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| -rw-r--r-- | Documentation/userspace-api/landlock.rst | 99 |
1 files changed, 74 insertions, 25 deletions
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index d8cd8cd9ce..2e38226770 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -8,13 +8,13 @@ Landlock: unprivileged access control ===================================== :Author: Mickaël Salaün -:Date: October 2022 +:Date: October 2023 The goal of Landlock is to enable to restrict ambient rights (e.g. global -filesystem access) for a set of processes. Because Landlock is a stackable -LSM, it makes possible to create safe security sandboxes as new security layers -in addition to the existing system-wide access-controls. This kind of sandbox -is expected to help mitigate the security impact of bugs or +filesystem or network access) for a set of processes. Because Landlock +is a stackable LSM, it makes possible to create safe security sandboxes as new +security layers in addition to the existing system-wide access-controls. This +kind of sandbox is expected to help mitigate the security impact of bugs or unexpected/malicious behaviors in user space applications. Landlock empowers any process, including unprivileged ones, to securely restrict themselves. @@ -28,20 +28,34 @@ appropriately <kernel_support>`. Landlock rules ============== -A Landlock rule describes an action on an object. An object is currently a -file hierarchy, and the related filesystem actions are defined with `access -rights`_. A set of rules is aggregated in a ruleset, which can then restrict +A Landlock rule describes an action on an object which the process intends to +perform. A set of rules is aggregated in a ruleset, which can then restrict the thread enforcing it, and its future children. +The two existing types of rules are: + +Filesystem rules + For these rules, the object is a file hierarchy, + and the related filesystem actions are defined with + `filesystem access rights`. + +Network rules (since ABI v4) + For these rules, the object is a TCP port, + and the related actions are defined with `network access rights`. + Defining and enforcing a security policy ---------------------------------------- -We first need to define the ruleset that will contain our rules. For this -example, the ruleset will contain rules that only allow read actions, but write -actions will be denied. The ruleset then needs to handle both of these kind of -actions. This is required for backward and forward compatibility (i.e. the -kernel and user space may not know each other's supported restrictions), hence -the need to be explicit about the denied-by-default access rights. +We first need to define the ruleset that will contain our rules. + +For this example, the ruleset will contain rules that only allow filesystem +read actions and establish a specific TCP connection. Filesystem write +actions and other TCP actions will be denied. + +The ruleset then needs to handle both these kinds of actions. This is +required for backward and forward compatibility (i.e. the kernel and user +space may not know each other's supported restrictions), hence the need +to be explicit about the denied-by-default access rights. .. code-block:: c @@ -62,6 +76,9 @@ the need to be explicit about the denied-by-default access rights. LANDLOCK_ACCESS_FS_MAKE_SYM | LANDLOCK_ACCESS_FS_REFER | LANDLOCK_ACCESS_FS_TRUNCATE, + .handled_access_net = + LANDLOCK_ACCESS_NET_BIND_TCP | + LANDLOCK_ACCESS_NET_CONNECT_TCP, }; Because we may not know on which kernel version an application will be @@ -70,9 +87,7 @@ should try to protect users as much as possible whatever the kernel they are using. To avoid binary enforcement (i.e. either all security features or none), we can leverage a dedicated Landlock command to get the current version of the Landlock ABI and adapt the handled accesses. Let's check if we should -remove the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE`` -access rights, which are only supported starting with the second and third -version of the ABI. +remove access rights which are only supported in higher versions of the ABI. .. code-block:: c @@ -92,6 +107,12 @@ version of the ABI. case 2: /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; + __attribute__((fallthrough)); + case 3: + /* Removes network support for ABI < 4 */ + ruleset_attr.handled_access_net &= + ~(LANDLOCK_ACCESS_NET_BIND_TCP | + LANDLOCK_ACCESS_NET_CONNECT_TCP); } This enables to create an inclusive ruleset that will contain our rules. @@ -143,10 +164,23 @@ for the ruleset creation, by filtering access rights according to the Landlock ABI version. In this example, this is not required because all of the requested ``allowed_access`` rights are already available in ABI 1. -We now have a ruleset with one rule allowing read access to ``/usr`` while -denying all other handled accesses for the filesystem. The next step is to -restrict the current thread from gaining more privileges (e.g. thanks to a SUID -binary). +For network access-control, we can add a set of rules that allow to use a port +number for a specific action: HTTPS connections. + +.. code-block:: c + + struct landlock_net_port_attr net_port = { + .allowed_access = LANDLOCK_ACCESS_NET_CONNECT_TCP, + .port = 443, + }; + + err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT, + &net_port, 0); + +The next step is to restrict the current thread from gaining more privileges +(e.g. through a SUID binary). We now have a ruleset with the first rule +allowing read access to ``/usr`` while denying all other handled accesses for +the filesystem, and a second rule allowing HTTPS connections. .. code-block:: c @@ -355,7 +389,7 @@ Access rights ------------- .. kernel-doc:: include/uapi/linux/landlock.h - :identifiers: fs_access + :identifiers: fs_access net_access Creating a new ruleset ---------------------- @@ -374,6 +408,7 @@ Extending a ruleset .. kernel-doc:: include/uapi/linux/landlock.h :identifiers: landlock_rule_type landlock_path_beneath_attr + landlock_net_port_attr Enforcing a ruleset ------------------- @@ -387,9 +422,9 @@ Current limitations Filesystem topology modification -------------------------------- -As for file renaming and linking, a sandboxed thread cannot modify its -filesystem topology, whether via :manpage:`mount(2)` or -:manpage:`pivot_root(2)`. However, :manpage:`chroot(2)` calls are not denied. +Threads sandboxed with filesystem restrictions cannot modify filesystem +topology, whether via :manpage:`mount(2)` or :manpage:`pivot_root(2)`. +However, :manpage:`chroot(2)` calls are not denied. Special filesystems ------------------- @@ -451,6 +486,14 @@ always allowed when using a kernel that only supports the first or second ABI. Starting with the Landlock ABI version 3, it is now possible to securely control truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right. +Network support (ABI < 4) +------------------------- + +Starting with the Landlock ABI version 4, it is now possible to restrict TCP +bind and connect actions to only a set of allowed ports thanks to the new +``LANDLOCK_ACCESS_NET_BIND_TCP`` and ``LANDLOCK_ACCESS_NET_CONNECT_TCP`` +access rights. + .. _kernel_support: Kernel support @@ -469,6 +512,12 @@ still enable it by adding ``lsm=landlock,[...]`` to Documentation/admin-guide/kernel-parameters.rst thanks to the bootloader configuration. +To be able to explicitly allow TCP operations (e.g., adding a network rule with +``LANDLOCK_ACCESS_NET_BIND_TCP``), the kernel must support TCP +(``CONFIG_INET=y``). Otherwise, sys_landlock_add_rule() returns an +``EAFNOSUPPORT`` error, which can safely be ignored because this kind of TCP +operation is already not possible. + Questions and answers ===================== |