595 lines
23 KiB
ReStructuredText
595 lines
23 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
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.. Copyright © 2019-2020 ANSSI
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.. Copyright © 2021-2022 Microsoft Corporation
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=====================================
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Landlock: unprivileged access control
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=====================================
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:Author: Mickaël Salaün
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:Date: October 2023
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The goal of Landlock is to enable to restrict ambient rights (e.g. global
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filesystem or network access) for a set of processes. Because Landlock
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is a stackable LSM, it makes possible to create safe security sandboxes as new
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security layers in addition to the existing system-wide access-controls. This
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kind of sandbox is expected to help mitigate the security impact of bugs or
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unexpected/malicious behaviors in user space applications. Landlock empowers
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any process, including unprivileged ones, to securely restrict themselves.
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We can quickly make sure that Landlock is enabled in the running system by
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looking for "landlock: Up and running" in kernel logs (as root):
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``dmesg | grep landlock || journalctl -kb -g landlock`` .
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Developers can also easily check for Landlock support with a
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:ref:`related system call <landlock_abi_versions>`.
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If Landlock is not currently supported, we need to
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:ref:`configure the kernel appropriately <kernel_support>`.
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Landlock rules
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==============
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A Landlock rule describes an action on an object which the process intends to
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perform. A set of rules is aggregated in a ruleset, which can then restrict
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the thread enforcing it, and its future children.
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The two existing types of rules are:
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Filesystem rules
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For these rules, the object is a file hierarchy,
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and the related filesystem actions are defined with
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`filesystem access rights`.
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Network rules (since ABI v4)
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For these rules, the object is a TCP port,
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and the related actions are defined with `network access rights`.
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Defining and enforcing a security policy
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----------------------------------------
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We first need to define the ruleset that will contain our rules.
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For this example, the ruleset will contain rules that only allow filesystem
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read actions and establish a specific TCP connection. Filesystem write
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actions and other TCP actions will be denied.
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The ruleset then needs to handle both these kinds of actions. This is
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required for backward and forward compatibility (i.e. the kernel and user
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space may not know each other's supported restrictions), hence the need
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to be explicit about the denied-by-default access rights.
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.. code-block:: c
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struct landlock_ruleset_attr ruleset_attr = {
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.handled_access_fs =
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LANDLOCK_ACCESS_FS_EXECUTE |
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LANDLOCK_ACCESS_FS_WRITE_FILE |
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LANDLOCK_ACCESS_FS_READ_FILE |
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LANDLOCK_ACCESS_FS_READ_DIR |
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LANDLOCK_ACCESS_FS_REMOVE_DIR |
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LANDLOCK_ACCESS_FS_REMOVE_FILE |
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LANDLOCK_ACCESS_FS_MAKE_CHAR |
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LANDLOCK_ACCESS_FS_MAKE_DIR |
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LANDLOCK_ACCESS_FS_MAKE_REG |
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LANDLOCK_ACCESS_FS_MAKE_SOCK |
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LANDLOCK_ACCESS_FS_MAKE_FIFO |
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LANDLOCK_ACCESS_FS_MAKE_BLOCK |
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LANDLOCK_ACCESS_FS_MAKE_SYM |
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LANDLOCK_ACCESS_FS_REFER |
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LANDLOCK_ACCESS_FS_TRUNCATE,
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.handled_access_net =
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LANDLOCK_ACCESS_NET_BIND_TCP |
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LANDLOCK_ACCESS_NET_CONNECT_TCP,
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};
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Because we may not know on which kernel version an application will be
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executed, it is safer to follow a best-effort security approach. Indeed, we
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should try to protect users as much as possible whatever the kernel they are
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using. To avoid binary enforcement (i.e. either all security features or
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none), we can leverage a dedicated Landlock command to get the current version
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of the Landlock ABI and adapt the handled accesses. Let's check if we should
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remove access rights which are only supported in higher versions of the ABI.
