then other fields may be filled in also. + + key_payload_reserve() should be called if the data length might change + before any changes are actually made. Note that if this succeeds, the type + is committed to changing the key because it's already been altered, so all + memory allocation must be done first. + + The key will have its semaphore write-locked before this method is called, + but this only deters other writers; any changes to the key's payload must + be made under RCU conditions, and call_rcu() must be used to dispose of + the old payload. + + key_payload_reserve() should be called before the changes are made, but + after all allocations and other potentially failing function calls are + made. + + It is safe to sleep in this method. + + + * ``int (*match_preparse)(struct key_match_data *match_data);`` + + This method is optional. It is called when a key search is about to be + performed. It is given the following structure:: + + struct key_match_data { + bool (*cmp)(const struct key *key, + const struct key_match_data *match_data); + const void *raw_data; + void *preparsed; + unsigned lookup_type; + }; + + On entry, raw_data will be pointing to the criteria to be used in matching + a key by the caller and should not be modified. ``(*cmp)()`` will be pointing + to the default matcher function (which does an exact description match + against raw_data) and lookup_type will be set to indicate a direct lookup. + + The following lookup_type values are available: + + * KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and + description to narrow down the search to a small number of keys. + + * KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the + keys in the keyring until one is matched. This must be used for any + search that's not doing a simple direct match on the key description. + + The method may set cmp to point to a function of its choice that does some + other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE + and may attach something to the preparsed pointer for use by ``(*cmp)()``. + ``(*cmp)()`` should return true if a key matches and false otherwise. + + If preparsed is set, it may be necessary to use the match_free() method to + clean it up. + + The method should return 0 if successful or a negative error code + otherwise. + + It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as + locks will be held over it. + + If match_preparse() is not provided, keys of this type will be matched + exactly by their description. + + + * ``void (*match_free)(struct key_match_data *match_data);`` + + This method is optional. If given, it called to clean up + match_data->preparsed after a successful call to match_preparse(). + + + * ``void (*revoke)(struct key *key);`` + + This method is optional. It is called to discard part of the payload + data upon a key being revoked. The caller will have the key semaphore + write-locked. + + It is safe to sleep in this method, though care should be taken to avoid + a deadlock against the key semaphore. + + + * ``void (*destroy)(struct key *key);`` + + This method is optional. It is called to discard the payload data on a key + when it is being destroyed. + + This method does not need to lock the key to access the payload; it can + consider the key as being inaccessible at this time. Note that the key's + type may have been changed before this function is called. + + It is not safe to sleep in this method; the caller may hold spinlocks. + + + * ``void (*describe)(const struct key *key, struct seq_file *p);`` + + This method is optional. It is called during /proc/keys reading to + summarise a key's description and payload in text form. + + This method will be called with the RCU read lock held. rcu_dereference() + should be used to read the payload pointer if the payload is to be + accessed. key->datalen cannot be trusted to stay consistent with the + contents of the payload. + + The description will not change, though the key's state may. + + It is not safe to sleep in this method; the RCU read lock is held by the + caller. + + + * ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);`` + + This method is optional. It is called by KEYCTL_READ to translate the + key's payload into something a blob of data for userspace to deal with. + Ideally, the blob should be in the same format as that passed in to the + instantiate and update methods. + + If successful, the blob size that could be produced should be returned + rather than the size copied. + + This method will be called with the key's semaphore read-locked. This will + prevent the key's payload changing. It is not necessary to use RCU locking + when accessing the key's payload. It is safe to sleep in this method, such + as might happen when the userspace buffer is accessed. + + + * ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);`` + + This method is optional. If provided, request_key() and friends will + invoke this function rather than upcalling to /sbin/request-key to operate + upon a key of this type. + + The aux parameter is as passed to request_key_async_with_auxdata() and + similar or is NULL otherwise. Also passed are the construction record for + the key to be operated upon and the operation type (currently only + "create"). + + This method is permitted to return before the upcall is complete, but the + following function must be called under all circumstances to complete the + instantiation process, whether or not it succeeds, whether or not there's + an error:: + + void complete_request_key(struct key_construction *cons, int error); + + The error parameter should be 0 on success, -ve on error. The + construction record is destroyed by this action and the authorisation key + will be revoked. If an error is indicated, the key under construction + will be negatively instantiated if it wasn't already instantiated. + + If this method returns an error, that error will be returned to the + caller of request_key*(). complete_request_key() must be called prior to + returning. + + The key under construction and the authorisation key can be found in the + key_construction struct pointed to by cons: + + * ``struct key *key;`` + + The key under construction. + + * ``struct key *authkey;`` + + The