Landlock LSM: kernel documentation

Author

: Mickaël Salaün

Date

: December 2022

Landlock’s goal is to create scoped access-control (i.e. sandboxing). To harden a whole system, this feature should be available to any process, including unprivileged ones. Because such process may be compromised or backdoored (i.e. untrusted), Landlock’s features must be safe to use from the kernel and other processes point of view. Landlock’s interface must therefore expose a minimal attack surface.

Landlock is designed to be usable by unprivileged processes while following the system security policy enforced by other access control mechanisms (e.g. DAC, LSM). Indeed, a Landlock rule shall not interfere with other access-controls enforced on the system, only add more restrictions.

Any user can enforce Landlock rulesets on their processes. They are merged and evaluated according to the inherited ones in a way that ensures that only more constraints can be added.

User space documentation can be found here: Documentation/userspace-api/landlock.rst.

Guiding principles for safe access controls

• A Landlock rule shall be focused on access control on kernel objects instead of syscall filtering (i.e. syscall arguments), which is the purpose of seccomp-bpf.
• To avoid multiple kinds of side-channel attacks (e.g. leak of security policies, CPU-based attacks), Landlock rules shall not be able to programmatically communicate with user space.
• Kernel access check shall not slow down access request from unsandboxed processes.
• Computation related to Landlock operations (e.g. enforcing a ruleset) shall only impact the processes requesting them.
• Resources (e.g. file descriptors) directly obtained from the kernel by a sandboxed process shall retain their scoped accesses (at the time of resource acquisition) whatever process use them. Cf. File descriptor access rights.

Design choices

Inode access rights

All access rights are tied to an inode and what can be accessed through it. Reading the content of a directory does not imply to be allowed to read the content of a listed inode. Indeed, a file name is local to its parent directory, and an inode can be referenced by multiple file names thanks to (hard) links. Being able to unlink a file only has a direct impact on the directory, not the unlinked inode. This is the reason why LANDLOCK_ACCESS_FS_REMOVE_FILE or LANDLOCK_ACCESS_FS_REFER are not allowed to be tied to files but only to directories.

File descriptor access rights

Access rights are checked and tied to file descriptors at open time. The underlying principle is that equivalent sequences of operations should lead to the same results, when they are executed under the same Landlock domain.

Taking the LANDLOCK_ACCESS_FS_TRUNCATE right as an example, it may be allowed to open a file for writing without being allowed to ftruncate{.interpreted-text role=“manpage”} the resulting file descriptor if the related file hierarchy doesn’t grant such access right. The following sequences of operations have the same semantic and should then have the same result:

• truncate(path);
• int fd = open(path, O_WRONLY); ftruncate(fd); close(fd);

Similarly to file access modes (e.g. O_RDWR), Landlock access rights attached to file descriptors are retained even if they are passed between processes (e.g. through a Unix domain socket). Such access rights will then be enforced even if the receiving process is not sandboxed by Landlock. Indeed, this is required to keep a consistent access control over the whole system, and this avoids unattended bypasses through file descriptor passing (i.e. confused deputy attack).

Tests

Userspace tests for backward compatibility, ptrace restrictions and filesystem support can be found here: tools/testing/selftests/landlock/.

Kernel structures

Object

::: {.kernel-doc identifiers=""} security/landlock/object.h :::

Filesystem

::: {.kernel-doc identifiers=""} security/landlock/fs.h :::

Ruleset and domain

A domain is a read-only ruleset tied to a set of subjects (i.e. tasks’ credentials). Each time a ruleset is enforced on a task, the current domain is duplicated and the ruleset is imported as a new layer of rules in the new domain. Indeed, once in a domain, each rule is tied to a layer level. To grant access to an object, at least one rule of each layer must allow the requested action on the object. A task can then only transit to a new domain that is the intersection of the constraints from the current domain and those of a ruleset provided by the task.

The definition of a subject is implicit for a task sandboxing itself, which makes the reasoning much easier and helps avoid pitfalls.

::: {.kernel-doc identifiers=""} security/landlock/ruleset.h :::