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 added in Linux 5.13.

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.

It is currently not possible to restrict some file-related actions accessible through these system call families: chdir(2), truncate(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 kind of actions. See below 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;
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 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.

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/