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.. code-block:: c
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int abi;
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abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
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if (abi < 0) {
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/* Degrades gracefully if Landlock is not handled. */
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perror("The running kernel does not enable to use Landlock");
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return 0;
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}
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switch (abi) {
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case 1:
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/* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
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ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
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__attribute__((fallthrough));
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case 2:
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/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
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ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
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__attribute__((fallthrough));
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case 3:
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/* Removes network support for ABI < 4 */
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ruleset_attr.handled_access_net &=
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~(LANDLOCK_ACCESS_NET_BIND_TCP |
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LANDLOCK_ACCESS_NET_CONNECT_TCP);
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}
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This enables to create an inclusive ruleset that will contain our rules.
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.. code-block:: c
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int ruleset_fd;
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ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
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if (ruleset_fd < 0) {
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perror("Failed to create a ruleset");
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return 1;
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}
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We can now add a new rule to this ruleset thanks to the returned file
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descriptor referring to this ruleset. The rule will only allow reading the
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file hierarchy ``/usr``. Without another rule, write actions would then be
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denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the
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``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file
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descriptor.
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.. code-block:: c
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int err;
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struct landlock_path_beneath_attr path_beneath = {
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.allowed_access =
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LANDLOCK_ACCESS_FS_EXECUTE |
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LANDLOCK_ACCESS_FS_READ_FILE |
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LANDLOCK_ACCESS_FS_READ_DIR,
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};
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path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC);
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if (path_beneath.parent_fd < 0) {
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perror("Failed to open file");
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close(ruleset_fd);
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return 1;
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}
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err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
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&path_beneath, 0);
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close(path_beneath.parent_fd);
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if (err) {
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perror("Failed to update ruleset");
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close(ruleset_fd);
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return 1;
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}
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It may also be required to create rules following the same logic as explained
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for the ruleset creation, by filtering access rights according to the Landlock
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ABI version. In this example, this is not required because all of the requested
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``allowed_access`` rights are already available in ABI 1.
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For network access-control, we can add a set of rules that allow to use a port
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number for a specific action: HTTPS connections.
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.. code-block:: c
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struct landlock_net_port_attr net_port = {
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.allowed_access = LANDLOCK_ACCESS_NET_CONNECT_TCP,
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.port = 443,
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};
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err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
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&net_port, 0);
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The next step is to restrict the current thread from gaining more privileges
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(e.g. through a SUID binary). We now have a ruleset with the first rule
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allowing read access to ``/usr`` while denying all other handled accesses for
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the filesystem, and a second rule allowing HTTPS connections.
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.. code-block:: c
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if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
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perror("Failed to restrict privileges");
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close(ruleset_fd);
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return 1;
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}
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The current thread is now ready to sandbox itself with the ruleset.
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.. code-block:: c
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if (landlock_restrict_self(ruleset_fd, 0)) {
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perror("Failed to enforce ruleset");
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close(ruleset_fd);
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return 1;
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}
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close(ruleset_fd);
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If the ``landlock_restrict_self`` system call succeeds, the current thread is
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now restricted and this policy will be enforced on all its subsequently created
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children as well. Once a thread is landlocked, there is no way to remove its
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security policy; only adding more restrictions is allowed. These threads are
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now in a new Landlock domain, merge of their parent one (if any) with the new
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ruleset.
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Full working code can be found in `samples/landlock/sandboxer.c`_.
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Good practices
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--------------
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It is recommended setting access rights to file hierarchy leaves as much as
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possible. For instance, it is better to be able to have ``~/doc/`` as a
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read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to
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``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy.
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Following this good practice leads to self-sufficient hierarchies that do not
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depend on their location (i.e. parent directories). This is particularly
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relevant when we want to allow linking or renaming. Indeed, having consistent
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access rights per directory enables to change the location of such directory
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without relying on the destination directory access rights (except those that
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are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER``
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documentation).
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Having self-sufficient hierarchies also helps to tighten the required access
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rights to the minimal set of data. This also helps avoid sinkhole directories,
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i.e. directories where data can be linked to but not linked from. However,
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this depends on data organization, which might not be controlled by developers.
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In this case, granting read-write access to ``~/tmp/``, instead of write-only
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access, would potentially allow to move ``~/tmp/`` to a non-readable directory
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and still keep the ability to list the content of ``~/tmp/``.