authorisation key. + + + * ``struct key_restriction *(*lookup_restriction)(const char *params);`` + + This optional method is used to enable userspace configuration of keyring + restrictions. The restriction parameter string (not including the key type + name) is passed in, and this method returns a pointer to a key_restriction + structure containing the relevant functions and data to evaluate each + attempted key link operation. If there is no match, -EINVAL is returned. + + +Request-Key Callback Service +============================ + +To create a new key, the kernel will attempt to execute the following command +line:: + + /sbin/request-key create <key> <uid> <gid> \ + <threadring> <processring> <sessionring> <callout_info> + +<key> is the key being constructed, and the three keyrings are the process +keyrings from the process that caused the search to be issued. These are +included for two reasons: + + 1 There may be an authentication token in one of the keyrings that is + required to obtain the key, eg: a Kerberos Ticket-Granting Ticket. + + 2 The new key should probably be cached in one of these rings. + +This program should set it UID and GID to those specified before attempting to +access any more keys. It may then look around for a user specific process to +hand the request off to (perhaps a path held in placed in another key by, for +example, the KDE desktop manager). + +The program (or whatever it calls) should finish construction of the key by +calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to +cache the key in one of the keyrings (probably the session ring) before +returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE +or KEYCTL_REJECT; this also permits the key to be cached in one of the +keyrings. + +If it returns with the key remaining in the unconstructed state, the key will +be marked as being negative, it will be added to the session keyring, and an +error will be returned to the key requestor. + +Supplementary information may be provided from whoever or whatever invoked this +service. This will be passed as the <callout_info> parameter. If no such +information was made available, then "-" will be passed as this parameter +instead. + + +Similarly, the kernel may attempt to update an expired or a soon to expire key +by executing:: + + /sbin/request-key update <key> <uid> <gid> \ + <threadring> <processring> <sessionring> + +In this case, the program isn't required to actually attach the key to a ring; +the rings are provided for reference. + + +Garbage Collection +================== + +Dead keys (for which the type has been removed) will be automatically unlinked +from those keyrings that point to them and deleted as soon as possible by a +background garbage collector. + +Similarly, revoked and expired keys will be garbage collected, but only after a +certain amount of time has passed. This time is set as a number of seconds in:: + + /proc/sys/kernel/keys/gc_delay diff --git a/Documentation/security/keys/ecryptfs.rst b/Documentation/security/keys/ecryptfs.rst new file mode 100644 index 000000000..4920f3a8e --- /dev/null +++ b/Documentation/security/keys/ecryptfs.rst @@ -0,0 +1,73 @@ +========================================== +Encrypted keys for the eCryptfs filesystem +========================================== + +ECryptfs is a stacked filesystem which transparently encrypts and decrypts each +file using a randomly generated File Encryption Key (FEK). + +Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK) +either in kernel space or in user space with a daemon called 'ecryptfsd'. In +the former case the operation is performed directly by the kernel CryptoAPI +using a key, the FEFEK, derived from a user prompted passphrase; in the latter +the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order +to support other mechanisms like public key cryptography, PKCS#11 and TPM based +operations. + +The data structure defined by eCryptfs to contain information required for the +FEK decryption is called authentication token and, currently, can be stored in a +kernel key of the 'user' type, inserted in the user's session specific keyring +by the userspace utility 'mount.ecryptfs' shipped with the package +'ecryptfs-utils'. + +The 'encrypted' key type has been extended with the introduction of the new +format 'ecryptfs' in order to be used in conjunction with the eCryptfs +filesystem. Encrypted keys of the newly introduced format store an +authentication token in its payload with a FEFEK randomly generated by the +kernel and protected by the parent master key. + +In order to avoid known-plaintext attacks, the datablob obtained through +commands 'keyctl print' or 'keyctl pipe' does not contain the overall +authentication token, which content is well known, but only the FEFEK in +encrypted form. + +The eCryptfs filesystem may really benefit from using encrypted keys in that the +required key can be securely generated by an Administrator and provided at boot +time after the unsealing of a 'trusted' key in order to perform the mount in a +controlled environment. Another advantage is that the key is not exposed to +threats of malicious software, because it is available in clear form only at +kernel level. + +Usage:: + + keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring + keyctl add encrypted name "load hex_blob" ring + keyctl update keyid "update key-type:master-key-name" + +Where:: + + name:= '<16 hexadecimal characters>' + key-type:= 'trusted' | 'user' + keylen:= 64 + + +Example of encrypted key usage with the eCryptfs filesystem: + +Create an encrypted key "1000100010001000" of length 64 bytes with format +'ecryptfs' and save it using a previously loaded user key "test":: + + $ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u + 19184530 + + $ keyctl print 19184530 + ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697 + dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2 + f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40 + 9d292e4bacded1258880122dd553a661 + + $ keyctl pipe 19184530 > ecryptfs.blob + +Mount an eCryptfs filesystem using the created encrypted key "1000100010001000" +into the '/secret' directory:: + + $ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\ + ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret diff --git a/Documentation/security/keys/index.rst b/Documentation/security/keys/index.rst new file mode 100644 index 000000000..647d58f25 --- /dev/null +++ b/Documentation/security/keys/index.rst @@ -0,0 +1,11 @@ +=========== +Kernel Keys +=========== + +.. toctree:: + :maxdepth: 1 + + core + ecryptfs + request-key + trusted-encrypted diff --git a/Documentation/security/keys/request-key.rst b/Documentation/security/keys/request-key.rst new file mode 100644 index 000000000..21e27238c --- /dev/null +++ b/Documentation/security/keys/request-key.rst @@ -0,0 +1,199 @@ +=================== +Key Request Service +=================== + +The key request service is part of the key retention service (refer to +Documentation/security/keys/core.rst). This document explains more fully how +the requesting algorithm works. + +The process starts by either the kernel requesting a service by calling +``request_key*()``:: + + struct key *request_key(const struct key_type *type, + const char *description, + const char *callout_info); + +or:: + + struct key *request_key_with_auxdata(const struct key_type *type, + const char *description, + const char *callout_info, + size_t callout_len, + void *aux); + +or:: + + struct key *request_key_async(const struct key_type *type, + const char *description, + const char *callout_info, + size_t callout_len); + +or:: + + struct key *request_key_async_with_auxdata(const struct key_type *type, + const char *description, + const char *callout_info, + size_t callout_len, + void *aux); + +Or by userspace invoking the request_key system call:: + + key_serial_t request_key(const char *type, + const char *description, + const char *callout_info, + key_serial_t dest_keyring); + +The main difference between the access points is that the in-kernel interface +does not need to link the key to a keyring to prevent it from being immediately +destroyed. The kernel interface returns a pointer directly to the key, and +it's up to the caller to destroy the key. + +The request_key*_with_auxdata() calls are like the in-kernel request_key*() +calls, except that they permit auxiliary data to be passed to the upcaller (the +default is NULL). This is only useful for those key types that define their +own upcall mechanism rather than using /sbin/request-key. + +The two async in-kernel calls may return keys that are still in the process of +being constructed. The two non-async ones will wait for construction to +complete first. + +The userspace interface links the key to a keyring associated with the process +to prevent the key from going away, and returns the serial number of the key to +the caller. + + +The following example assumes that the key types involved don't define their +own upcall mechanisms. If they do, then those should be substituted for the +forking and execution of /sbin/request-key. + + +The Process +=========== + +A request proceeds in the following manner: + + 1) Process A calls request_key() [the userspace syscall calls the kernel + interface]. + + 2) request_key() searches the process's subscribed keyrings to see if there's + a suitable key there. If there is, it returns the key. If there isn't, + and callout_info is not set, an error is returned. Otherwise the process + proceeds to the next step. + + 3) request_key() sees that A doesn't have the desired key yet, so it creates + two things: + + a) An uninstantiated key U of requested type and description. + + b) An authorisation key V that refers to key U and notes that process A + is the context in which key U should be instantiated and secured, and + from which associated key requests may be satisfied. + + 4) request_key() then forks and executes /sbin/request-key with a new session + keyring that contains a link to auth key V. + + 5) /sbin/request-key assumes the authority associated with key U. + + 6) /sbin/request-key execs an appropriate program to perform the actual + instantiation. + + 7) The program may want to access another key from A's context (say a + Kerberos TGT key). It just requests the appropriate key, and the keyring + search notes that the session keyring has auth key V in its bottom level. + + This will permit it to then search the keyrings of process A with the + UID, GID, groups and security info of process A as if it was process A, + and come up with key W. + + 8) The program then does what it must to get the data with which to + instantiate key U, using key W as a reference (perhaps it contacts a + Kerberos server using the TGT) and then instantiates key U. + + 9) Upon instantiating key U, auth key V is automatically revoked so that it + may not be used again. + + 10) The program then exits 0 and request_key() deletes key V and returns key + U to the caller. + +This also extends further. If key W (step 7 above) didn't exist, key W would +be created uninstantiated, another auth key (X) would be created (as per step +3) and another copy of /sbin/request-key spawned (as per step 4); but the +context specified by auth key X will still be process A, as it was in auth key +V. + +This is because process A's keyrings can't simply be attached to +/sbin/request-key at the appropriate places because (a) execve will discard two +of them, and (b) it requires the same UID/GID/Groups all the way through. + + +Negative Instantiation And Rejection +==================================== + +Rather than instantiating a key, it is possible for the possessor of an +authorisation key to negatively instantiate a key that's under construction. +This is a short duration placeholder that causes any attempt at re-requesting +the key whilst it exists to fail with error ENOKEY if negated or the specified +error if rejected. + +This is provided to prevent excessive repeated spawning of /sbin/request-key +processes for a key that will never be obtainable. + +Should the /sbin/request-key process exit anything other than 0 or die on a +signal, the key under construction will be automatically negatively +instantiated for a short amount of time. + + +The Search Algorithm +==================== + +A search of any particular keyring proceeds in the following fashion: + + 1) When the