landlock: Add audit documentation

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.. SPDX-License-Identifier: GPL-2.0

.. Copyright © 2025 Microsoft Corporation

================================

Landlock: system-wide management

================================

:Author: Mickaël Salaün

:Date: March 2025

Landlock can leverage the audit framework to log events.

User space documentation can be found here:

Documentation/userspace-api/landlock.rst.

Audit

=====

Denied access requests are logged by default for a sandboxed program if `audit`

is enabled.  This default behavior can be changed with the

sys_landlock_restrict_self() flags (cf.

Documentation/userspace-api/landlock.rst).  Landlock logs can also be masked

thanks to audit rules.  Landlock can generate 2 audit record types.

Record types

------------

AUDIT_LANDLOCK_ACCESS

    This record type identifies a denied access request to a kernel resource.

    The ``domain`` field indicates the ID of the domain that blocked the

    request.  The ``blockers`` field indicates the cause(s) of this denial

    (separated by a comma), and the following fields identify the kernel object

    (similar to SELinux).  There may be more than one of this record type per

    audit event.

    Example with a file link request generating two records in the same event::

        domain=195ba459b blockers=fs.refer path="/usr/bin" dev="vda2" ino=351

        domain=195ba459b blockers=fs.make_reg,fs.refer path="/usr/local" dev="vda2" ino=365

AUDIT_LANDLOCK_DOMAIN

    This record type describes the status of a Landlock domain.  The ``status``

    field can be either ``allocated`` or ``deallocated``.

    The ``allocated`` status is part of the same audit event and follows

    the first logged ``AUDIT_LANDLOCK_ACCESS`` record of a domain.  It identifies

    Landlock domain information at the time of the sys_landlock_restrict_self()

    call with the following fields:

    - the ``domain`` ID

    - the enforcement ``mode``

    - the domain creator's ``pid``

    - the domain creator's ``uid``

    - the domain creator's executable path (``exe``)

    - the domain creator's command line (``comm``)

    Example::

        domain=195ba459b status=allocated mode=enforcing pid=300 uid=0 exe="/root/sandboxer" comm="sandboxer"

    The ``deallocated`` status is an event on its own and it identifies a

    Landlock domain release.  After such event, it is guarantee that the

    related domain ID will never be reused during the lifetime of the system.

    The ``domain`` field indicates the ID of the domain which is released, and

    the ``denials`` field indicates the total number of denied access request,

    which might not have been logged according to the audit rules and

    sys_landlock_restrict_self()'s flags.

    Example::

        domain=195ba459b status=deallocated denials=3

Event samples

--------------

Here are two examples of log events (see serial numbers).

In this example a sandboxed program (``kill``) tries to send a signal to the

init process, which is denied because of the signal scoping restriction

(``LL_SCOPED=s``)::

  $ LL_FS_RO=/ LL_FS_RW=/ LL_SCOPED=s LL_FORCE_LOG=1 ./sandboxer kill 1

This command generates two events, each identified with a unique serial

number following a timestamp (``msg=audit(1729738800.268:30)``).  The first

event (serial ``30``) contains 4 records.  The first record

(``type=LANDLOCK_ACCESS``) shows an access denied by the domain `1a6fdc66f`.

The cause of this denial is signal scopping restriction

(``blockers=scope.signal``).  The process that would have receive this signal

is the init process (``opid=1 ocomm="systemd"``).

The second record (``type=LANDLOCK_DOMAIN``) describes (``status=allocated``)

domain `1a6fdc66f`.  This domain was created by process ``286`` executing the

``/root/sandboxer`` program launched by the root user.

The third record (``type=SYSCALL``) describes the syscall, its provided

arguments, its result (``success=no exit=-1``), and the process that called it.

The fourth record (``type=PROCTITLE``) shows the command's name as an

hexadecimal value.  This can be translated with ``python -c

'print(bytes.fromhex("6B696C6C0031"))'``.

Finally, the last record (``type=LANDLOCK_DOMAIN``) is also the only one from

the second event (serial ``31``).  It is not tied to a direct user space action

but an asynchronous one to free resources tied to a Landlock domain

(``status=deallocated``).  This can be useful to know that the following logs

will not concern the domain ``1a6fdc66f`` anymore.  This record also summarize

the number of requests this domain denied (``denials=1``), whether they were

logged or not.