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Layers of file path access rights
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---------------------------------
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Each time a thread enforces a ruleset on itself, it updates its Landlock domain
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with a new layer of policy. Indeed, this complementary policy is stacked with
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the potentially other rulesets already restricting this thread. A sandboxed
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thread can then safely add more constraints to itself with a new enforced
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ruleset.
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One policy layer grants access to a file path if at least one of its rules
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encountered on the path grants the access. A sandboxed thread can only access
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a file path if all its enforced policy layers grant the access as well as all
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the other system access controls (e.g. filesystem DAC, other LSM policies,
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etc.).
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Bind mounts and OverlayFS
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-------------------------
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Landlock enables to restrict access to file hierarchies, which means that these
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access rights can be propagated with bind mounts (cf.
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Documentation/filesystems/sharedsubtree.rst) but not with
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Documentation/filesystems/overlayfs.rst.
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A bind mount mirrors a source file hierarchy to a destination. The destination
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hierarchy is then composed of the exact same files, on which Landlock rules can
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be tied, either via the source or the destination path. These rules restrict
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access when they are encountered on a path, which means that they can restrict
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access to multiple file hierarchies at the same time, whether these hierarchies
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are the result of bind mounts or not.
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An OverlayFS mount point consists of upper and lower layers. These layers are
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combined in a merge directory, result of the mount point. This merge hierarchy
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may include files from the upper and lower layers, but modifications performed
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on the merge hierarchy only reflects on the upper layer. From a Landlock
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policy point of view, each OverlayFS layers and merge hierarchies are
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standalone and contains their own set of files and directories, which is
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different from bind mounts. A policy restricting an OverlayFS layer will not
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restrict the resulted merged hierarchy, and vice versa. Landlock users should
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then only think about file hierarchies they want to allow access to, regardless
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of the underlying filesystem.
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Inheritance
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-----------
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Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain
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restrictions from its parent. This is similar to the seccomp inheritance (cf.
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Documentation/userspace-api/seccomp_filter.rst) or any other LSM dealing with
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task's :manpage:`credentials(7)`. For instance, one process's thread may apply
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Landlock rules to itself, but they will not be automatically applied to other
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sibling threads (unlike POSIX thread credential changes, cf.
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:manpage:`nptl(7)`).
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When a thread sandboxes itself, we have the guarantee that the related security
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policy will stay enforced on all this thread's descendants. This allows
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creating standalone and modular security policies per application, which will
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automatically be composed between themselves according to their runtime parent
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policies.
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Ptrace restrictions
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-------------------
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A sandboxed process has less privileges than a non-sandboxed process and must
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then be subject to additional restrictions when manipulating another process.
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To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
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process, a sandboxed process should have a subset of the target process rules,
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which means the tracee must be in a sub-domain of the tracer.
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Truncating files
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----------------
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The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and
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``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes
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overlap in non-intuitive ways. It is recommended to always specify both of
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these together.
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A particularly surprising example is :manpage:`creat(2)`. The name suggests
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that this system call requires the rights to create and write files. However,
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it also requires the truncate right if an existing file under the same name is
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already present.
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It should also be noted that truncating files does not require the
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``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)`
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system call, this can also be done through :manpage:`open(2)` with the flags
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``O_RDONLY | O_TRUNC``.
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When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE``
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right is associated with the newly created file descriptor and will be used for
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subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavior is
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similar to opening a file for reading or writing, where permissions are checked
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during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and
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:manpage:`write(2)` calls.
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As a consequence, it is possible to have multiple open file descriptors for the
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same file, where one grants the right to truncate the file and the other does
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not. It is also possible to pass such file descriptors between processes,
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keeping their Landlock properties, even when these processes do not have an
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enforced Landlock ruleset.
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Compatibility
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=============
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Backward and forward compatibility
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----------------------------------
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Landlock is designed to be compatible with past and future versions of the
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kernel. This is achieved thanks to the system call attributes and the
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associated bitflags, particularly the ruleset's ``handled_access_fs``. Making
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handled access right explicit enables the kernel and user space to have a clear
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contract with each other. This is required to make sure sandboxing will not
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get stricter with a system update, which could break applications.
