Landlock(7)                                       Miscellaneous Information Manual                                      Landlock(7)

       Landlock - unprivileged access-control

       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 ac‐
       tions 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:

              Execute a file.

              Open a file with write access.

              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 direc‐
       tory itself, and the directories beneath it:

              Open a directory or list its content.

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

              Remove an empty directory or rename one.

              Unlink (or rename) a file.

              Create (or rename or link) a character device.

              Create (or rename) a directory.

              Create (or rename or link) a regular file.

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

              Create (or rename or link) a named pipe.

              Create (or rename or link) a block device.

              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.

       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 creden‐
       tial 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 automati‐
       cally 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.

       Landlock was added in Linux 5.13.

       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 pa‐
       rameter 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 ker‐
       nel 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.

       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_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 |

           ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
           if (ruleset_fd == -1) {
               perror("Failed to create a ruleset");

       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  de‐

           struct landlock_path_beneath_attr path_beneath = {0};
           int err;

           path_beneath.allowed_access =
                   LANDLOCK_ACCESS_FS_EXECUTE |
                   LANDLOCK_ACCESS_FS_READ_FILE |

           path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC);
           if (path_beneath.parent_fd == -1) {
               perror("Failed to open file");
           err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
                                   &path_beneath, 0);
           if (err) {
               perror("Failed to update ruleset");

       We  now  have a ruleset with one rule allowing read access to /usr while denying all other handled accesses for the filesys‐
       tem.  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");

       The current thread is now ready to sandbox itself with the ruleset.

           if (landlock_restrict_self(ruleset_fd, 0)) {
               perror("Failed to enforce ruleset");

       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 pol‐
       icy; 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 ⟨

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


Linux man-pages 6.03                                         2023-02-05                                                 Landlock(7)