.. code-block::

  type=LANDLOCK_ACCESS msg=audit(1729738800.268:30): domain=1a6fdc66f blockers=scope.signal opid=1 ocomm="systemd"

  type=LANDLOCK_DOMAIN msg=audit(1729738800.268:30): domain=1a6fdc66f status=allocated mode=enforcing pid=286 uid=0 exe="/root/sandboxer" comm="sandboxer"

  type=SYSCALL msg=audit(1729738800.268:30): arch=c000003e syscall=62 success=no exit=-1 [..] ppid=272 pid=286 auid=0 uid=0 gid=0 [...] comm="kill" [...]

  type=PROCTITLE msg=audit(1729738800.268:30): proctitle=6B696C6C0031

  type=LANDLOCK_DOMAIN msg=audit(1729738800.324:31): domain=1a6fdc66f status=deallocated denials=1

Here is another example showcasing filesystem access control::

  $ LL_FS_RO=/ LL_FS_RW=/tmp LL_FORCE_LOG=1 ./sandboxer sh -c "echo > /etc/passwd"

The related audit logs contains 8 records from 3 different events (serials 33,

34 and 35) created by the same domain `1a6fdc679`::

  type=LANDLOCK_ACCESS msg=audit(1729738800.221:33): domain=1a6fdc679 blockers=fs.write_file path="/dev/tty" dev="devtmpfs" ino=9

  type=LANDLOCK_DOMAIN msg=audit(1729738800.221:33): domain=1a6fdc679 status=allocated mode=enforcing pid=289 uid=0 exe="/root/sandboxer" comm="sandboxer"

  type=SYSCALL msg=audit(1729738800.221:33): arch=c000003e syscall=257 success=no exit=-13 [...] ppid=272 pid=289 auid=0 uid=0 gid=0 [...] comm="sh" [...]

  type=PROCTITLE msg=audit(1729738800.221:33): proctitle=7368002D63006563686F203E202F6574632F706173737764

  type=LANDLOCK_ACCESS msg=audit(1729738800.221:34): domain=1a6fdc679 blockers=fs.write_file path="/etc/passwd" dev="vda2" ino=143821

  type=SYSCALL msg=audit(1729738800.221:34): arch=c000003e syscall=257 success=no exit=-13 [...] ppid=272 pid=289 auid=0 uid=0 gid=0 [...] comm="sh" [...]

  type=PROCTITLE msg=audit(1729738800.221:34): proctitle=7368002D63006563686F203E202F6574632F706173737764

  type=LANDLOCK_DOMAIN msg=audit(1729738800.261:35): domain=1a6fdc679 status=deallocated denials=2

Event filtering

---------------

If you get spammed with audit logs related to Landlock, this is either an

attack attempt or a bug in the security policy.  We can put in place some

filters to limit noise with two complementary ways:

- with sys_landlock_restrict_self()'s flags if we can fix the sandboxed

  programs,

- or with audit rules (see :manpage:`auditctl(8)`).

Additional documentation

========================

* `Linux Audit Documentation`_

* Documentation/userspace-api/landlock.rst

* Documentation/security/landlock.rst

* https://landlock.io

.. Links

.. _Linux Audit Documentation:

   https://github.com/linux-audit/audit-documentation/wiki

==================================

:Author: Mickaël Salaün

:Date: December 2022

:Date: March 2025

Landlock's goal is to create scoped access-control (i.e. sandboxing).  To

harden a whole system, this feature should be available to any process,

  sandboxed process shall retain their scoped accesses (at the time of resource

  acquisition) whatever process uses them.

  Cf. `File descriptor access rights`_.

* Access denials shall be logged according to system and Landlock domain

  configurations.  Log entries must contain information about the cause of the

  denial and the owner of the related security policy.  Such log generation

  should have a negligible performance and memory impact on allowed requests.

Design choices

==============

.. kernel-doc:: security/landlock/ruleset.h

    :identifiers:

Additional documentation

========================

* Documentation/userspace-api/landlock.rst

* Documentation/admin-guide/LSM/landlock.rst

* https://landlock.io

.. Links

.. _tools/testing/selftests/landlock/:

   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/

:manpage:`signal(7)` sending by setting ``LANDLOCK_SCOPE_SIGNAL`` to the

``scoped`` ruleset attribute.

Logging (ABI < 7)

-----------------

Starting with the Landlock ABI version 7, it is possible to control logging of

Landlock audit events with the ``LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF``,

``LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON``, and

``LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF`` flags passed to

sys_landlock_restrict_self().  See Documentation/admin-guide/LSM/landlock.rst

for more details on audit.

.. _kernel_support:

Kernel support

issues, especially when untrusted processes can manipulate them (cf.

`Controlling access to user namespaces <https://lwn.net/Articles/673597/>`_).

How to disable Landlock audit records?

--------------------------------------

You might want to put in place filters as explained here:

Documentation/admin-guide/LSM/landlock.rst

Additional documentation

========================

* Documentation/admin-guide/LSM/landlock.rst

* Documentation/security/landlock.rst

* https://landlock.io

S:	Supported

W:	https://landlock.io

T:	git https://git.kernel.org/pub/scm/linux/kernel/git/mic/linux.git

F:	Documentation/admin-guide/LSM/landlock.rst

F:	Documentation/security/landlock.rst

F:	Documentation/userspace-api/landlock.rst

F:	fs/ioctl.c