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102 lines
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.. SPDX-License-Identifier: GPL-2.0
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.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
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.. Copyright © 2019-2020 ANSSI
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==================================
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Landlock LSM: kernel documentation
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==================================
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:Author: Mickaël Salaün
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:Date: May 2022
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Landlock's goal is to create scoped access-control (i.e. sandboxing). To
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harden a whole system, this feature should be available to any process,
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including unprivileged ones. Because such process may be compromised or
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backdoored (i.e. untrusted), Landlock's features must be safe to use from the
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kernel and other processes point of view. Landlock's interface must therefore
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expose a minimal attack surface.
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Landlock is designed to be usable by unprivileged processes while following the
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system security policy enforced by other access control mechanisms (e.g. DAC,
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LSM). Indeed, a Landlock rule shall not interfere with other access-controls
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enforced on the system, only add more restrictions.
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Any user can enforce Landlock rulesets on their processes. They are merged and
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evaluated according to the inherited ones in a way that ensures that only more
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constraints can be added.
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User space documentation can be found here:
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Documentation/userspace-api/landlock.rst.
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Guiding principles for safe access controls
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===========================================
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* A Landlock rule shall be focused on access control on kernel objects instead
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of syscall filtering (i.e. syscall arguments), which is the purpose of
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seccomp-bpf.
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* To avoid multiple kinds of side-channel attacks (e.g. leak of security
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policies, CPU-based attacks), Landlock rules shall not be able to
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programmatically communicate with user space.
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* Kernel access check shall not slow down access request from unsandboxed
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processes.
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* Computation related to Landlock operations (e.g. enforcing a ruleset) shall
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only impact the processes requesting them.
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Design choices
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==============
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Filesystem access rights
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------------------------
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All access rights are tied to an inode and what can be accessed through it.
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Reading the content of a directory doesn't imply to be allowed to read the
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content of a listed inode. Indeed, a file name is local to its parent
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directory, and an inode can be referenced by multiple file names thanks to
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(hard) links. Being able to unlink a file only has a direct impact on the
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directory, not the unlinked inode. This is the reason why
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`LANDLOCK_ACCESS_FS_REMOVE_FILE` or `LANDLOCK_ACCESS_FS_REFER` are not allowed
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to be tied to files but only to directories.
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Tests
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=====
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Userspace tests for backward compatibility, ptrace restrictions and filesystem
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support can be found here: `tools/testing/selftests/landlock/`_.
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Kernel structures
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=================
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Object
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------
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.. kernel-doc:: security/landlock/object.h
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:identifiers:
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Filesystem
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----------
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.. kernel-doc:: security/landlock/fs.h
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:identifiers:
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Ruleset and domain
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------------------
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A domain is a read-only ruleset tied to a set of subjects (i.e. tasks'
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credentials). Each time a ruleset is enforced on a task, the current domain is
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duplicated and the ruleset is imported as a new layer of rules in the new
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domain. Indeed, once in a domain, each rule is tied to a layer level. To
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grant access to an object, at least one rule of each layer must allow the
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requested action on the object. A task can then only transit to a new domain
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that is the intersection of the constraints from the current domain and those
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of a ruleset provided by the task.
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The definition of a subject is implicit for a task sandboxing itself, which
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makes the reasoning much easier and helps avoid pitfalls.
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.. kernel-doc:: security/landlock/ruleset.h
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:identifiers:
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.. Links
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.. _tools/testing/selftests/landlock/:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/
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