Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+============================
+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.
+
+     To link a key into the destination keyring the key must grant link
+     permission on the key to the caller and the keyring must grant write
+     permission.
+
+     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.
+
+
+  *  Move a key from one keyring to another::
+
+	long keyctl(KEYCTL_MOVE,
+		    key_serial_t id,
+		    key_serial_t from_ring_id,
+		    key_serial_t to_ring_id,
+		    unsigned int flags);
+
+     Move the key specified by "id" from the keyring specified by
+     "from_ring_id" to the keyring specified by "to_ring_id".  If the two
+     keyrings are the same, nothing is done.
+
+     "flags" can have KEYCTL_MOVE_EXCL set in it to cause the operation to fail
+     with EEXIST if a matching key exists in the destination keyring, otherwise
+     such a key will be replaced.
+
+     A process must have link permission on the key for this function to be
+     successful and write permission on both keyrings.  Any errors that can
+     occur from KEYCTL_LINK also apply on the destination keyring here.
+
+
+  *  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.rst for specific restrictions
+     applicable to the asymmetric key type.
+
+
+  *  Query an asymmetric key::
+
+	long keyctl(KEYCTL_PKEY_QUERY,
+		    key_serial_t key_id, unsigned long reserved,
+		    const char *params,
+		    struct keyctl_pkey_query *info);
+
+     Get information about an asymmetric key.  Specific algorithms and
+     encodings may be queried by using the ``params`` argument.  This is a
+     string containing a space- or tab-separated string of key-value pairs.
+     Currently supported keys include ``enc`` and ``hash``.  The information
+     is returned in the keyctl_pkey_query struct::
+
+	__u32	supported_ops;
+	__u32	key_size;
+	__u16	max_data_size;
+	__u16	max_sig_size;
+	__u16	max_enc_size;
+	__u16	max_dec_size;
+	__u32	__spare[10];
+
+     ``supported_ops`` contains a bit mask of flags indicating which ops are
+     supported.  This is constructed from a bitwise-OR of::
+
+	KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
+
+     ``key_size`` indicated the size of the key in bits.
+
+     ``max_*_size`` indicate the maximum sizes in bytes of a blob of data to be
+     signed, a signature blob, a blob to be encrypted and a blob to be
+     decrypted.
+
+     ``__spare[]`` must be set to 0.  This is intended for future use to hand
+     over one or more passphrases needed unlock a key.
+
+     If successful, 0 is returned.  If the key is not an asymmetric key,
+     EOPNOTSUPP is returned.
+
+
+  *  Encrypt, decrypt, sign or verify a blob using an asymmetric key::
+
+	long keyctl(KEYCTL_PKEY_ENCRYPT,
+		    const struct keyctl_pkey_params *params,
+		    const char *info,
+		    const void *in,
+		    void *out);
+
+	long keyctl(KEYCTL_PKEY_DECRYPT,
+		    const struct keyctl_pkey_params *params,
+		    const char *info,
+		    const void *in,
+		    void *out);
+
+	long keyctl(KEYCTL_PKEY_SIGN,
+		    const struct keyctl_pkey_params *params,
+		    const char *info,
+		    const void *in,
+		    void *out);
+
+	long keyctl(KEYCTL_PKEY_VERIFY,
+		    const struct keyctl_pkey_params *params,
+		    const char *info,
+		    const void *in,
+		    const void *in2);
+
+     Use an asymmetric key to perform a public-key cryptographic operation a
+     blob of data.  For encryption and verification, the asymmetric key may
+     only need the public parts to be available, but for decryption and signing
+     the private parts are required also.
+
+     The parameter block pointed to by params contains a number of integer
+     values::
+
+	__s32		key_id;
+	__u32		in_len;
+	__u32		out_len;
+	__u32		in2_len;
+
+     ``key_id`` is the ID of the asymmetric key to be used.  ``in_len`` and
+     ``in2_len`` indicate the amount of data in the in and in2 buffers and
+     ``out_len`` indicates the size of the out buffer as appropriate for the
+     above operations.
