On 22/01/2021 09:33, Michael Kerrisk (man-pages) wrote: > Hello Mickaël, Hi Michael, > > It would be great to have some manual pages for these system calls > before release... Can you prepare something? Yes, I will start some pages based on the current documentation, but it will be some time before seeing Landlock in a (stable) release. Could you please take a look at the UAPI (patch 8/12)? > > Thanks, > > Michael > > On Thu, 21 Jan 2021 at 21:51, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: >> >> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >> >> This documentation can be built with the Sphinx framework. >> >> Cc: James Morris <jmorris@xxxxxxxxx> >> Cc: Jann Horn <jannh@xxxxxxxxxx> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx> >> Cc: Serge E. Hallyn <serge@xxxxxxxxxx> >> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@xxxxxxxxxxx> >> --- >> >> Changes since v25: >> * Explain the behavior of layered access rights. >> * Explain how bind mounts and overayfs mounts are handled by Landlock: >> merged overlayfs mount points have their own inodes, which makes these >> hierarchies independent from its upper and lower layers, unlike bind >> mounts which share the same inodes between the source hierarchy and >> the mount point hierarchy. >> New overlayfs mount and bind mount tests check these behaviors. >> * Synchronize with the new syscalls.c file and update syscall names. >> * Fix spelling. >> * Remove Reviewed-by Jann Horn because of the above changes. >> >> Changes since v24: >> * Add Reviewed-by Jann Horn. >> * Add a paragraph to explain how the ruleset layers work. >> * Bump date. >> >> Changes since v23: >> * Explain limitations for the maximum number of stacked ruleset, and the >> memory usage restrictions. >> >> Changes since v22: >> * Fix spelling and remove obsolete sentence (spotted by Jann Horn). >> * Bump date. >> >> Changes since v21: >> * Move the user space documentation to userspace-api/landlock.rst and >> the kernel documentation to security/landlock.rst . >> * Add license headers. >> * Add last update dates. >> * Update MAINTAINERS file. >> * Add (back) links to git.kernel.org . >> * Fix spelling. >> >> Changes since v20: >> * Update examples and documentation with the new syscalls. >> >> 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.rst | 79 ++++++ >> Documentation/userspace-api/index.rst | 1 + >> Documentation/userspace-api/landlock.rst | 306 +++++++++++++++++++++++ >> MAINTAINERS | 2 + >> 5 files changed, 389 insertions(+) >> create mode 100644 Documentation/security/landlock.rst >> create mode 100644 Documentation/userspace-api/landlock.rst >> >> diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst >> index 8129405eb2cc..16335de04e8c 100644 >> --- a/Documentation/security/index.rst >> +++ b/Documentation/security/index.rst >> @@ -16,3 +16,4 @@ Security Documentation >> siphash >> tpm/index >> digsig >> + landlock >> diff --git a/Documentation/security/landlock.rst b/Documentation/security/landlock.rst >> new file mode 100644 >> index 000000000000..244e616d3d7a >> --- /dev/null >> +++ b/Documentation/security/landlock.rst >> @@ -0,0 +1,79 @@ >> +.. SPDX-License-Identifier: GPL-2.0 >> +.. Copyright © 2017-2020 Mickaël Salaün <mic@xxxxxxxxxxx> >> +.. Copyright © 2019-2020 ANSSI >> + >> +================================== >> +Landlock LSM: kernel documentation >> +================================== >> + >> +:Author: Mickaël Salaün >> +:Date: January 2021 >> + >> +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. >> + >> +User space documentation can be found here: :doc:`/userspace-api/landlock`. >> + >> +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 >> +that 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://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/landlock/ >> diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst >> index acd2cc2a538d..01f1748ab569 100644 >> --- a/Documentation/userspace-api/index.rst >> +++ b/Documentation/userspace-api/index.rst >> @@ -18,6 +18,7 @@ place where this information is gathered. >> >> no_new_privs >> seccomp_filter >> + landlock >> unshare >> spec_ctrl >> accelerators/ocxl >> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst >> new file mode 100644 >> index 000000000000..06c16f2b038b >> --- /dev/null >> +++ b/Documentation/userspace-api/landlock.rst >> @@ -0,0 +1,306 @@ >> +.. SPDX-License-Identifier: GPL-2.0 >> +.. Copyright © 2017-2020 Mickaël Salaün <mic@xxxxxxxxxxx> >> +.. Copyright © 2019-2020 ANSSI >> + >> +===================================== >> +Landlock: unprivileged access control >> +===================================== >> + >> +:Author: Mickaël Salaün >> +:Date: January 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 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 >> +---------------------------------------- >> + >> +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_enforce_ruleset_self(ruleset_fd, 0)) { >> + perror("Failed to enforce ruleset"); >> + close(ruleset_fd); >> + return 1; >> + } >> + close(ruleset_fd); >> + >> +If the `landlock_enforce_ruleset_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. >> + >> +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_enforce_ruleset_self >> + >> +Current limitations >> +=================== >> + >> +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_enforce_ruleset_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`. >> + >> +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 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 >> +flags. >> + >> +Filesystem layout modification >> +------------------------------ >> + >> +As for file renaming and linking, a sandboxed thread cannot modify its >> +filesystem layout, 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/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 flags. >> + >> +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 >> diff --git a/MAINTAINERS b/MAINTAINERS >> index 572e4288c60f..68276c73c33e 100644 >> --- a/MAINTAINERS >> +++ b/MAINTAINERS >> @@ -9942,6 +9942,8 @@ L: linux-security-module@xxxxxxxxxxxxxxx >> S: Supported >> W: https://landlock.io >> T: git https://github.com/landlock-lsm/linux.git >> +F: Documentation/security/landlock.rst >> +F: Documentation/userspace-api/landlock.rst >> F: include/uapi/linux/landlock.h >> F: security/landlock/ >> K: landlock >> -- >> 2.30.0 >> > >
