1.. SPDX-License-Identifier: GPL-2.0 2.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net> 3.. Copyright © 2019-2020 ANSSI 4 5================================== 6Landlock LSM: kernel documentation 7================================== 8 9:Author: Mickaël Salaün 10:Date: March 2025 11 12Landlock's goal is to create scoped access-control (i.e. sandboxing). To 13harden a whole system, this feature should be available to any process, 14including unprivileged ones. Because such a process may be compromised or 15backdoored (i.e. untrusted), Landlock's features must be safe to use from the 16kernel and other processes point of view. Landlock's interface must therefore 17expose a minimal attack surface. 18 19Landlock is designed to be usable by unprivileged processes while following the 20system security policy enforced by other access control mechanisms (e.g. DAC, 21LSM). A Landlock rule shall not interfere with other access-controls enforced 22on the system, only add more restrictions. 23 24Any user can enforce Landlock rulesets on their processes. They are merged and 25evaluated against inherited rulesets in a way that ensures that only more 26constraints can be added. 27 28User space documentation can be found here: 29Documentation/userspace-api/landlock.rst. 30 31Guiding principles for safe access controls 32=========================================== 33 34* A Landlock rule shall be focused on access control on kernel objects instead 35 of syscall filtering (i.e. syscall arguments), which is the purpose of 36 seccomp-bpf. 37* To avoid multiple kinds of side-channel attacks (e.g. leak of security 38 policies, CPU-based attacks), Landlock rules shall not be able to 39 programmatically communicate with user space. 40* Kernel access check shall not slow down access request from unsandboxed 41 processes. 42* Computation related to Landlock operations (e.g. enforcing a ruleset) shall 43 only impact the processes requesting them. 44* Resources (e.g. file descriptors) directly obtained from the kernel by a 45 sandboxed process shall retain their scoped accesses (at the time of resource 46 acquisition) whatever process uses them. 47 Cf. `File descriptor access rights`_. 48* Access denials shall be logged according to system and Landlock domain 49 configurations. Log entries must contain information about the cause of the 50 denial and the owner of the related security policy. Such log generation 51 should have a negligible performance and memory impact on allowed requests. 52 53Design choices 54============== 55 56Inode access rights 57------------------- 58 59All access rights are tied to an inode and what can be accessed through it. 60Reading the content of a directory does not imply to be allowed to read the 61content of a listed inode. Indeed, a file name is local to its parent 62directory, and an inode can be referenced by multiple file names thanks to 63(hard) links. Being able to unlink a file only has a direct impact on the 64directory, not the unlinked inode. This is the reason why 65``LANDLOCK_ACCESS_FS_REMOVE_FILE`` or ``LANDLOCK_ACCESS_FS_REFER`` are not 66allowed to be tied to files but only to directories. 67 68File descriptor access rights 69----------------------------- 70 71Access rights are checked and tied to file descriptors at open time. The 72underlying principle is that equivalent sequences of operations should lead to 73the same results, when they are executed under the same Landlock domain. 74 75Taking the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right as an example, it may be 76allowed to open a file for writing without being allowed to 77:manpage:`ftruncate` the resulting file descriptor if the related file 78hierarchy doesn't grant that access right. The following sequences of 79operations have the same semantic and should then have the same result: 80 81* ``truncate(path);`` 82* ``int fd = open(path, O_WRONLY); ftruncate(fd); close(fd);`` 83 84Similarly to file access modes (e.g. ``O_RDWR``), Landlock access rights 85attached to file descriptors are retained even if they are passed between 86processes (e.g. through a Unix domain socket). Such access rights will then be 87enforced even if the receiving process is not sandboxed by Landlock. Indeed, 88this is required to keep access controls consistent over the whole system, and 89this avoids unattended bypasses through file descriptor passing (i.e. confused 90deputy attack). 91 92Tests 93===== 94 95Userspace tests for backward compatibility, ptrace restrictions and filesystem 96support can be found here: `tools/testing/selftests/landlock/`_. 97 98Kernel structures 99================= 100 101Object 102------ 103 104.. kernel-doc:: security/landlock/object.h 105 :identifiers: 106 107Filesystem 108---------- 109 110.. kernel-doc:: security/landlock/fs.h 111 :identifiers: 112 113Ruleset and domain 114------------------ 115 116A domain is a read-only ruleset tied to a set of subjects (i.e. tasks' 117credentials). Each time a ruleset is enforced on a task, the current domain is 118duplicated and the ruleset is imported as a new layer of rules in the new 119domain. Indeed, once in a domain, each rule is tied to a layer level. To 120grant access to an object, at least one rule of each layer must allow the 121requested action on the object. A task can then only transit to a new domain 122that is the intersection of the constraints from the current domain and those 123of a ruleset provided by the task. 124 125The definition of a subject is implicit for a task sandboxing itself, which 126makes the reasoning much easier and helps avoid pitfalls. 127 128.. kernel-doc:: security/landlock/ruleset.h 129 :identifiers: 130 131Additional documentation 132======================== 133 134* Documentation/userspace-api/landlock.rst 135* Documentation/admin-guide/LSM/landlock.rst 136* https://landlock.io 137 138.. Links 139.. _tools/testing/selftests/landlock/: 140 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/ 141