+
+     For a given operation, the in and out buffers are used as follows::
+
+	Operation ID		in,in_len	out,out_len	in2,in2_len
+	=======================	===============	===============	===============
+	KEYCTL_PKEY_ENCRYPT	Raw data	Encrypted data	-
+	KEYCTL_PKEY_DECRYPT	Encrypted data	Raw data	-
+	KEYCTL_PKEY_SIGN	Raw data	Signature	-
+	KEYCTL_PKEY_VERIFY	Raw data	-		Signature
+
+     ``info`` is a string of key=value pairs that supply supplementary
+     information.  These include:
+
+	``enc=<encoding>`` The encoding of the encrypted/signature blob.  This
+			can be "pkcs1" for RSASSA-PKCS1-v1.5 or
+			RSAES-PKCS1-v1.5; "pss" for "RSASSA-PSS"; "oaep" for
+			"RSAES-OAEP".  If omitted or is "raw", the raw output
+			of the encryption function is specified.
+
+	``hash=<algo>``	If the data buffer contains the output of a hash
+			function and the encoding includes some indication of
+			which hash function was used, the hash function can be
+			specified with this, eg. "hash=sha256".
+
+     The ``__spare[]`` space in the parameter block must be set to 0.  This is
+     intended, amongst other things, to allow the passing of passphrases
+     required to unlock a key.
+
+     If successful, encrypt, decrypt and sign all return the amount of data
+     written into the output buffer.  Verification returns 0 on success.
+
+
+  *  Watch a key or keyring for changes::
+
+	long keyctl(KEYCTL_WATCH_KEY, key_serial_t key, int queue_fd,
+		    const struct watch_notification_filter *filter);
+
+     This will set or remove a watch for changes on the specified key or
+     keyring.
+
+     "key" is the ID of the key to be watched.
+
+     "queue_fd" is a file descriptor referring to an open pipe which
+     manages the buffer into which notifications will be delivered.
+
+     "filter" is either NULL to remove a watch or a filter specification to
+     indicate what events are required from the key.
+
+     See Documentation/core-api/watch_queue.rst for more information.
+
+     Note that only one watch may be emplaced for any particular { key,
+     queue_fd } combination.
+
+     Notification records look like::
+
+	struct key_notification {
+		struct watch_notification watch;
+		__u32	key_id;
+		__u32	aux;
+	};
+
+     In this, watch::type will be "WATCH_TYPE_KEY_NOTIFY" and subtype will be
+     one of::
+
+	NOTIFY_KEY_INSTANTIATED
+	NOTIFY_KEY_UPDATED
+	NOTIFY_KEY_LINKED
+	NOTIFY_KEY_UNLINKED
+	NOTIFY_KEY_CLEARED
+	NOTIFY_KEY_REVOKED
+	NOTIFY_KEY_INVALIDATED
+	NOTIFY_KEY_SETATTR
+
+     Where these indicate a key being instantiated/rejected, updated, a link
+     being made in a keyring, a link being removed from a keyring, a keyring
+     being cleared, a key being revoked, a key being invalidated or a key
+     having one of its attributes changed (user, group, perm, timeout,
+     restriction).
+
+     If a watched key is deleted, a basic watch_notification will be issued
+     with "type" set to WATCH_TYPE_META and "subtype" set to
+     watch_meta_removal_notification.  The watchpoint ID will be set in the
+     "info" field.
+
+     This needs to be configured by enabling:
+
+	"Provide key/keyring change notifications" (KEY_NOTIFICATIONS)
+
+
+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 in a specific domain, call::
+
+	struct key *request_key_tag(const struct key_type *type,
+				    const char *description,
+				    struct key_tag *domain_tag,
+				    const char *callout_info);
+
+    This is identical to request_key(), except that a domain tag may be
+    specifies that causes search algorithm to only match keys matching that
+    tag.  The domain_tag may be NULL, specifying a global domain that is
+    separate from any nominated domain.
+
+
+ *  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,
+					     struct key_tag *domain_tag,
+					     const void *callout_info,
+					     size_t callout_len,
+					     void *aux);
+
+    This is identical to request_key_tag(), 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).
+
+
+ *  To search for a key under RCU conditions, call::
+
+	struct key *request_key_rcu(const struct key_type *type,
+				    const char *description,
+				    struct key_tag *domain_tag);
+
+    which is similar to request_key_tag() except that it does not check for
+    keys that are under construction and it will not call out to userspace to
+    construct a key if it can't find a match.
+
+
+ *  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,
+				 bool recurse)
+
+    This searches the specified keyring only (recurse == false) or keyring tree
+    (recurse == true) 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.