key management code searches for a key (keyring_search_aux) it + firstly calls key_permission(SEARCH) on the keyring it's starting with, + if this denies permission, it doesn't search further. + + 2) It considers all the non-keyring keys within that keyring and, if any key + matches the criteria specified, calls key_permission(SEARCH) on it to see + if the key is allowed to be found. If it is, that key is returned; if + not, the search continues, and the error code is retained if of higher + priority than the one currently set. + + 3) It then considers all the keyring-type keys in the keyring it's currently + searching. It calls key_permission(SEARCH) on each keyring, and if this + grants permission, it recurses, executing steps (2) and (3) on that + keyring. + +The process stops immediately a valid key is found with permission granted to +use it. Any error from a previous match attempt is discarded and the key is +returned. + +When search_process_keyrings() is invoked, it performs the following searches +until one succeeds: + + 1) If extant, the process's thread keyring is searched. + + 2) If extant, the process's process keyring is searched. + + 3) The process's session keyring is searched. + + 4) If the process has assumed the authority associated with a request_key() + authorisation key then: + + a) If extant, the calling process's thread keyring is searched. + + b) If extant, the calling process's process keyring is searched. + + c) The calling process's session keyring is searched. + +The moment one succeeds, all pending errors are discarded and the found key is +returned. + +Only if all these fail does the whole thing fail with the highest priority +error. Note that several errors may have come from LSM. + +The error priority is:: + + EKEYREVOKED > EKEYEXPIRED > ENOKEY + +EACCES/EPERM are only returned on a direct search of a specific keyring where +the basal keyring does not grant Search permission. diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst new file mode 100644 index 000000000..3bb24e09a --- /dev/null +++ b/Documentation/security/keys/trusted-encrypted.rst @@ -0,0 +1,175 @@ +========================== +Trusted and Encrypted Keys +========================== + +Trusted and Encrypted Keys are two new key types added to the existing kernel +key ring service. Both of these new types are variable length symmetric keys, +and in both cases all keys are created in the kernel, and user space sees, +stores, and loads only encrypted blobs. Trusted Keys require the availability +of a Trusted Platform Module (TPM) chip for greater security, while Encrypted +Keys can be used on any system. All user level blobs, are displayed and loaded +in hex ascii for convenience, and are integrity verified. + +Trusted Keys use a TPM both to generate and to seal the keys. Keys are sealed +under a 2048 bit RSA key in the TPM, and optionally sealed to specified PCR +(integrity measurement) values, and only unsealed by the TPM, if PCRs and blob +integrity verifications match. A loaded Trusted Key can be updated with new +(future) PCR values, so keys are easily migrated to new pcr values, such as +when the kernel and initramfs are updated. The same key can have many saved +blobs under different PCR values, so multiple boots are easily supported. + +By default, trusted keys are sealed under the SRK, which has the default +authorization value (20 zeros). This can be set at takeownership time with the +trouser's utility: "tpm_takeownership -u -z". + +Usage:: + + keyctl add trusted name "new keylen [options]" ring + keyctl add trusted name "load hex_blob [pcrlock=pcrnum]" ring + keyctl update key "update [options]" + keyctl print keyid + + options: + keyhandle= ascii hex value of sealing key default 0x40000000 (SRK) + keyauth= ascii hex auth for sealing key default 0x00...i + (40 ascii zeros) + blobauth= ascii hex auth for sealed data default 0x00... + (40 ascii zeros) + pcrinfo= ascii hex of PCR_INFO or PCR_INFO_LONG (no default) + pcrlock= pcr number to be extended to "lock" blob + migratable= 0|1 indicating permission to reseal to new PCR values, + default 1 (resealing allowed) + hash= hash algorithm name as a string. For TPM 1.x the only + allowed value is sha1. For TPM 2.x the allowed values + are sha1, sha256, sha384, sha512 and sm3-256. + policydigest= digest for the authorization policy. must be calculated + with the same hash algorithm as specified by the 'hash=' + option. + policyhandle= handle to an authorization policy session that defines the + same policy and with the same hash algorithm as was used to + seal the key. + +"keyctl print" returns an ascii hex copy of the sealed key, which is in standard +TPM_STORED_DATA format. The key length for new keys are always in bytes. +Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit +within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding. + +Encrypted keys do not depend on a TPM, and are faster, as they use AES for +encryption/decryption. New keys are created from kernel generated random +numbers, and are encrypted/decrypted using a specified 'master' key. The +'master' key can either be a trusted-key or user-key type. The main +disadvantage of encrypted keys is that if they are not rooted in a trusted key, +they are only as secure as the user key encrypting them. The master user key +should therefore be loaded in as secure a way as possible, preferably early in +boot. + +The decrypted portion of encrypted keys can contain either a simple symmetric +key or a more complex structure. The format of the more complex structure is +application specific, which is identified by 'format'. + +Usage:: + + keyctl add encrypted name "new [format] key-type:master-key-name keylen" + ring + keyctl add encrypted name "load hex_blob" ring + keyctl update keyid "update key-type:master-key-name" + +Where:: + + format:= 'default | ecryptfs' + key-type:= 'trusted' | 'user' + + +Examples of trusted and encrypted key usage: + +Create and save a trusted key named "kmk" of length 32 bytes:: + + $ keyctl add trusted kmk "new 32" @u + 440502848 + + $ keyctl show + Session Keyring + -3 --alswrv 500 500 keyring: _ses + 97833714 --alswrv 500 -1 \_ keyring: _uid.500 + 440502848 --alswrv 