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Developers can subscribe to the `Landlock mailing list
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<https://subspace.kernel.org/lists.linux.dev.html>`_ to knowingly update and
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test their applications with the latest available features. In the interest of
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users, and because they may use different kernel versions, it is strongly
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encouraged to follow a best-effort security approach by checking the Landlock
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ABI version at runtime and only enforcing the supported features.
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.. _landlock_abi_versions:
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Landlock ABI versions
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---------------------
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The Landlock ABI version can be read with the sys_landlock_create_ruleset()
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system call:
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.. code-block:: c
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int abi;
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abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
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if (abi < 0) {
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switch (errno) {
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case ENOSYS:
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printf("Landlock is not supported by the current kernel.\n");
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break;
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case EOPNOTSUPP:
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printf("Landlock is currently disabled.\n");
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break;
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}
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return 0;
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}
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if (abi >= 2) {
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printf("Landlock supports LANDLOCK_ACCESS_FS_REFER.\n");
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}
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The following kernel interfaces are implicitly supported by the first ABI
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version. Features only supported from a specific version are explicitly marked
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as such.
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Kernel interface
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================
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Access rights
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-------------
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: fs_access net_access
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Creating a new ruleset
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----------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_create_ruleset
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: landlock_ruleset_attr
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Extending a ruleset
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-------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_add_rule
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.. kernel-doc:: include/uapi/linux/landlock.h
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:identifiers: landlock_rule_type landlock_path_beneath_attr
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landlock_net_port_attr
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Enforcing a ruleset
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-------------------
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.. kernel-doc:: security/landlock/syscalls.c
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:identifiers: sys_landlock_restrict_self
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Current limitations
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===================
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Filesystem topology modification
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--------------------------------
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Threads sandboxed with filesystem restrictions cannot modify filesystem
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topology, whether via :manpage:`mount(2)` or :manpage:`pivot_root(2)`.
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However, :manpage:`chroot(2)` calls are not denied.
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Special filesystems
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-------------------
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Access to regular files and directories can be restricted by Landlock,
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according to the handled accesses of a ruleset. However, files that do not
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come from a user-visible filesystem (e.g. pipe, socket), but can still be
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accessed through ``/proc/<pid>/fd/*``, cannot currently be explicitly
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restricted. Likewise, some special kernel filesystems such as nsfs, which can
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be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly
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restricted. However, thanks to the `ptrace restrictions`_, access to such
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sensitive ``/proc`` files are automatically restricted according to domain
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hierarchies. Future Landlock evolutions could still enable to explicitly
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restrict such paths with dedicated ruleset flags.
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Ruleset layers
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--------------
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There is a limit of 16 layers of stacked rulesets. This can be an issue for a
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task willing to enforce a new ruleset in complement to its 16 inherited
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rulesets. Once this limit is reached, sys_landlock_restrict_self() returns
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E2BIG. It is then strongly suggested to carefully build rulesets once in the
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life of a thread, especially for applications able to launch other applications
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that may also want to sandbox themselves (e.g. shells, container managers,
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etc.).
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Memory usage
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------------
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Kernel memory allocated to create rulesets is accounted and can be restricted
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by the Documentation/admin-guide/cgroup-v1/memory.rst.
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Previous limitations
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====================
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File renaming and linking (ABI < 2)
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-----------------------------------
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Because Landlock targets unprivileged access controls, it needs to properly
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handle composition of rules. Such property also implies rules nesting.
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Properly handling multiple layers of rulesets, each one of them able to
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restrict access to files, also implies inheritance of the ruleset restrictions
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from a parent to its hierarchy. Because files are identified and restricted by
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their hierarchy, moving or linking a file from one directory to another implies
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propagation of the hierarchy constraints, or restriction of these actions
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according to the potentially lost constraints. To protect against privilege
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escalations through renaming or linking, and for the sake of simplicity,
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Landlock previously limited linking and renaming to the same directory.
