1 .. SPDX-License-Identifier: GPL-2.0
2 .. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
3 .. Copyright © 2019-2020 ANSSI
4 .. Copyright © 2021-2022 Microsoft Corporation
6 =====================================
7 Landlock: unprivileged access control
8 =====================================
10 :Author: Mickaël Salaün
13 The goal of Landlock is to enable to restrict ambient rights (e.g. global
14 filesystem access) for a set of processes. Because Landlock is a stackable
15 LSM, it makes possible to create safe security sandboxes as new security layers
16 in addition to the existing system-wide access-controls. This kind of sandbox
17 is expected to help mitigate the security impact of bugs or
18 unexpected/malicious behaviors in user space applications. Landlock empowers
19 any process, including unprivileged ones, to securely restrict themselves.
21 We can quickly make sure that Landlock is enabled in the running system by
22 looking for "landlock: Up and running" in kernel logs (as root): ``dmesg | grep
23 landlock || journalctl -kg landlock`` . Developers can also easily check for
24 Landlock support with a :ref:`related system call <landlock_abi_versions>`. If
25 Landlock is not currently supported, we need to :ref:`configure the kernel
26 appropriately <kernel_support>`.
31 A Landlock rule describes an action on an object. An object is currently a
32 file hierarchy, and the related filesystem actions are defined with `access
33 rights`_. A set of rules is aggregated in a ruleset, which can then restrict
34 the thread enforcing it, and its future children.
36 Defining and enforcing a security policy
37 ----------------------------------------
39 We first need to define the ruleset that will contain our rules. For this
40 example, the ruleset will contain rules that only allow read actions, but write
41 actions will be denied. The ruleset then needs to handle both of these kind of
42 actions. This is required for backward and forward compatibility (i.e. the
43 kernel and user space may not know each other's supported restrictions), hence
44 the need to be explicit about the denied-by-default access rights.
48 struct landlock_ruleset_attr ruleset_attr = {
50 LANDLOCK_ACCESS_FS_EXECUTE |
51 LANDLOCK_ACCESS_FS_WRITE_FILE |
52 LANDLOCK_ACCESS_FS_READ_FILE |
53 LANDLOCK_ACCESS_FS_READ_DIR |
54 LANDLOCK_ACCESS_FS_REMOVE_DIR |
55 LANDLOCK_ACCESS_FS_REMOVE_FILE |
56 LANDLOCK_ACCESS_FS_MAKE_CHAR |
57 LANDLOCK_ACCESS_FS_MAKE_DIR |
58 LANDLOCK_ACCESS_FS_MAKE_REG |
59 LANDLOCK_ACCESS_FS_MAKE_SOCK |
60 LANDLOCK_ACCESS_FS_MAKE_FIFO |
61 LANDLOCK_ACCESS_FS_MAKE_BLOCK |
62 LANDLOCK_ACCESS_FS_MAKE_SYM |
63 LANDLOCK_ACCESS_FS_REFER,
66 Because we may not know on which kernel version an application will be
67 executed, it is safer to follow a best-effort security approach. Indeed, we
68 should try to protect users as much as possible whatever the kernel they are
69 using. To avoid binary enforcement (i.e. either all security features or
70 none), we can leverage a dedicated Landlock command to get the current version
71 of the Landlock ABI and adapt the handled accesses. Let's check if we should
72 remove the ``LANDLOCK_ACCESS_FS_REFER`` access right which is only supported
73 starting with the second version of the ABI.
79 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
81 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
84 This enables to create an inclusive ruleset that will contain our rules.
90 ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
92 perror("Failed to create a ruleset");
96 We can now add a new rule to this ruleset thanks to the returned file
97 descriptor referring to this ruleset. The rule will only allow reading the
98 file hierarchy ``/usr``. Without another rule, write actions would then be
99 denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the
100 ``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file
106 struct landlock_path_beneath_attr path_beneath = {
108 LANDLOCK_ACCESS_FS_EXECUTE |
109 LANDLOCK_ACCESS_FS_READ_FILE |
110 LANDLOCK_ACCESS_FS_READ_DIR,
113 path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC);
114 if (path_beneath.parent_fd < 0) {
115 perror("Failed to open file");
119 err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
121 close(path_beneath.parent_fd);
123 perror("Failed to update ruleset");
128 It may also be required to create rules following the same logic as explained
129 for the ruleset creation, by filtering access rights according to the Landlock
130 ABI version. In this example, this is not required because
131 ``LANDLOCK_ACCESS_FS_REFER`` is not allowed by any rule.
