A Landlock security policy is a set of access rights (e.g., open a file in read-only, make a directory, etc.) tied to a file hierarchy. Such policy can be configured and enforced by processes for themselves using three system calls:
To be able to use these system calls, the running kernel must support Landlock and it must be enabled at boot time.
A file can only receive these access rights:
A directory can receive access rights related to files or directories. The following access right is applied to the directory itself, and the directories beneath it:
However, the following access rights only apply to the content of a directory, not the directory itself:
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.).
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 reflect on the upper layer. From a Landlock policy point of view, each of the OverlayFS layers and merge hierarchies is standalone and contains its own set of files and directories, which is different from a bind mount. 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.
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 run-time parent policies.
A particularly surprising example is 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 truncate(2) system call, this can also be done through 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 ftruncate(2). The behavior is similar to opening a file for reading or writing, where permissions are checked during open(2), but not during the subsequent read(2) and 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.
To determine which Landlock features are available, users should query the Landlock ABI version:
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Users should use the Landlock ABI version rather than the kernel version to determine which features are available. The mainline kernel versions listed here are only included for orientation. Kernels from other sources may contain backported features, and their version numbers may not match.
To query the running kernel's Landlock ABI version, programs may pass the LANDLOCK_CREATE_RULESET_VERSION flag to landlock_create_ruleset(2).
When building fallback mechanisms for compatibility with older kernels, users are advised to consider the special semantics of the LANDLOCK_ACCESS_FS_REFER access right: In ABI v1, linking and moving of files between different directories is always forbidden, so programs relying on such operations are only compatible with Landlock ABI v2 and higher.
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 kinds of actions. See the DESCRIPTION section for the description of filesystem actions.
struct landlock_ruleset_attr attr = {0};
int ruleset_fd;
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;
To be compatible with older Linux versions, we detect the available Landlock ABI version, and only use the available subset of access rights:
/*
* Table of available file system access rights by ABI version,
* numbers hardcoded to keep the example short.
*/
__u64 landlock_fs_access_rights[] = {
(LANDLOCK_ACCESS_FS_MAKE_SYM << 1) - 1, /* v1 */
(LANDLOCK_ACCESS_FS_REFER << 1) - 1, /* v2: add "refer" */
(LANDLOCK_ACCESS_FS_TRUNCATE << 1) - 1, /* v3: add "truncate" */
};
int abi = landlock_create_ruleset(NULL, 0,
LANDLOCK_CREATE_RULESET_VERSION);
if (abi == -1) {
/*
* Kernel too old, not compiled with Landlock,
* or Landlock was not enabled at boot time.
*/
perror("Unable to use Landlock");
return; /* Graceful fallback: Do nothing. */
}
abi = MIN(abi, 3);
/* Only use the available rights in the ruleset. */
attr.handled_access_fs &= landlock_fs_access_rights[abi - 1];
The available access rights for each ABI version are listed in the VERSIONS section.
If our program needed to create hard links or rename files between different directories (LANDLOCK_ACCESS_FS_REFER), we would require the following change to the backwards compatibility logic: Directory reparenting is not possible in a process restricted with Landlock ABI version 1. Therefore, if the program needed to do file reparenting, and if only Landlock ABI version 1 was available, we could not restrict the process.
Now that the ruleset attributes are determined, we create the Landlock ruleset and acquire a file descriptor as a handle to it, using landlock_create_ruleset(2):
ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
if (ruleset_fd == -1) {
perror("Failed to create a ruleset");
exit(EXIT_FAILURE);
}
We can now add a new rule to the ruleset through the ruleset's file descriptor. The requested access rights must be a subset of the access rights which were specified in attr.handled_access_fs at ruleset creation time.
In this example, 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.
struct landlock_path_beneath_attr path_beneath = {0};
int err;
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 == -1) {
perror("Failed to open file");
close(ruleset_fd);
exit(EXIT_FAILURE);
}
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);
exit(EXIT_FAILURE);
}
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 set-user-ID binary).
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
perror("Failed to restrict privileges");
close(ruleset_fd);
exit(EXIT_FAILURE);
}
The current thread is now ready to sandbox itself with the ruleset.
if (landlock_restrict_self(ruleset_fd, 0)) {
perror("Failed to enforce ruleset");
close(ruleset_fd);
exit(EXIT_FAILURE);
}
close(ruleset_fd);
If the landlock_restrict_self(2) 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