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Starting with the Landlock ABI version 2, it is now possible to securely
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control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER``
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access right.
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File truncation (ABI < 3)
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-------------------------
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File truncation could not be denied before the third Landlock ABI, so it is
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always allowed when using a kernel that only supports the first or second ABI.
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Starting with the Landlock ABI version 3, it is now possible to securely control
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truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
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|
Network support (ABI < 4)
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|
-------------------------
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|
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Starting with the Landlock ABI version 4, it is now possible to restrict TCP
|
|
bind and connect actions to only a set of allowed ports thanks to the new
|
|
``LANDLOCK_ACCESS_NET_BIND_TCP`` and ``LANDLOCK_ACCESS_NET_CONNECT_TCP``
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access rights.
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|
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|
.. _kernel_support:
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|
|
Kernel support
|
|
==============
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|
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|
Build time configuration
|
|
------------------------
|
|
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|
Landlock was first introduced in Linux 5.13 but it must be configured at build
|
|
time with ``CONFIG_SECURITY_LANDLOCK=y``. Landlock must also be enabled at boot
|
|
time as the other security modules. The list of security modules enabled by
|
|
default is set with ``CONFIG_LSM``. The kernel configuration should then
|
|
contains ``CONFIG_LSM=landlock,[...]`` with ``[...]`` as the list of other
|
|
potentially useful security modules for the running system (see the
|
|
``CONFIG_LSM`` help).
|
|
|
|
Boot time configuration
|
|
-----------------------
|
|
|
|
If the running kernel does not have ``landlock`` in ``CONFIG_LSM``, then we can
|
|
enable Landlock by adding ``lsm=landlock,[...]`` to
|
|
Documentation/admin-guide/kernel-parameters.rst in the boot loader
|
|
configuration.
|
|
|
|
For example, if the current built-in configuration is:
|
|
|
|
.. code-block:: console
|
|
|
|
$ zgrep -h "^CONFIG_LSM=" "/boot/config-$(uname -r)" /proc/config.gz 2>/dev/null
|
|
CONFIG_LSM="lockdown,yama,integrity,apparmor"
|
|
|
|
...and if the cmdline doesn't contain ``landlock`` either:
|
|
|
|
.. code-block:: console
|
|
|
|
$ sed -n 's/.*\(\<lsm=\S\+\).*/\1/p' /proc/cmdline
|
|
lsm=lockdown,yama,integrity,apparmor
|
|
|
|
...we should configure the boot loader to set a cmdline extending the ``lsm``
|
|
list with the ``landlock,`` prefix::
|
|
|
|
lsm=landlock,lockdown,yama,integrity,apparmor
|
|
|
|
After a reboot, we can check that Landlock is up and running by looking at
|
|
kernel logs:
|
|
|
|
.. code-block:: console
|
|
|
|
# dmesg | grep landlock || journalctl -kb -g landlock
|
|
[ 0.000000] Command line: [...] lsm=landlock,lockdown,yama,integrity,apparmor
|
|
[ 0.000000] Kernel command line: [...] lsm=landlock,lockdown,yama,integrity,apparmor
|
|
[ 0.000000] LSM: initializing lsm=lockdown,capability,landlock,yama,integrity,apparmor
|
|
[ 0.000000] landlock: Up and running.
|
|
|
|
The kernel may be configured at build time to always load the ``lockdown`` and
|
|
``capability`` LSMs. In that case, these LSMs will appear at the beginning of
|
|
the ``LSM: initializing`` log line as well, even if they are not configured in
|
|
the boot loader.
|
|
|
|
Network support
|
|
---------------
|
|
|
|
To be able to explicitly allow TCP operations (e.g., adding a network rule with
|
|
``LANDLOCK_ACCESS_NET_BIND_TCP``), the kernel must support TCP
|
|
(``CONFIG_INET=y``). Otherwise, sys_landlock_add_rule() returns an
|
|
``EAFNOSUPPORT`` error, which can safely be ignored because this kind of TCP
|
|
operation is already not possible.
|
|
|
|
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
|