.. SPDX-License-Identifier: GPL-2.0
.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
.. Copyright © 2019-2020 ANSSI
.. Copyright © 2021 Microsoft Corporation

=====================================
Landlock: unprivileged access control
=====================================

:Author: Mickaël Salaün
:Date: March 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 with `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_restrict_self(ruleset_fd, 0)) {
        perror("Failed to enforce ruleset");
        close(ruleset_fd);
        return 1;
    }
    close(ruleset_fd);

If the `landlock_restrict_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.
Documentation/filesystems/sharedsubtree.rst) but not with
Documentation/filesystems/overlayfs.rst.

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.  Landlock users should
then only think about file hierarchies they want to allow access to, regardless
of the underlying filesystem.

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.
Documentation/userspace-api/seccomp_filter.rst) 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_restrict_self

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 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 sake 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 topology modification
--------------------------------

As for file renaming and linking, a sandboxed thread cannot modify its
filesystem topology, 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/<pid>/fd/*``, cannot currently be explicitly
restricted.  Likewise, some special kernel filesystems such as nsfs, which can
be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly
restricted.  However, thanks to the `ptrace restrictions`_, access to such
sensitive ``/proc`` files are automatically restricted according to domain
hierarchies.  Future Landlock evolutions could still enable to explicitly
restrict such paths with dedicated ruleset flags.

Ruleset layers
--------------

There is a limit of 16 layers of stacked rulesets.  This can be an issue for a
task willing to enforce a new ruleset in complement to its 16 inherited
rulesets.  Once this limit is reached, sys_landlock_restrict_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 Documentation/admin-guide/cgroup-v1/memory.rst.

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
========================

* Documentation/security/landlock.rst
* 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
