Landlock(7)

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.

  Landlock rules
A Landlock rule describes an action on an object. An object is currently a file hierarchy, and the related filesystem actions are defined with access rights (see landlock_add_rule(2)). A set of rules is aggregated in a ruleset, which can then restrict the thread enforcing it, and its future children.

  Filesystem actions
These flags enable to restrict a sandboxed process to a set of actions on files and directories. Files or directories opened before the sandboxing are not subject to these restrictions. See landlock_add_rule(2) and landlock_create_ruleset(2) for more context.

A file can only receive these access rights:

LANDLOCK_ACCESS_FS_EXECUTE
    Execute a file.

LANDLOCK_ACCESS_FS_WRITE_FILE
    Open a file with write access.

LANDLOCK_ACCESS_FS_READ_FILE
    Open a file with read access.

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:

LANDLOCK_ACCESS_FS_READ_DIR
    Open a directory or list its content.

However, the following access rights only apply to the content of a directory, not the directory itself:

LANDLOCK_ACCESS_FS_REMOVE_DIR
    Remove an empty directory or rename one.

LANDLOCK_ACCESS_FS_REMOVE_FILE
    Unlink (or rename) a file.

LANDLOCK_ACCESS_FS_MAKE_CHAR
    Create (or rename or link) a character device.

LANDLOCK_ACCESS_FS_MAKE_DIR
    Create (or rename) a directory.

LANDLOCK_ACCESS_FS_MAKE_REG
    Create (or rename or link) a regular file.

LANDLOCK_ACCESS_FS_MAKE_SOCK
    Create (or rename or link) a UNIX domain socket.

LANDLOCK_ACCESS_FS_MAKE_FIFO
    Create (or rename or link) a named pipe.

LANDLOCK_ACCESS_FS_MAKE_BLOCK
    Create (or rename or link) a block device.

LANDLOCK_ACCESS_FS_MAKE_SYM
    Create (or rename or link) a symbolic link.

  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 composed 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 restricting access to file hierarchies, which means that these access rights can be propagated with bind mounts (cf. mount_namespaces(7)) but not with OverlayFS.

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.

  Inheritance
Every new thread resulting from a clone(2) inherits Landlock domain restrictions from its parent. This is similar to the seccomp(2) inheritance or any other LSM dealing with tasks' 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. 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 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.

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 kinds 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/