.. SPDX-License-Identifier: GPL-2.0 .. Copyright © 2017-2020 Mickaël Salaün .. Copyright © 2019-2020 ANSSI .. Copyright © 2021-2022 Microsoft Corporation ===================================== Landlock: unprivileged access control ===================================== :Author: Mickaël Salaün :Date: October 2023 The goal of Landlock is to enable to restrict ambient rights (e.g. global filesystem or network 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. We can quickly make sure that Landlock is enabled in the running system by looking for "landlock: Up and running" in kernel logs (as root): ``dmesg | grep landlock || journalctl -kb -g landlock`` . Developers can also easily check for Landlock support with a :ref:`related system call `. If Landlock is not currently supported, we need to :ref:`configure the kernel appropriately `. Landlock rules ============== A Landlock rule describes an action on an object which the process intends to perform. A set of rules is aggregated in a ruleset, which can then restrict the thread enforcing it, and its future children. The two existing types of rules are: Filesystem rules For these rules, the object is a file hierarchy, and the related filesystem actions are defined with `filesystem access rights`. Network rules (since ABI v4) For these rules, the object is a TCP port, and the related actions are defined with `network access rights`. Defining and enforcing a security policy ---------------------------------------- We first need to define the ruleset that will contain our rules. For this example, the ruleset will contain rules that only allow filesystem read actions and establish a specific TCP connection. Filesystem write actions and other TCP actions will be denied. The ruleset then needs to handle both these kinds of actions. This is required for backward and forward compatibility (i.e. the kernel and user space may not know each other's supported restrictions), hence the need to be explicit about the denied-by-default access rights. .. code-block:: c 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 | LANDLOCK_ACCESS_FS_REFER | LANDLOCK_ACCESS_FS_TRUNCATE, .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP | LANDLOCK_ACCESS_NET_CONNECT_TCP, }; Because we may not know on which kernel version an application will be executed, it is safer to follow a best-effort security approach. Indeed, we should try to protect users as much as possible whatever the kernel they are using. To avoid binary enforcement (i.e. either all security features or none), we can leverage a dedicated Landlock command to get the current version of the Landlock ABI and adapt the handled accesses. Let's check if we should remove access rights which are only supported in higher versions of the ABI. .. code-block:: c int abi; abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); if (abi < 0) { /* Degrades gracefully if Landlock is not handled. */ perror("The running kernel does not enable to use Landlock"); return 0; } switch (abi) { case 1: /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; __attribute__((fallthrough)); case 2: /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; __attribute__((fallthrough)); case 3: /* Removes network support for ABI < 4 */ ruleset_attr.handled_access_net &= ~(LANDLOCK_ACCESS_NET_BIND_TCP | LANDLOCK_ACCESS_NET_CONNECT_TCP); } This enables to create an inclusive ruleset that will contain our rules. .. code-block:: c int ruleset_fd; 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; } It may also be required to create rules following the same logic as explained for the ruleset creation, by filtering access rights according to the Landlock ABI version. In this example, this is not required because all of the requested ``allowed_access`` rights are already available in ABI 1. For network access-control, we can add a set of rules that allow to use a port number for a specific action: HTTPS connections. .. code-block:: c struct landlock_net_port_attr net_port = { .allowed_access = LANDLOCK_ACCESS_NET_CONNECT_TCP, .port = 443, }; err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT, &net_port, 0); The next step is to restrict the current thread from gaining more privileges (e.g. through a SUID binary). We now have a ruleset with the first rule allowing read access to ``/usr`` while denying all other handled accesses for the filesystem, and a second rule allowing HTTPS connections. .. 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`_. Good practices -------------- It is recommended setting access rights to file hierarchy leaves as much as possible. For instance, it is better to be able to have ``~/doc/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to ``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy. Following this good practice leads to self-sufficient hierarchies that do not depend on their location (i.e. parent directories). This is particularly relevant when we want to allow linking or renaming. Indeed, having consistent access rights per directory enables to change the location of such directory without relying on the destination directory access rights (except those that are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER`` documentation). Having self-sufficient hierarchies also helps to tighten the required access rights to the minimal set of data. This also helps avoid sinkhole directories, i.e. directories where data can be linked to but not linked from. However, this depends on data organization, which might not be controlled by developers. In this case, granting read-write access to ``~/tmp/``, instead of write-only access, would potentially allow to move ``~/tmp/`` to a non-readable directory and still keep the ability to list the content of ``~/tmp/``. 