Re: [PATCH v27 12/12] landlock: Add user and kernel documentation

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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/




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