Hello Mickaël, It would be great to have some manual pages for these system calls before release... Can you prepare something? 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 > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
