.. 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