This documentation can be built with the Sphinx framework. Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@xxxxxxxxxxx> Cc: James Morris <jmorris@xxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Serge E. Hallyn <serge@xxxxxxxxxx> --- Changes since v19: * Update examples and documentation with the new syscalls. 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@xxxxxxxxxxx/ --- Documentation/security/index.rst | 1 + Documentation/security/landlock/index.rst | 18 ++ Documentation/security/landlock/kernel.rst | 69 ++++++ Documentation/security/landlock/user.rst | 271 +++++++++++++++++++++ include/uapi/linux/landlock.h | 49 +--- security/landlock/syscall.c | 27 +- 6 files changed, 388 insertions(+), 47 deletions(-) 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 8129405eb2cc..e3f2bf4fef77 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -16,3 +16,4 @@ Security Documentation siphash tpm/index digsig + 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..f382d830cbc0 --- /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. enforcing 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-v20/tools/testing/selftests/landlock/ diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst new file mode 100644 index 000000000000..605ff5991fe7 --- /dev/null +++ b/Documentation/security/landlock/user.rst @@ -0,0 +1,271 @@ +================================= +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 sys_landlock_get_features(). +syscall. + +.. code-block:: c + + struct landlock_attr_features attr_features; + + if (landlock_get_features(&attr_features, sizeof(attr_features), 0)) { + 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_create_ruleset(&ruleset, sizeof(ruleset), 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 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 = { + .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_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH, + &path_beneath, sizeof(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 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 + + if (landlock_enforce_ruleset(ruleset_fd, 0)) { + perror("Failed to enforce ruleset"); + close(ruleset_fd); + return 1; + } + close(ruleset_fd); + +If the `landlock_enforce_ruleset` 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. + +Kernel interface +================ + +Access rights +------------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: fs_access + + +Fetching the supported features +------------------------------- + +.. kernel-doc:: security/landlock/syscall.c + :identifiers: sys_landlock_get_features + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_attr_features + +Creating a new ruleset +---------------------- + +.. kernel-doc:: security/landlock/syscall.c + :identifiers: sys_landlock_create_ruleset + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_attr_ruleset + +Extending a ruleset +------------------- + +.. kernel-doc:: security/landlock/syscall.c + :identifiers: sys_landlock_add_rule + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_rule_type landlock_attr_path_beneath + +Enforcing a ruleset +------------------- + +.. kernel-doc:: security/landlock/syscall.c + :identifiers: sys_landlock_enforce_ruleset + +.. kernel-doc:: include/uapi/linux/landlock.h + :identifiers: landlock_target_type + +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-v20/samples/landlock/sandboxer.c diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h index eb2a5e0d61a4..7d3bbde738b6 100644 --- a/include/uapi/linux/landlock.h +++ b/include/uapi/linux/landlock.h @@ -11,18 +11,6 @@ #include <linux/types.h> -#if 0 -/** - * DOC: options_intro - * - * These options may be used as second argument of sys_landlock(). Each - * command have a dedicated set of options, represented as bitmasks. For two - * different commands, their options may overlap. Each command have at least - * one option defining the used attribute type. This also enables to always - * have a usable &struct landlock_attr_features (i.e. filled with bits). - */ -#endif - /** * enum landlock_rule_type - Landlock rule type * @@ -52,31 +40,7 @@ enum landlock_target_type { /** * struct landlock_attr_features - Receives the supported features * - * This struct should be allocated by user space but it will be filled by the - * kernel to indicate the subset of Landlock features effectively handled by - * the running kernel. This enables backward compatibility for applications - * which are developed on a newer kernel than the one running the application. - * This helps avoid hard errors that may entirely disable the use of Landlock - * features because some of them may not be supported. Indeed, because - * Landlock is a security feature, even if the kernel doesn't support all the - * requested features, user space applications should still use the subset - * which is supported by the running kernel. Indeed, a partial security policy - * can still improve the security of the application and better protect the - * user (i.e. best-effort approach). The %LANDLOCK_CMD_GET_FEATURES command - * and &struct landlock_attr_features are future-proof because the future - * unknown fields requested by user space (i.e. a larger &struct - * landlock_attr_features) can still be filled with zeros. - * - * The Landlock commands will fail if an unsupported option or access is - * requested. By firstly requesting the supported options and accesses, it is - * quite easy for the developer to binary AND these returned bitmasks with the - * used options and accesses from the attribute structs (e.g. &struct - * landlock_attr_ruleset), and even infer the supported Landlock commands. - * Indeed, because each command must support at least one option, the options_* - * fields are always filled if the related commands are supported. The - * supported attributes are also discoverable thanks to the size_* fields. All - * this data enable to create applications doing their best to sandbox - * themselves regardless of the running kernel. + * Argument of sys_landlock_get_features(). */ struct landlock_attr_features { /** @@ -138,8 +102,7 @@ struct landlock_attr_features { /** * struct landlock_attr_ruleset- Defines a new ruleset * - * Used as first attribute for the %LANDLOCK_CMD_CREATE_RULESET command and - * with the %LANDLOCK_OPT_CREATE_RULESET option. + * Argument of sys_landlock_create_ruleset(). */ struct landlock_attr_ruleset { /** @@ -147,15 +110,17 @@ struct landlock_attr_ruleset { * that is handled by this ruleset and should then be forbidden if no * rule explicitly allow them. This is needed for backward * compatibility reasons. The user space code should check the - * effectively supported actions thanks to %LANDLOCK_CMD_GET_FEATURES - * and &struct landlock_attr_features, and then adjust the arguments of - * the next calls to sys_landlock() accordingly. + * effectively supported actions thanks to sys_landlock_get_features() + * and then adjust the arguments of the next calls to + * sys_landlock_create_ruleset() accordingly. */ __u64 handled_access_fs; }; /** * struct landlock_attr_path_beneath - Defines a path hierarchy + * + * Argument of sys_landlock_add_rule(). */ struct landlock_attr_path_beneath { /** diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c index 7bf4dc175dee..bba66db211a1 100644 --- a/security/landlock/syscall.c +++ b/security/landlock/syscall.c @@ -132,15 +132,32 @@ static void build_check_abi(void) /** * sys_landlock_get_features - Identify the supported Landlock features * - * @features_ptr: Pointer to a &struct landlock_attr_features to be filled by - * the supported features. + * @features_ptr: Pointer to a &struct landlock_attr_features (allocated by + * user space) to be filled by the supported features. * @features_size: Size of the pointed &struct landlock_attr_features (needed * for backward and forward compatibility). * @options: Must be 0. * - * This system call enables to ask the kernel for supported Landlock features. - * This is important to build user space code compatible with older and newer - * kernels. + * This system call enables to ask for the Landlock features effectively + * handled by the running kernel. This enables backward compatibility for + * applications which are developed on a newer kernel than the one running the + * application. This helps avoid hard errors that may entirely disable the use + * of Landlock features because some of them may not be supported. Indeed, + * because Landlock is a security feature, even if the kernel doesn't support + * all the requested features, user space applications should still use the + * subset which is supported by the running kernel. Indeed, a partial security + * policy can still improve the security of the application and better protect + * the user (i.e. best-effort approach). Handling of &struct + * landlock_attr_features with sys_landlock_get_features() is future-proof + * because the future unknown fields requested by user space (i.e. a larger + * &struct landlock_attr_features) can still be filled with zeros. + * + * The other Landlock syscalls will fail if an unsupported option or access is + * requested. By firstly requesting the supported options and accesses, it is + * quite easy for the developer to binary AND these returned bitmasks with the + * used options and accesses from the attribute structs (e.g. &struct + * landlock_attr_ruleset). This enables to create applications doing their + * best to sandbox themselves regardless of the running kernel. * * Possible returned errors are: * -- 2.28.0.rc2