+
+
+  *  ``asym_eds_op`` and ``asym_verify_signature``::
+
+       int (*asym_eds_op)(struct kernel_pkey_params *params,
+			  const void *in, void *out);
+       int (*asym_verify_signature)(struct kernel_pkey_params *params,
+				    const void *in, const void *in2);
+
+     These methods are optional.  If provided the first allows a key to be
+     used to encrypt, decrypt or sign a blob of data, and the second allows a
+     key to verify a signature.
+
+     In all cases, the following information is provided in the params block::
+
+	struct kernel_pkey_params {
+		struct key	*key;
+		const char	*encoding;
+		const char	*hash_algo;
+		char		*info;
+		__u32		in_len;
+		union {
+			__u32	out_len;
+			__u32	in2_len;
+		};
+		enum kernel_pkey_operation op : 8;
+	};
+
+     This includes the key to be used; a string indicating the encoding to use
+     (for instance, "pkcs1" may be used with an RSA key to indicate
+     RSASSA-PKCS1-v1.5 or RSAES-PKCS1-v1.5 encoding or "raw" if no encoding);
+     the name of the hash algorithm used to generate the data for a signature
+     (if appropriate); the sizes of the input and output (or second input)
+     buffers; and the ID of the operation to be performed.
+
+     For a given operation ID, the input and output buffers are used as
+     follows::
+
+	Operation ID		in,in_len	out,out_len	in2,in2_len
+	=======================	===============	===============	===============
+	kernel_pkey_encrypt	Raw data	Encrypted data	-
+	kernel_pkey_decrypt	Encrypted data	Raw data	-
+	kernel_pkey_sign	Raw data	Signature	-
+	kernel_pkey_verify	Raw data	-		Signature
+
+     asym_eds_op() deals with encryption, decryption and signature creation as
+     specified by params->op.  Note that params->op is also set for
+     asym_verify_signature().
+
+     Encrypting and signature creation both take raw data in the input buffer
+     and return the encrypted result in the output buffer.  Padding may have
+     been added if an encoding was set.  In the case of signature creation,
+     depending on the encoding, the padding created may need to indicate the
+     digest algorithm - the name of which should be supplied in hash_algo.
+
+     Decryption takes encrypted data in the input buffer and returns the raw
+     data in the output buffer.  Padding will get checked and stripped off if
+     an encoding was set.
+
+     Verification takes raw data in the input buffer and the signature in the
+     second input buffer and checks that the one matches the other.  Padding
+     will be validated.  Depending on the encoding, the digest algorithm used
+     to generate the raw data may need to be indicated in hash_algo.
+
+     If successful, asym_eds_op() should return the number of bytes written
+     into the output buffer.  asym_verify_signature() should return 0.
+
+     A variety of errors may be returned, including EOPNOTSUPP if the operation
+     is not supported; EKEYREJECTED if verification fails; ENOPKG if the
+     required crypto isn't available.
+
+
+  *  ``asym_query``::
+
+       int (*asym_query)(const struct kernel_pkey_params *params,
+			 struct kernel_pkey_query *info);
+
+     This method is optional.  If provided it allows information about the
+     public or asymmetric key held in the key to be determined.
+
+     The parameter block is as for asym_eds_op() and co. but in_len and out_len
+     are unused.  The encoding and hash_algo fields should be used to reduce
+     the returned buffer/data sizes as appropriate.
+
+     If successful, the following information is filled in::
+
+	struct kernel_pkey_query {
+		__u32		supported_ops;
+		__u32		key_size;
+		__u16		max_data_size;
+		__u16		max_sig_size;
+		__u16		max_enc_size;
+		__u16		max_dec_size;
+	};
+
+     The supported_ops field will contain a bitmask indicating what operations
+     are supported by the key, including encryption of a blob, decryption of a
+     blob, signing a blob and verifying the signature on a blob.  The following
+     constants are defined for this::
+
+	KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}
+
+     The key_size field is the size of the key in bits.  max_data_size and
+     max_sig_size are the maximum raw data and signature sizes for creation and
+     verification of a signature; max_enc_size and max_dec_size are the maximum
+     raw data and signature sizes for encryption and decryption.  The
+     max_*_size fields are measured in bytes.
+
+     If successful, 0 will be returned.  If the key doesn't support this,
+     EOPNOTSUPP will be 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