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diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst new file mode 100644 index 000000000000..62c9361a3c7f --- /dev/null +++ b/Documentation/userspace-api/landlock.rst @@ -0,0 +1,311 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net> +.. Copyright © 2019-2020 ANSSI +.. Copyright © 2021 Microsoft Corporation + +===================================== +Landlock: unprivileged access control +===================================== + +:Author: Mickaël Salaün +:Date: March 2021 + +The goal of Landlock is to enable to restrict ambient rights (e.g. global +filesystem access) for a set of processes. Because Landlock is a stackable +LSM, it makes possible to create safe security sandboxes as new security layers +in addition to the existing system-wide access-controls. This kind of sandbox +is expected to help mitigate the security impact of bugs or +unexpected/malicious behaviors in user space applications. Landlock empowers +any process, including unprivileged ones, to securely restrict themselves. + +Landlock rules +============== + +A Landlock rule describes an action on an object. An object is currently a +file hierarchy, and the related filesystem actions are defined with `access +rights`_. A set of rules is aggregated in a ruleset, which can then restrict +the thread enforcing it, and its future children. + +Defining and enforcing a security policy +---------------------------------------- + +We first need to create the ruleset that will contain our rules. For this +example, the ruleset will contain rules that only allow read actions, but write +actions will be denied. The ruleset then needs to handle both of these kind of +actions. + +.. code-block:: c + + int ruleset_fd; + struct landlock_ruleset_attr ruleset_attr = { + .handled_access_fs = + LANDLOCK_ACCESS_FS_EXECUTE | + LANDLOCK_ACCESS_FS_WRITE_FILE | + LANDLOCK_ACCESS_FS_READ_FILE | + LANDLOCK_ACCESS_FS_READ_DIR | + LANDLOCK_ACCESS_FS_REMOVE_DIR | + LANDLOCK_ACCESS_FS_REMOVE_FILE | + LANDLOCK_ACCESS_FS_MAKE_CHAR | + LANDLOCK_ACCESS_FS_MAKE_DIR | + LANDLOCK_ACCESS_FS_MAKE_REG | + LANDLOCK_ACCESS_FS_MAKE_SOCK | + LANDLOCK_ACCESS_FS_MAKE_FIFO | + LANDLOCK_ACCESS_FS_MAKE_BLOCK | + LANDLOCK_ACCESS_FS_MAKE_SYM, + }; + + ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0); + if (ruleset_fd < 0) { + perror("Failed to create a ruleset"); + return 1; + } + +We can now add a new rule to this ruleset thanks to the returned file +descriptor referring to this ruleset. The rule will only allow reading the +file hierarchy ``/usr``. Without another rule, write actions would then be +denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the +``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file +descriptor. + +.. code-block:: c + + int err; + struct landlock_path_beneath_attr path_beneath = { + .allowed_access = + LANDLOCK_ACCESS_FS_EXECUTE | + LANDLOCK_ACCESS_FS_READ_FILE | + LANDLOCK_ACCESS_FS_READ_DIR, + }; + + path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC); + if (path_beneath.parent_fd < 0) { + perror("Failed to open file"); + close(ruleset_fd); + return 1; + } + err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH, + &path_beneath, 0); + close(path_beneath.parent_fd); + if (err) { + perror("Failed to update ruleset"); + close(ruleset_fd); + return 1; + } + +We now have a ruleset with one rule allowing read access to ``/usr`` while +denying all other handled accesses for the filesystem. The next step is to +restrict the current thread from gaining more privileges (e.g. thanks to a SUID +binary). + +.. code-block:: c + + if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { + perror("Failed to restrict privileges"); + close(ruleset_fd); + return 1; + } + +The current thread is now ready to sandbox itself with the ruleset. + +.. code-block:: c + + if (landlock_restrict_self(ruleset_fd, 0)) { + perror("Failed to enforce ruleset"); + close(ruleset_fd); + return 1; + } + close(ruleset_fd); + +If the `landlock_restrict_self` system call succeeds, the current thread is now +restricted and this policy will be enforced on all its subsequently created +children as well. Once a thread is landlocked, there is no way to remove its +security policy; only adding more restrictions is allowed. These threads are +now in a new Landlock domain, merge of their parent one (if any) with the new +ruleset. + +Full working code can be found in `samples/landlock/sandboxer.c`_. + +Layers of file path access rights +--------------------------------- + +Each time a thread enforces a ruleset on itself, it updates its Landlock domain +with a new layer of policy. Indeed, this complementary policy is stacked with +the potentially other rulesets already restricting this thread. A sandboxed +thread can then safely add more constraints to itself with a new enforced +ruleset. + +One policy layer grants access to a file path if at least one of its rules +encountered on the path grants the access. A sandboxed thread can only access +a file path if all its enforced policy layers grant the access as well as all +the other system access controls (e.g. filesystem DAC, other LSM policies, +etc.). + +Bind mounts and OverlayFS +------------------------- + +Landlock enables to restrict access to file hierarchies, which means that these +access rights can be propagated with bind mounts (cf. +:doc:`/filesystems/sharedsubtree`) but not with :doc:`/filesystems/overlayfs`. + +A bind mount mirrors a source file hierarchy to a destination. The destination +hierarchy is then composed of the exact same files, on which Landlock rules can +be tied, either via the source or the destination path. These rules restrict +access when they are encountered on a path, which means that they can restrict +access to multiple file hierarchies at the same time, whether these hierarchies +are the result of bind mounts or not. + +An OverlayFS mount point consists of upper and lower layers. These layers are +combined in a merge directory, result of the mount point. This merge hierarchy +may include files from the upper and