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: December 2022 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 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). Indeed, a Landlock rule shall not interfere with other access-controls 22enforced on the system, only add more restrictions. 23 24Any user can enforce Landlock rulesets on their processes. They are merged and 25evaluated according to the inherited ones 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 use them. 47 Cf. `File descriptor access rights`_. 48 49Design choices 50============== 51 52Inode access rights 53------------------- 54 55All access rights are tied to an inode and what can be accessed through it. 56Reading the content of a directory does not imply to be allowed to read the 57content of a listed inode. Indeed, a file name is local to its parent 58directory, and an inode can be referenced by multiple file names thanks to 59(hard) links. Being able to unlink a file only has a direct impact on the 60directory, not the unlinked inode. This is the reason why 61``LANDLOCK_ACCESS_FS_REMOVE_FILE`` or ``LANDLOCK_ACCESS_FS_REFER`` are not 62allowed to be tied to files but only to directories. 63 64File descriptor access rights 65----------------------------- 66 67Access rights are checked and tied to file descriptors at open time. The 68underlying principle is that equivalent sequences of operations should lead to 69the same results, when they are executed under the same Landlock domain. 70 71Taking the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right as an example, it may be 72allowed to open a file for writing without being allowed to 73:manpage:`ftruncate` the resulting file descriptor if the related file 74hierarchy doesn't grant such access right. The following sequences of 75operations have the same semantic and should then have the same result: 76 77* ``truncate(path);`` 78* ``int fd = open(path, O_WRONLY); ftruncate(fd); close(fd);`` 79 80Similarly to file access modes (e.g. ``O_RDWR``), Landlock access rights 81attached to file descriptors are retained even if they are passed between 82processes (e.g. through a Unix domain socket). Such access rights will then be 83enforced even if the receiving process is not sandboxed by Landlock. Indeed, 84this is required to keep a consistent access control over the whole system, and 85this avoids unattended bypasses through file descriptor passing (i.e. confused 86deputy attack). 87 88Tests 89===== 90 91Userspace tests for backward compatibility, ptrace restrictions and filesystem 92support can be found here: `tools/testing/selftests/landlock/`_. 93 94Kernel structures 95================= 96 97Object 98------ 99 100.. kernel-doc:: security/landlock/object.h 101 :identifiers: 102 103Filesystem 104---------- 105 106.. kernel-doc:: security/landlock/fs.h 107 :identifiers: 108 109Ruleset and domain 110------------------ 111 112A domain is a read-only ruleset tied to a set of subjects (i.e. tasks' 113credentials). Each time a ruleset is enforced on a task, the current domain is 114duplicated and the ruleset is imported as a new layer of rules in the new 115domain. Indeed, once in a domain, each rule is tied to a layer level. To 116grant access to an object, at least one rule of each layer must allow the 117requested action on the object. A task can then only transit to a new domain 118that is the intersection of the constraints from the current domain and those 119of a ruleset provided by the task. 120 121The definition of a subject is implicit for a task sandboxing itself, which 122makes the reasoning much easier and helps avoid pitfalls. 123 124.. kernel-doc:: security/landlock/ruleset.h 125 :identifiers: 126 127.. Links 128.. _tools/testing/selftests/landlock/: 129 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/ 130