500 500 \_ trusted: kmk + + $ keyctl print 440502848 + 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915 + 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b + 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722 + a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec + d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d + dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0 + f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b + e4a8aea2b607ec96931e6f4d4fe563ba + + $ keyctl pipe 440502848 > kmk.blob + +Load a trusted key from the saved blob:: + + $ keyctl add trusted kmk "load `cat kmk.blob`" @u + 268728824 + + $ keyctl print 268728824 + 0101000000000000000001005d01b7e3f4a6be5709930f3b70a743cbb42e0cc95e18e915 + 3f60da455bbf1144ad12e4f92b452f966929f6105fd29ca28e4d4d5a031d068478bacb0b + 27351119f822911b0a11ba3d3498ba6a32e50dac7f32894dd890eb9ad578e4e292c83722 + a52e56a097e6a68b3f56f7a52ece0cdccba1eb62cad7d817f6dc58898b3ac15f36026fec + d568bd4a706cb60bb37be6d8f1240661199d640b66fb0fe3b079f97f450b9ef9c22c6d5d + dd379f0facd1cd020281dfa3c70ba21a3fa6fc2471dc6d13ecf8298b946f65345faa5ef0 + f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b + e4a8aea2b607ec96931e6f4d4fe563ba + +Reseal a trusted key under new pcr values:: + + $ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`" + $ keyctl print 268728824 + 010100000000002c0002800093c35a09b70fff26e7a98ae786c641e678ec6ffb6b46d805 + 77c8a6377aed9d3219c6dfec4b23ffe3000001005d37d472ac8a44023fbb3d18583a4f73 + d3a076c0858f6f1dcaa39ea0f119911ff03f5406df4f7f27f41da8d7194f45c9f4e00f2e + df449f266253aa3f52e55c53de147773e00f0f9aca86c64d94c95382265968c354c5eab4 + 9638c5ae99c89de1e0997242edfb0b501744e11ff9762dfd951cffd93227cc513384e7e6 + e782c29435c7ec2edafaa2f4c1fe6e7a781b59549ff5296371b42133777dcc5b8b971610 + 94bc67ede19e43ddb9dc2baacad374a36feaf0314d700af0a65c164b7082401740e489c9 + 7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef + df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8 + +The initial consumer of trusted keys is EVM, which at boot time needs a high +quality symmetric key for HMAC protection of file metadata. The use of a +trusted key provides strong guarantees that the EVM key has not been +compromised by a user level problem, and when sealed to specific boot PCR +values, protects against boot and offline attacks. Create and save an +encrypted key "evm" using the above trusted key "kmk": + +option 1: omitting 'format':: + + $ keyctl add encrypted evm "new trusted:kmk 32" @u + 159771175 + +option 2: explicitly defining 'format' as 'default':: + + $ keyctl add encrypted evm "new default trusted:kmk 32" @u + 159771175 + + $ keyctl print 159771175 + default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3 + 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0 + 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc + + $ keyctl pipe 159771175 > evm.blob + +Load an encrypted key "evm" from saved blob:: + + $ keyctl add encrypted evm "load `cat evm.blob`" @u + 831684262 + + $ keyctl print 831684262 + default trusted:kmk 32 2375725ad57798846a9bbd240de8906f006e66c03af53b1b3 + 82dbbc55be2a44616e4959430436dc4f2a7a9659aa60bb4652aeb2120f149ed197c564e0 + 24717c64 5972dcb82ab2dde83376d82b2e3c09ffc + +Other uses for trusted and encrypted keys, such as for disk and file encryption +are anticipated. In particular the new format 'ecryptfs' has been defined in +in order to use encrypted keys to mount an eCryptfs filesystem. More details +about the usage can be found in the file +``Documentation/security/keys/ecryptfs.rst``. diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst new file mode 100644 index 000000000..e1ca698e0 --- /dev/null +++ b/Documentation/security/self-protection.rst @@ -0,0 +1,317 @@ +====================== +Kernel Self-Protection +====================== + +Kernel self-protection is the design and implementation of systems and +structures within the Linux kernel to protect against security flaws in +the kernel itself. This covers a wide range of issues, including removing +entire classes of bugs, blocking security flaw exploitation methods, +and actively detecting attack attempts. Not all topics are explored in +this document, but it should serve as a reasonable starting point and +answer any frequently asked questions. (Patches welcome, of course!) + +In the worst-case scenario, we assume an unprivileged local attacker +has arbitrary read and write access to the kernel's memory. In many +cases, bugs being exploited will not provide this level of access, +but with systems in place that defend against the worst case we'll +cover the more limited cases as well. A higher bar, and one that should +still be kept in mind, is protecting the kernel against a _privileged_ +local attacker, since the root user has access to a vastly increased +attack surface. (Especially when they have the ability to load arbitrary +kernel modules.) + +The goals for successful self-protection systems would be that they +are effective, on by default, require no opt-in by developers, have no +performance impact, do not impede kernel debugging, and have tests. It +is uncommon that all these goals can be met, but it is worth explicitly +mentioning them, since these aspects need to be explored, dealt with, +and/or accepted. + + +Attack Surface Reduction +======================== + +The most fundamental defense against security exploits is to reduce the +areas of the kernel that can be used to redirect execution. This ranges +from limiting the exposed APIs available to userspace, making in-kernel +APIs hard to use incorrectly, minimizing the areas of writable kernel +memory, etc. + +Strict kernel memory permissions +-------------------------------- + +When all of kernel memory is writable, it becomes trivial for attacks +to redirect execution flow. To reduce the availability of these targets +the kernel needs to protect its memory with a tight set of permissions. + +Executable code and read-only data must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Any areas of the kernel with executable memory must not be writable. +While this obviously includes the kernel text itself, we must consider +all additional places too: kernel modules, JIT