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12 KiB
ReStructuredText
313 lines
12 KiB
ReStructuredText
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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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.. Copyright © 2021 Microsoft Corporation
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=====================================
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Landlock: unprivileged access control
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=====================================
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:Author: Mickaël Salaün
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:Date: March 2021
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The goal of Landlock is to enable to restrict ambient rights (e.g. global
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filesystem access) for a set of processes. Because Landlock is a stackable
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LSM, it makes possible to create safe security sandboxes as new security layers
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in addition to the existing system-wide access-controls. This kind of sandbox
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is expected to help mitigate the security impact of bugs or
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unexpected/malicious behaviors in user space applications. Landlock empowers
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any process, including unprivileged ones, to securely restrict themselves.
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Landlock rules
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==============
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A Landlock rule describes an action on an object. An object is currently a
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file hierarchy, and the related filesystem actions are defined with `access
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rights`_. A set of rules is aggregated in a ruleset, which can then restrict
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the thread enforcing it, and its future children.
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Defining and enforcing a security policy
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----------------------------------------
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We first need to create the ruleset that will contain our rules. For this
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example, the ruleset will contain rules that only allow read actions, but write
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actions will be denied. The ruleset then needs to handle both of these kind of
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actions.
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.. code-block:: c
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int ruleset_fd;
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struct landlock_ruleset_attr ruleset_attr = {
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.handled_access_fs =
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LANDLOCK_ACCESS_FS_EXECUTE |
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LANDLOCK_ACCESS_FS_WRITE_FILE |
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LANDLOCK_ACCESS_FS_READ_FILE |
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LANDLOCK_ACCESS_FS_READ_DIR |
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LANDLOCK_ACCESS_FS_REMOVE_DIR |
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LANDLOCK_ACCESS_FS_REMOVE_FILE |
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LANDLOCK_ACCESS_FS_MAKE_CHAR |
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LANDLOCK_ACCESS_FS_MAKE_DIR |
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LANDLOCK_ACCESS_FS_MAKE_REG |
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LANDLOCK_ACCESS_FS_MAKE_SOCK |
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LANDLOCK_ACCESS_FS_MAKE_FIFO |
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LANDLOCK_ACCESS_FS_MAKE_BLOCK |
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LANDLOCK_ACCESS_FS_MAKE_SYM,
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};
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ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
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if (ruleset_fd < 0) {
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perror("Failed to create a ruleset");
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return 1;
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}
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We can now add a new rule to this ruleset thanks to the returned file
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descriptor referring to this ruleset. The rule will only allow reading the
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file hierarchy ``/usr``. Without another rule, write actions would then be
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denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the
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``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file
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descriptor.
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.. code-block:: c
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int err;
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struct landlock_path_beneath_attr path_beneath = {
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.allowed_access =
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LANDLOCK_ACCESS_FS_EXECUTE |
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LANDLOCK_ACCESS_FS_READ_FILE |
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LANDLOCK_ACCESS_FS_READ_DIR,
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};
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path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC);
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if (path_beneath.parent_fd < 0) {
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perror("Failed to open file");
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close(ruleset_fd);
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return 1;
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}
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err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
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&path_beneath, 0);
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close(path_beneath.parent_fd);
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if (err) {
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perror("Failed to update ruleset");
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close(ruleset_fd);
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return 1;
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}
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We now have a ruleset with one rule allowing read access to ``/usr`` while
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denying all other handled accesses for the filesystem. The next step is to
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restrict the current thread from gaining more privileges (e.g. thanks to a SUID
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binary).
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.. code-block:: c
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if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
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perror("Failed to restrict privileges");
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close(ruleset_fd);
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return 1;
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}
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The current thread is now ready to sandbox itself with the ruleset.
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.. code-block:: c
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if (landlock_restrict_self(ruleset_fd, 0)) {
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perror("Failed to enforce ruleset");
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close(ruleset_fd);
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return 1;
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}
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close(ruleset_fd);
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If the `landlock_restrict_self` system call succeeds, the current thread is now
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restricted and this policy will be enforced on all its subsequently created
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children as well. Once a thread is landlocked, there is no way to remove its
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security policy; only adding more restrictions is allowed. These threads are
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now in a new Landlock domain, merge of their parent one (if any) with the new
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ruleset.
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Full working code can be found in `samples/landlock/sandboxer.c`_.
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Layers of file path access rights
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---------------------------------
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Each time a thread enforces a ruleset on itself, it updates its Landlock domain
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with a new layer of policy. Indeed, this complementary policy is stacked with
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the potentially other rulesets already restricting this thread. A sandboxed
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thread can then safely add more constraints to itself with a new enforced
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ruleset.
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One policy layer grants access to a file path if at least one of its rules
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encountered on the path grants the access. A sandboxed thread can only access
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a file path if all its enforced policy layers grant the access as well as all
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the other system access controls (e.g. filesystem DAC, other LSM policies,
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etc.).
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Bind mounts and OverlayFS
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-------------------------
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Landlock enables to restrict access to file hierarchies, which means that these
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access rights can be propagated with bind mounts (cf.
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Documentation/filesystems/sharedsubtree.rst) but not with
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Documentation/filesystems/overlayfs.rst.
