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: May 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 45Design choices 46============== 47 48Filesystem access rights 49------------------------ 50 51All access rights are tied to an inode and what can be accessed through it. 52Reading the content of a directory doesn't imply to be allowed to read the 53content of a listed inode. Indeed, a file name is local to its parent 54directory, and an inode can be referenced by multiple file names thanks to 55(hard) links. Being able to unlink a file only has a direct impact on the 56directory, not the unlinked inode. This is the reason why 57`LANDLOCK_ACCESS_FS_REMOVE_FILE` or `LANDLOCK_ACCESS_FS_REFER` are not allowed 58to be tied to files but only to directories. 59 60Tests 61===== 62 63Userspace tests for backward compatibility, ptrace restrictions and filesystem 64support can be found here: `tools/testing/selftests/landlock/`_. 65 66Kernel structures 67================= 68 69Object 70------ 71 72.. kernel-doc:: security/landlock/object.h 73 :identifiers: 74 75Filesystem 76---------- 77 78.. kernel-doc:: security/landlock/fs.h 79 :identifiers: 80 81Ruleset and domain 82------------------ 83 84A domain is a read-only ruleset tied to a set of subjects (i.e. tasks' 85credentials). Each time a ruleset is enforced on a task, the current domain is 86duplicated and the ruleset is imported as a new layer of rules in the new 87domain. Indeed, once in a domain, each rule is tied to a layer level. To 88grant access to an object, at least one rule of each layer must allow the 89requested action on the object. A task can then only transit to a new domain 90that is the intersection of the constraints from the current domain and those 91of a ruleset provided by the task. 92 93The definition of a subject is implicit for a task sandboxing itself, which 94makes the reasoning much easier and helps avoid pitfalls. 95 96.. kernel-doc:: security/landlock/ruleset.h 97 :identifiers: 98 99.. Links 100.. _tools/testing/selftests/landlock/: 101 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/ 102