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. Truncating files ---------------- The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and ``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes overlap in non-intuitive ways. It is recommended to always specify both of these together. A particularly surprising example is :manpage:`creat(2)`. The name suggests that this system call requires the rights to create and write files. However, it also requires the truncate right if an existing file under the same name is already present. It should also be noted that truncating files does not require the ``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)` system call, this can also be done through :manpage:`open(2)` with the flags ``O_RDONLY | O_TRUNC``. When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right is associated with the newly created file descriptor and will be used for subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavior is similar to opening a file for reading or writing, where permissions are checked during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and :manpage:`write(2)` calls. As a consequence, it is possible to have multiple open file descriptors for the same file, where one grants the right to truncate the file and the other does not. It is also possible to pass such file descriptors between processes, keeping their Landlock properties, even when these processes do not have an enforced Landlock ruleset. Compatibility ============= Backward and forward compatibility ---------------------------------- Landlock is designed to be compatible with past and future versions of the kernel. This is achieved thanks to the system call attributes and the associated bitflags, particularly the ruleset's ``handled_access_fs``. Making handled access right explicit enables the kernel and user space to have a clear contract with each other. This is required to make sure sandboxing will not get stricter with a system update, which could break applications. Developers can subscribe to the `Landlock mailing list `_ to knowingly update and test their applications with the latest available features. In the interest of users, and because they may use different kernel versions, it is strongly encouraged to follow a best-effort security approach by checking the Landlock ABI version at runtime and only enforcing the supported features. .. _landlock_abi_versions: Landlock ABI versions --------------------- The Landlock ABI version can be read with the sys_landlock_create_ruleset() system call: .. code-block:: c int abi; abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); if (abi < 0) { switch (errno) { case ENOSYS: printf("Landlock is not supported by the current kernel.\n"); break; case EOPNOTSUPP: printf("Landlock is currently disabled.\n"); break; } return 0; } if (abi >= 2) { printf("Landlock supports LANDLOCK_ACCESS_FS_REFER.\n"); } The following kernel interfaces are implicitly supported by the first ABI version. Features only supported from a specific version are explicitly marked as such. Kernel interface ================ Access rights ------------- .. kernel-doc:: include/uapi/linux/landlock.h :identifiers: fs_access net_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 landlock_net_port_attr Enforcing a ruleset ------------------- .. kernel-doc:: security/landlock/syscalls.c :identifiers: sys_landlock_restrict_self Current limitations =================== Filesystem topology modification -------------------------------- Threads sandboxed with filesystem restrictions cannot modify 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//fd/*``, cannot currently be explicitly restricted. Likewise, some special kernel filesystems such as nsfs, which can be accessed through ``/proc//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. Previous limitations ==================== File renaming and linking (ABI < 2) ----------------------------------- Because Landlock targets unprivileged access controls, it needs to properly handle composition of rules. Such property also implies rules nesting. Properly handling multiple layers of rulesets, each one of them able to restrict access to files, also implies inheritance of 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 propagation of the hierarchy constraints, or restriction of these actions according to the potentially lost constraints. To protect against privilege escalations through renaming or linking, and for the sake of simplicity, Landlock previously limited linking and renaming to the same directory. Starting with the Landlock ABI version 2, it is now possible to securely control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER`` access right. File truncation (ABI < 3) ------------------------- File truncation could not be denied before the third Landlock ABI, so it is always allowed when using a kernel that only supports the first or second ABI. Starting with the Landlock ABI version 3, it is now possible to securely control truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right. Network support (ABI < 4) ------------------------- 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`` access rights. .. _kernel_support: Kernel support ============== Build time configuration ------------------------ 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/.*\(\`_). 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 `_). 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