133 We now have a ruleset with one rule allowing read access to ``/usr`` while
134 denying all other handled accesses for the filesystem. The next step is to
135 restrict the current thread from gaining more privileges (e.g. thanks to a SUID
140 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
141 perror("Failed to restrict privileges");
146 The current thread is now ready to sandbox itself with the ruleset.
150 if (landlock_restrict_self(ruleset_fd, 0)) {
151 perror("Failed to enforce ruleset");
157 If the ``landlock_restrict_self`` system call succeeds, the current thread is
158 now restricted and this policy will be enforced on all its subsequently created
159 children as well. Once a thread is landlocked, there is no way to remove its
160 security policy; only adding more restrictions is allowed. These threads are
161 now in a new Landlock domain, merge of their parent one (if any) with the new
164 Full working code can be found in `samples/landlock/sandboxer.c`_.
169 It is recommended setting access rights to file hierarchy leaves as much as
170 possible. For instance, it is better to be able to have ``~/doc/`` as a
171 read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to
172 ``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy.
173 Following this good practice leads to self-sufficient hierarchies that do not
174 depend on their location (i.e. parent directories). This is particularly
175 relevant when we want to allow linking or renaming. Indeed, having consistent
176 access rights per directory enables to change the location of such directory
177 without relying on the destination directory access rights (except those that
178 are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER``
180 Having self-sufficient hierarchies also helps to tighten the required access
181 rights to the minimal set of data. This also helps avoid sinkhole directories,
182 i.e. directories where data can be linked to but not linked from. However,
183 this depends on data organization, which might not be controlled by developers.
184 In this case, granting read-write access to ``~/tmp/``, instead of write-only
185 access, would potentially allow to move ``~/tmp/`` to a non-readable directory
186 and still keep the ability to list the content of ``~/tmp/``.
188 Layers of file path access rights
189 ---------------------------------
191 Each time a thread enforces a ruleset on itself, it updates its Landlock domain
192 with a new layer of policy. Indeed, this complementary policy is stacked with
193 the potentially other rulesets already restricting this thread. A sandboxed
194 thread can then safely add more constraints to itself with a new enforced
197 One policy layer grants access to a file path if at least one of its rules
198 encountered on the path grants the access. A sandboxed thread can only access
199 a file path if all its enforced policy layers grant the access as well as all
200 the other system access controls (e.g. filesystem DAC, other LSM policies,
203 Bind mounts and OverlayFS
204 -------------------------
206 Landlock enables to restrict access to file hierarchies, which means that these
207 access rights can be propagated with bind mounts (cf.
208 Documentation/filesystems/sharedsubtree.rst) but not with
209 Documentation/filesystems/overlayfs.rst.
211 A bind mount mirrors a source file hierarchy to a destination. The destination
212 hierarchy is then composed of the exact same files, on which Landlock rules can
213 be tied, either via the source or the destination path. These rules restrict
214 access when they are encountered on a path, which means that they can restrict
215 access to multiple file hierarchies at the same time, whether these hierarchies
216 are the result of bind mounts or not.
218 An OverlayFS mount point consists of upper and lower layers. These layers are
219 combined in a merge directory, result of the mount point. This merge hierarchy
220 may include files from the upper and lower layers, but modifications performed
221 on the merge hierarchy only reflects on the upper layer. From a Landlock
222 policy point of view, each OverlayFS layers and merge hierarchies are
223 standalone and contains their own set of files and directories, which is
224 different from bind mounts. A policy restricting an OverlayFS layer will not
225 restrict the resulted merged hierarchy, and vice versa. Landlock users should
226 then only think about file hierarchies they want to allow access to, regardless
227 of the underlying filesystem.
232 Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain
233 restrictions from its parent. This is similar to the seccomp inheritance (cf.
234 Documentation/userspace-api/seccomp_filter.rst) or any other LSM dealing with
235 task's :manpage:`credentials(7)`. For instance, one process's thread may apply
236 Landlock rules to itself, but they will not be automatically applied to other
237 sibling threads (unlike POSIX thread credential changes, cf.
240 When a thread sandboxes itself, we have the guarantee that the related security
241 policy will stay enforced on all this thread's descendants. This allows
242 creating standalone and modular security policies per application, which will
243 automatically be composed between themselves according to their runtime parent
249 A sandboxed process has less privileges than a non-sandboxed process and must
250 then be subject to additional restrictions when manipulating another process.
251 To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
252 process, a sandboxed process should have a subset of the target process rules,
253 which means the tracee must be in a sub-domain of the tracer.
258 Backward and forward compatibility
259 ----------------------------------
261 Landlock is designed to be compatible with past and future versions of the
262 kernel. This is achieved thanks to the system call attributes and the
263 associated bitflags, particularly the ruleset's ``handled_access_fs``. Making
264 handled access right explicit enables the kernel and user space to have a clear
265 contract with each other. This is required to make sure sandboxing will not
266 get stricter with a system update, which could break applications.
