NAME¶

Landlock - unprivileged access-control

DESCRIPTION¶

Landlock is an access-control system that enables any processes to securely restrict themselves and their future children. Because Landlock is a stackable Linux Security Module (LSM), it makes it 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, and unexpected or malicious behaviors in applications.

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:

•

landlock_create_ruleset(2) creates a new ruleset;

•

landlock_add_rule(2) adds a new rule to a ruleset;

•

landlock_restrict_self(2) enforces a ruleset on the calling thread.

To be able to use these system calls, the running kernel must support Landlock and it must be enabled at boot time.

VERSIONS¶

Landlock was introduced in Linux 5.13.

To determine which Landlock features are available, users should query the Landlock ABI version:

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.

NOTES¶

Landlock is enabled by CONFIG_SECURITY_LANDLOCK. The lsm=lsm1,...,lsmN command line parameter controls the sequence of the initialization of Linux Security Modules. It must contain the string landlock to enable Landlock. If the command line parameter is not specified, the initialization falls back to the value of the deprecated security= command line parameter and further to the value of CONFIG_LSM. We can check that Landlock is enabled by looking for landlock: Up and running. in kernel logs.

CAVEATS¶

It is currently not possible to restrict some file-related actions accessible through these system call families: chdir(2), stat(2), flock(2), chmod(2), chown(2), setxattr(2), utime(2), ioctl(2), fcntl(2), access(2). Future Landlock evolutions will enable to restrict them.

EXAMPLES¶

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 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 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/landlock/sandboxer.c

SEE ALSO¶

landlock_create_ruleset(2), landlock_add_rule(2), landlock_restrict_self(2)

https://landlock.io/