memory, etc. (There are +temporary exceptions to this rule to support things like instruction +alternatives, breakpoints, kprobes, etc. If these must exist in a +kernel, they are implemented in a way where the memory is temporarily +made writable during the update, and then returned to the original +permissions.) + +In support of this are ``CONFIG_STRICT_KERNEL_RWX`` and +``CONFIG_STRICT_MODULE_RWX``, which seek to make sure that code is not +writable, data is not executable, and read-only data is neither writable +nor executable. + +Most architectures have these options on by default and not user selectable. +For some architectures like arm that wish to have these be selectable, +the architecture Kconfig can select ARCH_OPTIONAL_KERNEL_RWX to enable +a Kconfig prompt. ``CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT`` determines +the default setting when ARCH_OPTIONAL_KERNEL_RWX is enabled. + +Function pointers and sensitive variables must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Vast areas of kernel memory contain function pointers that are looked +up by the kernel and used to continue execution (e.g. descriptor/vector +tables, file/network/etc operation structures, etc). The number of these +variables must be reduced to an absolute minimum. + +Many such variables can be made read-only by setting them "const" +so that they live in the .rodata section instead of the .data section +of the kernel, gaining the protection of the kernel's strict memory +permissions as described above. + +For variables that are initialized once at ``__init`` time, these can +be marked with the (new and under development) ``__ro_after_init`` +attribute. + +What remains are variables that are updated rarely (e.g. GDT). These +will need another infrastructure (similar to the temporary exceptions +made to kernel code mentioned above) that allow them to spend the rest +of their lifetime read-only. (For example, when being updated, only the +CPU thread performing the update would be given uninterruptible write +access to the memory.) + +Segregation of kernel memory from userspace memory +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The kernel must never execute userspace memory. The kernel must also never +access userspace memory without explicit expectation to do so. These +rules can be enforced either by support of hardware-based restrictions +(x86's SMEP/SMAP, ARM's PXN/PAN) or via emulation (ARM's Memory Domains). +By blocking userspace memory in this way, execution and data parsing +cannot be passed to trivially-controlled userspace memory, forcing +attacks to operate entirely in kernel memory. + +Reduced access to syscalls +-------------------------- + +One trivial way to eliminate many syscalls for 64-bit systems is building +without ``CONFIG_COMPAT``. However, this is rarely a feasible scenario. + +The "seccomp" system provides an opt-in feature made available to +userspace, which provides a way to reduce the number of kernel entry +points available to a running process. This limits the breadth of kernel +code that can be reached, possibly reducing the availability of a given +bug to an attack. + +An area of improvement would be creating viable ways to keep access to +things like compat, user namespaces, BPF creation, and perf limited only +to trusted processes. This would keep the scope of kernel entry points +restricted to the more regular set of normally available to unprivileged +userspace. + +Restricting access to kernel modules +------------------------------------ + +The kernel should never allow an unprivileged user the ability to +load specific kernel modules, since that would provide a facility to +unexpectedly extend the available attack surface. (The on-demand loading +of modules via their predefined subsystems, e.g. MODULE_ALIAS_*, is +considered "expected" here, though additional consideration should be +given even to these.) For example, loading a filesystem module via an +unprivileged socket API is nonsense: only the root or physically local +user should trigger filesystem module loading. (And even this can be up +for debate in some scenarios.) + +To protect against even privileged users, systems may need to either +disable module loading entirely (e.g. monolithic kernel builds or +modules_disabled sysctl), or provide signed modules (e.g. +``CONFIG_MODULE_SIG_FORCE``, or dm-crypt with LoadPin), to keep from having +root load arbitrary kernel code via the module loader interface. + + +Memory integrity +================ + +There are many memory structures in the kernel that are regularly abused +to gain execution control during an attack, By far the most commonly +understood is that of the stack buffer overflow in which the return +address stored on the stack is overwritten. Many other examples of this +kind of attack exist, and protections exist to defend against them. + +Stack buffer overflow +--------------------- + +The classic stack buffer overflow involves writing past the expected end +of a variable stored on the stack, ultimately writing a controlled value +to the stack frame's stored return address. The most widely used defense +is the presence of a stack canary between the stack variables and the +return address (``CONFIG_STACKPROTECTOR``), which is verified just before +the function returns. Other defenses include things like shadow stacks. + +Stack depth overflow +-------------------- + +A less well understood attack is using a bug that triggers the +kernel to consume stack memory with deep function calls or large stack +allocations. With this attack it is possible to write beyond the end of +the kernel's preallocated stack space and into sensitive structures. Two +important changes need to be made for better protections: moving the +sensitive thread_info structure elsewhere, and adding a faulting memory +hole at the bottom of the stack to catch these overflows. + +Heap memory integrity +--------------------- + +The structures used to track heap free lists can be sanity-checked during +allocation and freeing to make sure they aren't being used to manipulate +other memory areas. + +Counter integrity +----------------- + +Many places in the kernel use atomic counters to track