268 Developers can subscribe to the `Landlock mailing list
269 <https://subspace.kernel.org/lists.linux.dev.html>`_ to knowingly update and
270 test their applications with the latest available features. In the interest of
271 users, and because they may use different kernel versions, it is strongly
272 encouraged to follow a best-effort security approach by checking the Landlock
273 ABI version at runtime and only enforcing the supported features.
275 .. _landlock_abi_versions:
277 Landlock ABI versions
278 ---------------------
280 The Landlock ABI version can be read with the sys_landlock_create_ruleset()
287 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
291 printf("Landlock is not supported by the current kernel.\n");
294 printf("Landlock is currently disabled.\n");
300 printf("Landlock supports LANDLOCK_ACCESS_FS_REFER.\n");
303 The following kernel interfaces are implicitly supported by the first ABI
304 version. Features only supported from a specific version are explicitly marked
313 .. kernel-doc:: include/uapi/linux/landlock.h
314 :identifiers: fs_access
316 Creating a new ruleset
317 ----------------------
319 .. kernel-doc:: security/landlock/syscalls.c
320 :identifiers: sys_landlock_create_ruleset
322 .. kernel-doc:: include/uapi/linux/landlock.h
323 :identifiers: landlock_ruleset_attr
328 .. kernel-doc:: security/landlock/syscalls.c
329 :identifiers: sys_landlock_add_rule
331 .. kernel-doc:: include/uapi/linux/landlock.h
332 :identifiers: landlock_rule_type landlock_path_beneath_attr
337 .. kernel-doc:: security/landlock/syscalls.c
338 :identifiers: sys_landlock_restrict_self
343 Filesystem topology modification
344 --------------------------------
346 As for file renaming and linking, a sandboxed thread cannot modify its
347 filesystem topology, whether via :manpage:`mount(2)` or
348 :manpage:`pivot_root(2)`. However, :manpage:`chroot(2)` calls are not denied.
353 Access to regular files and directories can be restricted by Landlock,
354 according to the handled accesses of a ruleset. However, files that do not
355 come from a user-visible filesystem (e.g. pipe, socket), but can still be
356 accessed through ``/proc/<pid>/fd/*``, cannot currently be explicitly
357 restricted. Likewise, some special kernel filesystems such as nsfs, which can
358 be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly
359 restricted. However, thanks to the `ptrace restrictions`_, access to such
360 sensitive ``/proc`` files are automatically restricted according to domain
361 hierarchies. Future Landlock evolutions could still enable to explicitly
362 restrict such paths with dedicated ruleset flags.
367 There is a limit of 16 layers of stacked rulesets. This can be an issue for a
368 task willing to enforce a new ruleset in complement to its 16 inherited
369 rulesets. Once this limit is reached, sys_landlock_restrict_self() returns
370 E2BIG. It is then strongly suggested to carefully build rulesets once in the
371 life of a thread, especially for applications able to launch other applications
372 that may also want to sandbox themselves (e.g. shells, container managers,
378 Kernel memory allocated to create rulesets is accounted and can be restricted
379 by the Documentation/admin-guide/cgroup-v1/memory.rst.
384 File renaming and linking (ABI < 2)
385 -----------------------------------
387 Because Landlock targets unprivileged access controls, it needs to properly
388 handle composition of rules. Such property also implies rules nesting.
389 Properly handling multiple layers of rulesets, each one of them able to
390 restrict access to files, also implies inheritance of the ruleset restrictions
391 from a parent to its hierarchy. Because files are identified and restricted by
392 their hierarchy, moving or linking a file from one directory to another implies
393 propagation of the hierarchy constraints, or restriction of these actions
394 according to the potentially lost constraints. To protect against privilege
395 escalations through renaming or linking, and for the sake of simplicity,
396 Landlock previously limited linking and renaming to the same directory.
397 Starting with the Landlock ABI version 2, it is now possible to securely
398 control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER``
406 Landlock was first introduced in Linux 5.13 but it must be configured at build
407 time with ``CONFIG_SECURITY_LANDLOCK=y``. Landlock must also be enabled at boot
408 time as the other security modules. The list of security modules enabled by
409 default is set with ``CONFIG_LSM``. The kernel configuration should then
410 contains ``CONFIG_LSM=landlock,[...]`` with ``[...]`` as the list of other
411 potentially useful security modules for the running system (see the
412 ``CONFIG_LSM`` help).
414 If the running kernel does not have ``landlock`` in ``CONFIG_LSM``, then we can
415 still enable it by adding ``lsm=landlock,[...]`` to
416 Documentation/admin-guide/kernel-parameters.rst thanks to the bootloader
419 Questions and answers
420 =====================
422 What about user space sandbox managers?
423 ---------------------------------------
425 Using user space process to enforce restrictions on kernel resources can lead
426 to race conditions or inconsistent evaluations (i.e. `Incorrect mirroring of
427 the OS code and state
428 <https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_).
430 What about namespaces and containers?
431 -------------------------------------
433 Namespaces can help create sandboxes but they are not designed for
434 access-control and then miss useful features for such use case (e.g. no
435 fine-grained restrictions). Moreover, their complexity can lead to security
436 issues, especially when untrusted processes can manipulate them (cf.
437 `Controlling access to user namespaces <https://lwn.net/Articles/673597/>`_).
439 Additional documentation
440 ========================
442 * Documentation/security/landlock.rst
443 * https://landlock.io
446 .. _samples/landlock/sandboxer.c:
447 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/landlock/sandboxer.c