lower layers, but modifications performed +on the merge hierarchy only reflects on the upper layer. From a Landlock +policy point of view, each OverlayFS layers and merge hierarchies are +standalone and contains their own set of files and directories, which is +different from bind mounts. A policy restricting an OverlayFS layer will not +restrict the resulted merged hierarchy, and vice versa. Landlock users should +then only think about file hierarchies they want to allow access to, regardless +of the underlying filesystem. + +Inheritance +----------- + +Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain +restrictions from its parent. This is similar to the seccomp inheritance (cf. +:doc:`/userspace-api/seccomp_filter`) or any other LSM dealing with task's +:manpage:`credentials(7)`. For instance, one process's thread may apply +Landlock rules to itself, but they will not be automatically applied to other +sibling threads (unlike POSIX thread credential changes, cf. +:manpage:`nptl(7)`). + +When a thread sandboxes itself, we have the guarantee that the related security +policy will stay enforced on all this thread's descendants. This allows +creating standalone and modular security policies per application, which will +automatically be composed between themselves according to their runtime parent +policies. + +Ptrace restrictions +------------------- + +A sandboxed process has less privileges than a non-sandboxed process and must +then be subject to additional restrictions when manipulating another process. +To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target +process, a sandboxed process should have a subset of the target process rules, +which means the tracee must be in a sub-domain of the tracer. + +Kernel interface +================ + +Access rights +------------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: fs_access + +Creating a new ruleset +---------------------- + +.. kernel-doc:: security/landlock/syscalls.c + :identifiers: sys_landlock_create_ruleset + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_ruleset_attr + +Extending a ruleset +------------------- + +.. kernel-doc:: security/landlock/syscalls.c + :identifiers: sys_landlock_add_rule + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_rule_type landlock_path_beneath_attr + +Enforcing a ruleset +------------------- + +.. kernel-doc:: security/landlock/syscalls.c + :identifiers: sys_landlock_restrict_self + +Current limitations +=================== + +File renaming and linking +------------------------- + +Because Landlock targets unprivileged access controls, it is needed to properly +handle composition of rules. Such property also implies rules nesting. +Properly handling multiple layers of ruleset, each one of them able to restrict +access to files, also implies to inherit the ruleset restrictions from a parent +to its hierarchy. Because files are identified and restricted by their +hierarchy, moving or linking a file from one directory to another implies to +propagate the hierarchy constraints. To protect against privilege escalations +through renaming or linking, and for the sake of simplicity, Landlock currently +limits linking and renaming to the same directory. Future Landlock evolutions +will enable more flexibility for renaming and linking, with dedicated ruleset +flags. + +Filesystem topology modification +-------------------------------- + +As for file renaming and linking, a sandboxed thread cannot modify its +filesystem topology, whether via :manpage:`mount(2)` or +:manpage:`pivot_root(2)`. However, :manpage:`chroot(2)` calls are not denied. + +Special filesystems +------------------- + +Access to regular files and directories can be restricted by Landlock, +according to the handled accesses of a ruleset. However, files that do not +come from a user-visible filesystem (e.g. pipe, socket), but can still be +accessed through ``/proc/<pid>/fd/*``, cannot currently be explicitly +restricted. Likewise, some special kernel filesystems such as nsfs, which can +be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly +restricted. However, thanks to the `ptrace restrictions`_, access to such +sensitive ``/proc`` files are automatically restricted according to domain +hierarchies. Future Landlock evolutions could still enable to explicitly +restrict such paths with dedicated ruleset flags. + +Ruleset layers +-------------- + +There is a limit of 64 layers of stacked rulesets. This can be an issue for a +task willing to enforce a new ruleset in complement to its 64 inherited +rulesets. Once this limit is reached, sys_landlock_restrict_self() returns +E2BIG. It is then strongly suggested to carefully build rulesets once in the +life of a thread, especially for applications able to launch other applications +that may also want to sandbox themselves (e.g. shells, container managers, +etc.). + +Memory usage +------------ + +Kernel memory allocated to create rulesets is accounted and can be restricted +by the :doc:`/admin-guide/cgroup-v1/memory`. + +Questions and answers +===================== + +What about user space sandbox managers? +--------------------------------------- + +Using user space process to enforce restrictions on kernel resources can lead +to race conditions or inconsistent evaluations (i.e. `Incorrect mirroring of +the OS code and state +<https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_). + +What about namespaces and containers? +------------------------------------- + +Namespaces can help create sandboxes but they are not designed for +access-control and then miss useful features for such use case (e.g. no +fine-grained restrictions). Moreover, their complexity can lead to security +issues, especially when untrusted processes can manipulate them (cf. +`Controlling access to user namespaces <https://lwn.net/Articles/673597/>`_). + +Additional documentation +======================== + +* :doc:`/security/landlock` +* https://landlock.io + +.. Links +.. _samples/landlock/sandboxer.c: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/landlock/sandboxer.c |