object references +or perform similar lifetime management. When these counters can be made +to wrap (over or under) this traditionally exposes a use-after-free +flaw. By trapping atomic wrapping, this class of bug vanishes. + +Size calculation overflow detection +----------------------------------- + +Similar to counter overflow, integer overflows (usually size calculations) +need to be detected at runtime to kill this class of bug, which +traditionally leads to being able to write past the end of kernel buffers. + + +Probabilistic defenses +====================== + +While many protections can be considered deterministic (e.g. read-only +memory cannot be written to), some protections provide only statistical +defense, in that an attack must gather enough information about a +running system to overcome the defense. While not perfect, these do +provide meaningful defenses. + +Canaries, blinding, and other secrets +------------------------------------- + +It should be noted that things like the stack canary discussed earlier +are technically statistical defenses, since they rely on a secret value, +and such values may become discoverable through an information exposure +flaw. + +Blinding literal values for things like JITs, where the executable +contents may be partially under the control of userspace, need a similar +secret value. + +It is critical that the secret values used must be separate (e.g. +different canary per stack) and high entropy (e.g. is the RNG actually +working?) in order to maximize their success. + +Kernel Address Space Layout Randomization (KASLR) +------------------------------------------------- + +Since the location of kernel memory is almost always instrumental in +mounting a successful attack, making the location non-deterministic +raises the difficulty of an exploit. (Note that this in turn makes +the value of information exposures higher, since they may be used to +discover desired memory locations.) + +Text and module base +~~~~~~~~~~~~~~~~~~~~ + +By relocating the physical and virtual base address of the kernel at +boot-time (``CONFIG_RANDOMIZE_BASE``), attacks needing kernel code will be +frustrated. Additionally, offsetting the module loading base address +means that even systems that load the same set of modules in the same +order every boot will not share a common base address with the rest of +the kernel text. + +Stack base +~~~~~~~~~~ + +If the base address of the kernel stack is not the same between processes, +or even not the same between syscalls, targets on or beyond the stack +become more difficult to locate. + +Dynamic memory base +~~~~~~~~~~~~~~~~~~~ + +Much of the kernel's dynamic memory (e.g. kmalloc, vmalloc, etc) ends up +being relatively deterministic in layout due to the order of early-boot +initializations. If the base address of these areas is not the same +between boots, targeting them is frustrated, requiring an information +exposure specific to the region. + +Structure layout +~~~~~~~~~~~~~~~~ + +By performing a per-build randomization of the layout of sensitive +structures, attacks must either be tuned to known kernel builds or expose +enough kernel memory to determine structure layouts before manipulating +them. + + +Preventing Information Exposures +================================ + +Since the locations of sensitive structures are the primary target for +attacks, it is important to defend against exposure of both kernel memory +addresses and kernel memory contents (since they may contain kernel +addresses or other sensitive things like canary values). + +Kernel addresses +---------------- + +Printing kernel addresses to userspace leaks sensitive information about +the kernel memory layout. Care should be exercised when using any printk +specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb] +in certain circumstances [*]). Any file written to using one of these +specifiers should be readable only by privileged processes. + +Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1 +addresses printed with the specifier %p are hashed before printing. + +[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is +printed. If KALLSYMS is not enabled the raw address is printed. + +Unique identifiers +------------------ + +Kernel memory addresses must never be used as identifiers exposed to +userspace. Instead, use an atomic counter, an idr, or similar unique +identifier. + +Memory initialization +--------------------- + +Memory copied to userspace must always be fully initialized. If not +explicitly memset(), this will require changes to the compiler to make +sure structure holes are cleared. + +Memory poisoning +---------------- + +When releasing memory, it is best to poison the contents (clear stack on +syscall return, wipe heap memory on a free), to avoid reuse attacks that +rely on the old contents of memory. This frustrates many uninitialized +variable attacks, stack content exposures, heap content exposures, and +use-after-free attacks. + +Destination tracking +-------------------- + +To help kill classes of bugs that result in kernel addresses being +written to userspace, the destination of writes needs to be tracked. If +the buffer is destined for userspace (e.g. seq_file backed ``/proc`` files), +it should automatically censor sensitive values. diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst new file mode 100644 index 000000000..af77a7bbb --- /dev/null +++ b/Documentation/security/tpm/index.rst @@ -0,0 +1,7 @@ +===================================== +Trusted Platform Module documentation +===================================== + +.. toctree:: + + tpm_vtpm_proxy diff --git a/Documentation/security/tpm/tpm_vtpm_proxy.rst b/Documentation/security/tpm/tpm_vtpm_proxy.rst new file mode 100644 index 000000000..ea08e76b1 --- /dev/null +++ b/Documentation/security/tpm/tpm_vtpm_proxy.rst @@ -0,0 +1,50 @@ +============================================= +Virtual TPM Proxy Driver for Linux Containers +============================================= + +| Authors: +| Stefan Berger <stefanb@linux.vnet.ibm.com> + +This document describes the virtual Trusted Platform Module (vTPM) +proxy device driver for Linux containers. + +Introduction +============ + +The goal