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A bind mount mirrors a source file hierarchy to a destination. The destination
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hierarchy is then composed of the exact same files, on which Landlock rules can
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be tied, either via the source or the destination path. These rules restrict
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access when they are encountered on a path, which means that they can restrict
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access to multiple file hierarchies at the same time, whether these hierarchies
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are the result of bind mounts or not.
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An OverlayFS mount point consists of upper and lower layers. These layers are
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combined in a merge directory, result of the mount point. This merge hierarchy
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may include files from the upper and lower layers, but modifications performed
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on the merge hierarchy only reflects on the upper layer. From a Landlock
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policy point of view, each OverlayFS layers and merge hierarchies are
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standalone and contains their own set of files and directories, which is
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different from bind mounts. A policy restricting an OverlayFS layer will not
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restrict the resulted merged hierarchy, and vice versa. Landlock users should
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then only think about file hierarchies they want to allow access to, regardless
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of the underlying filesystem.
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Inheritance
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-----------
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Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain
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restrictions from its parent. This is similar to the seccomp inheritance (cf.
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Documentation/userspace-api/seccomp_filter.rst) or any other LSM dealing with
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task's :manpage:`credentials(7)`. For instance, one process's thread may apply
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Landlock rules to itself, but they will not be automatically applied to other
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sibling threads (unlike POSIX thread credential changes, cf.
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:manpage:`nptl(7)`).
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When a thread sandboxes itself, we have the guarantee that the related security
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policy will stay enforced on all this thread's descendants. This allows
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creating standalone and modular security policies per application, which will
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automatically be composed between themselves according to their runtime parent
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policies.
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Ptrace restrictions
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-------------------
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A sandboxed process has less privileges than a non-sandboxed process and must
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then be subject to additional restrictions when manipulating another process.
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To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
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process, a sandboxed process should have a subset of the target process rules,
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which means the tracee must be in a sub-domain of the tracer.
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Kernel interface
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================
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Access rights
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-------------
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: fs_access
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Creating a new ruleset
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----------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_create_ruleset
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: landlock_ruleset_attr
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Extending a ruleset
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-------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_add_rule
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: landlock_rule_type landlock_path_beneath_attr
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Enforcing a ruleset
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-------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_restrict_self
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Current limitations
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===================
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File renaming and linking
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-------------------------
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Because Landlock targets unprivileged access controls, it is needed to properly
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handle composition of rules. Such property also implies rules nesting.
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Properly handling multiple layers of ruleset, each one of them able to restrict
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access to files, also implies to inherit the ruleset restrictions from a parent
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to its hierarchy. Because files are identified and restricted by their
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hierarchy, moving or linking a file from one directory to another implies to
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propagate the hierarchy constraints. To protect against privilege escalations
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through renaming or linking, and for the sake of simplicity, Landlock currently
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limits linking and renaming to the same directory. Future Landlock evolutions
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will enable more flexibility for renaming and linking, with dedicated ruleset
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flags.
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Filesystem topology modification
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--------------------------------
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As for file renaming and linking, a sandboxed thread cannot modify its
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filesystem topology, whether via :manpage:`mount(2)` or
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:manpage:`pivot_root(2)`. However, :manpage:`chroot(2)` calls are not denied.
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Special filesystems
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-------------------
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Access to regular files and directories can be restricted by Landlock,
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according to the handled accesses of a ruleset. However, files that do not
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come from a user-visible filesystem (e.g. pipe, socket), but can still be
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accessed through ``/proc/<pid>/fd/*``, cannot currently be explicitly
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restricted. Likewise, some special kernel filesystems such as nsfs, which can
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be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly
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restricted. However, thanks to the `ptrace restrictions`_, access to such
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sensitive ``/proc`` files are automatically restricted according to domain
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hierarchies. Future Landlock evolutions could still enable to explicitly
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restrict such paths with dedicated ruleset flags.
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Ruleset layers
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--------------
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There is a limit of 16 layers of stacked rulesets. This can be an issue for a
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task willing to enforce a new ruleset in complement to its 16 inherited
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rulesets. Once this limit is reached, sys_landlock_restrict_self() returns
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E2BIG. It is then strongly suggested to carefully build rulesets once in the
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life of a thread, especially for applications able to launch other applications
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that may also want to sandbox themselves (e.g. shells, container managers,
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etc.).
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Memory usage
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------------
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Kernel memory allocated to create rulesets is accounted and can be restricted
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by the Documentation/admin-guide/cgroup-v1/memory.rst.
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Questions and answers
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=====================
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What about user space sandbox managers?
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---------------------------------------
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Using user space process to enforce restrictions on kernel resources can lead
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to race conditions or inconsistent evaluations (i.e. `Incorrect mirroring of
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the OS code and state
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<https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_).
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What about namespaces and containers?
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-------------------------------------
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Namespaces can help create sandboxes but they are not designed for
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access-control and then miss useful features for such use case (e.g. no
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fine-grained restrictions). Moreover, their complexity can lead to security
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issues, especially when untrusted processes can manipulate them (cf.
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`Controlling access to user namespaces <https://lwn.net/Articles/673597/>`_).
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Additional documentation
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========================
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* Documentation/security/landlock.rst
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* https://landlock.io
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.. Links
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.. _samples/landlock/sandboxer.c:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/landlock/sandboxer.c
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