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Signed-off-by: Mickaël Salaün mic@digikod.net Reviewed-by: Vincent Dagonneau vincent.dagonneau@ssi.gouv.fr Cc: James Morris jmorris@namei.org Cc: Jann Horn jannh@google.com Cc: Kees Cook keescook@chromium.org Cc: Serge E. Hallyn serge@hallyn.com ---
Changes since v15: * Add current limitations.
Changes since v14: * Fix spelling (contributed by Randy Dunlap). * Extend documentation about inheritance and explain layer levels. * Remove the use of now-removed access rights. * Use GitHub links. * Improve kernel documentation. * Add section for tests. * Update example.
Changes since v13: * Rewrote the documentation according to the major revamp.
Previous changes: https://lore.kernel.org/lkml/20191104172146.30797-8-mic@digikod.net/ --- Documentation/security/index.rst | 1 + Documentation/security/landlock/index.rst | 18 ++ Documentation/security/landlock/kernel.rst | 69 ++++++ Documentation/security/landlock/user.rst | 268 +++++++++++++++++++++ 4 files changed, 356 insertions(+) create mode 100644 Documentation/security/landlock/index.rst create mode 100644 Documentation/security/landlock/kernel.rst create mode 100644 Documentation/security/landlock/user.rst
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst index fc503dd689a7..4d213e76ddf4 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -15,3 +15,4 @@ Security Documentation self-protection siphash tpm/index + landlock/index diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst new file mode 100644 index 000000000000..2520f8f33f5e --- /dev/null +++ b/Documentation/security/landlock/index.rst @@ -0,0 +1,18 @@ +========================================= +Landlock LSM: unprivileged access control +========================================= + +:Author: Mickaël Salaün + +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. + +.. toctree:: + + user + kernel diff --git a/Documentation/security/landlock/kernel.rst b/Documentation/security/landlock/kernel.rst new file mode 100644 index 000000000000..ca15073c7a06 --- /dev/null +++ b/Documentation/security/landlock/kernel.rst @@ -0,0 +1,69 @@ +============================== +Landlock: kernel documentation +============================== + +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. + +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. enforce a ruleset) shall + only impact the processes requesting them. + +Tests +===== + +Userspace tests for backward compatibility, ptrace restrictions and filesystem +support can be found here: `tools/testing/selftests/landlock/`_. + +Kernel structures +================= + +Object +------ + +.. kernel-doc:: security/landlock/object.h + :identifiers: + +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 +which 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:: security/landlock/ruleset.h + :identifiers: + +.. Links +.. _tools/testing/selftests/landlock/: https://github.com/landlock-lsm/linux/tree/landlock-v18/tools/testing/selfte... diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst new file mode 100644 index 000000000000..e5174738e2a7 --- /dev/null +++ b/Documentation/security/landlock/user.rst @@ -0,0 +1,268 @@ +================================= +Landlock: userspace documentation +================================= + +Landlock rules +============== + +A Landlock rule enables to describe an action on an object. An object is +currently a file hierarchy, and the related filesystem actions are defined in +`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 +---------------------------------------- + +Before defining a security policy, an application should first probe for the +features supported by the running kernel, which is important to be compatible +with older kernels. This can be done thanks to the `landlock` syscall (cf. +:ref:`syscall`). + +.. code-block:: c + + struct landlock_attr_features attr_features; + + if (landlock(LANDLOCK_CMD_GET_FEATURES, LANDLOCK_OPT_GET_FEATURES, + sizeof(attr_features), &attr_features)) { + perror("Failed to probe the Landlock supported features"); + return 1; + } + +Then, we need to create the ruleset that will contain our rules. For this +example, the ruleset will contain rules which only allow read actions, but +write actions will be denied. The ruleset then needs to handle both of these +kind of actions. To have a backward compatibility, these actions should be +ANDed with the supported ones. + +.. code-block:: c + + int ruleset_fd; + struct landlock_attr_ruleset ruleset = { + .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.handled_access_fs &= attr_features.access_fs; + ruleset_fd = landlock(LANDLOCK_CMD_CREATE_RULESET, + LANDLOCK_OPT_CREATE_RULESET, sizeof(ruleset), &ruleset); + 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 enable to read 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_attr_path_beneath with this file +descriptor. + +.. code-block:: c + + int err; + struct landlock_attr_path_beneath path_beneath = { + .ruleset_fd = ruleset_fd, + .allowed_access = + LANDLOCK_ACCESS_FS_EXECUTE | + LANDLOCK_ACCESS_FS_READ_FILE | + LANDLOCK_ACCESS_FS_READ_DIR, + }; + + path_beneath.allowed_access &= attr_features.access_fs; + 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(LANDLOCK_CMD_ADD_RULE, LANDLOCK_OPT_ADD_RULE_PATH_BENEATH, + sizeof(path_beneath), &path_beneath); + 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 accesses featured in ``attr_features.access_fs`` to everything else +on 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 + + struct landlock_attr_enforce attr_enforce = { + .ruleset_fd = ruleset_fd, + }; + + if (landlock(LANDLOCK_CMD_ENFORCE_RULESET, LANDLOCK_OPT_ENFORCE_RULESET, + sizeof(attr_enforce), &attr_enforce)) { + perror("Failed to enforce ruleset"); + close(ruleset_fd); + return 1; + } + close(ruleset_fd); + +If the last `landlock` 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`_. + +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 sandbox itself, we have the grantee that the related security +policy will stay enforced on all this thread's descendants. This enables to +create 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. + +.. _syscall: + +The `landlock` syscall and its arguments +======================================== + +.. kernel-doc:: security/landlock/syscall.c + :identifiers: sys_landlock + +Commands +-------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_cmd + +Options +------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: options_intro + options_get_features options_create_ruleset + options_add_rule options_enforce_ruleset + +Attributes +---------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_attr_features landlock_attr_ruleset + landlock_attr_path_beneath landlock_attr_enforce + +Access rights +------------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: fs_access + +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 imply 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 imply to +propagate the hierarchy constraints. To protect against privilege escalations +through renaming or linking, and for the sack 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 +options. + +OverlayFS +--------- + +An OverlayFS mount point consists of upper and lower layers. It is currently +not possible to reliably infer which underlying file hierarchy matches an +OverlayFS path composed of such layers. It is then not currently possible to +track the source of an indirect access-request, and then not possible to +properly identify and allow an unified OverlayFS hierarchy. Restricting files +in an OverlayFS mount point works, but files allowed in one layer may not be +allowed in a related OverlayFS mount point. A future Landlock evolution will +make possible to properly work with OverlayFS, according to a dedicated ruleset +option. + + +Special filesystems +------------------- + +Access to regular files and directories can be restricted by Landlock, +according to the handled accesses of a ruleset. However, files which do not +come from a user-visible filesystem (e.g. pipe, socket), but can still be +accessed through /proc/self/fd/, cannot currently be restricted. Likewise, +some special kernel filesystems such as nsfs which can be accessed through +/proc/self/ns/, cannot currently be restricted. For now, these kind of special +paths are then always allowed. Future Landlock evolutions will enable to +restrict such paths, with dedicated ruleset options. + +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 +======================== + +See https://landlock.io + +.. Links +.. _samples/landlock/sandboxer.c: https://github.com/landlock-lsm/linux/tree/landlock-v18/samples/landlock/san...