of this work is to provide TPM functionality to each Linux +container. This allows programs to interact with a TPM in a container +the same way they interact with a TPM on the physical system. Each +container gets its own unique, emulated, software TPM. + +Design +====== + +To make an emulated software TPM available to each container, the container +management stack needs to create a device pair consisting of a client TPM +character device ``/dev/tpmX`` (with X=0,1,2...) and a 'server side' file +descriptor. The former is moved into the container by creating a character +device with the appropriate major and minor numbers while the file descriptor +is passed to the TPM emulator. Software inside the container can then send +TPM commands using the character device and the emulator will receive the +commands via the file descriptor and use it for sending back responses. + +To support this, the virtual TPM proxy driver provides a device ``/dev/vtpmx`` +that is used to create device pairs using an ioctl. The ioctl takes as +an input flags for configuring the device. The flags for example indicate +whether TPM 1.2 or TPM 2 functionality is supported by the TPM emulator. +The result of the ioctl are the file descriptor for the 'server side' +as well as the major and minor numbers of the character device that was created. +Besides that the number of the TPM character device is returned. If for +example ``/dev/tpm10`` was created, the number (``dev_num``) 10 is returned. + +Once the device has been created, the driver will immediately try to talk +to the TPM. All commands from the driver can be read from the file descriptor +returned by the ioctl. The commands should be responded to immediately. + +UAPI +==== + +.. kernel-doc:: include/uapi/linux/vtpm_proxy.h + +.. kernel-doc:: drivers/char/tpm/tpm_vtpm_proxy.c + :functions: vtpmx_ioc_new_dev diff --git a/Documentation/security/tpm/xen-tpmfront.txt b/Documentation/security/tpm/xen-tpmfront.txt new file mode 100644 index 000000000..69346de87 --- /dev/null +++ b/Documentation/security/tpm/xen-tpmfront.txt @@ -0,0 +1,113 @@ +Virtual TPM interface for Xen + +Authors: Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA) + +This document describes the virtual Trusted Platform Module (vTPM) subsystem for +Xen. The reader is assumed to have familiarity with building and installing Xen, +Linux, and a basic understanding of the TPM and vTPM concepts. + +INTRODUCTION + +The goal of this work is to provide a TPM functionality to a virtual guest +operating system (in Xen terms, a DomU). This allows programs to interact with +a TPM in a virtual system the same way they interact with a TPM on the physical +system. Each guest gets its own unique, emulated, software TPM. However, each +of the vTPM's secrets (Keys, NVRAM, etc) are managed by a vTPM Manager domain, +which seals the secrets to the Physical TPM. If the process of creating each of +these domains (manager, vTPM, and guest) is trusted, the vTPM subsystem extends +the chain of trust rooted in the hardware TPM to virtual machines in Xen. Each +major component of vTPM is implemented as a separate domain, providing secure +separation guaranteed by the hypervisor. The vTPM domains are implemented in +mini-os to reduce memory and processor overhead. + +This mini-os vTPM subsystem was built on top of the previous vTPM work done by +IBM and Intel corporation. + + +DESIGN OVERVIEW +--------------- + +The architecture of vTPM is described below: + ++------------------+ +| Linux DomU | ... +| | ^ | +| v | | +| xen-tpmfront | ++------------------+ + | ^ + v | ++------------------+ +| mini-os/tpmback | +| | ^ | +| v | | +| vtpm-stubdom | ... +| | ^ | +| v | | +| mini-os/tpmfront | ++------------------+ + | ^ + v | ++------------------+ +| mini-os/tpmback | +| | ^ | +| v | | +| vtpmmgr-stubdom | +| | ^ | +| v | | +| mini-os/tpm_tis | ++------------------+ + | ^ + v | ++------------------+ +| Hardware TPM | ++------------------+ + + * Linux DomU: The Linux based guest that wants to use a vTPM. There may be + more than one of these. + + * xen-tpmfront.ko: Linux kernel virtual TPM frontend driver. This driver + provides vTPM access to a Linux-based DomU. + + * mini-os/tpmback: Mini-os TPM backend driver. The Linux frontend driver + connects to this backend driver to facilitate communications + between the Linux DomU and its vTPM. This driver is also + used by vtpmmgr-stubdom to communicate with vtpm-stubdom. + + * vtpm-stubdom: A mini-os stub domain that implements a vTPM. There is a + one to one mapping between running vtpm-stubdom instances and + logical vtpms on the system. The vTPM Platform Configuration + Registers (PCRs) are normally all initialized to zero. + + * mini-os/tpmfront: Mini-os TPM frontend driver. The vTPM mini-os domain + vtpm-stubdom uses this driver to communicate with + vtpmmgr-stubdom. This driver is also used in mini-os + domains such as pv-grub that talk to the vTPM domain. + + * vtpmmgr-stubdom: A mini-os domain that implements the vTPM manager. There is + only one vTPM manager and it should be running during the + entire lifetime of the machine. This domain regulates + access to the physical TPM on the system and secures the + persistent state of each vTPM. + + * mini-os/tpm_tis: Mini-os TPM version 1.2 TPM Interface Specification (TIS) + driver. This driver used by vtpmmgr-stubdom to talk directly to + the hardware TPM. Communication is facilitated by mapping + hardware memory pages into vtpmmgr-stubdom. + + * Hardware TPM: The physical TPM that is soldered onto the motherboard. + + +INTEGRATION WITH XEN +-------------------- + +Support for the vTPM driver was added in Xen using the libxl toolstack in Xen +4.3. See the Xen documentation (docs/misc/vtpm.txt) for details on setting up +the vTPM and vTPM Manager stub domains. Once the stub domains are running, a +vTPM device is set up in the same manner as a disk or network device in the +domain's configuration file. + +In order to use features such as IMA that require a TPM to be loaded prior to +the initrd, the xen-tpmfront driver must be compiled in to the kernel. If not +using such features, the driver can be compiled as a module and will be loaded +as usual. |