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 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.

When opening files for writing, you will often additionally need the LANDLOCK_ACCESS_FS_TRUNCATE right. In many cases, these system calls truncate existing files when overwriting them (e.g., creat(2)).

LANDLOCK_ACCESS_FS_READ_FILE

Open a file with read access.

LANDLOCK_ACCESS_FS_TRUNCATE

Truncate a file with truncate(2), ftruncate(2), creat(2), or open(2) with O_TRUNC. Whether an opened file can be truncated with ftruncate(2) is determined during open(2), in the same way as read and write permissions are checked during open(2) using LANDLOCK_ACCESS_FS_READ_FILE and LANDLOCK_ACCESS_FS_WRITE_FILE. This access right is available since the third version of the Landlock ABI.

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.

LANDLOCK_ACCESS_FS_REFER

Link or rename a file from or to a different directory (i.e., reparent a file hierarchy).

This access right is available since the second version of the Landlock ABI.

This is the only access right which is denied by default by any ruleset, even if the right is not specified as handled at ruleset creation time. The only way to make a ruleset grant this right is to explicitly allow it for a specific directory by adding a matching rule to the ruleset.

In particular, when using the first Landlock ABI version, Landlock will always deny attempts to reparent files between different directories.

In addition to the source and destination directories having the LANDLOCK_ACCESS_FS_REFER access right, the attempted link or rename operation must meet the following constraints:

• The reparented file may not gain more access rights in the destination directory than it previously had in the source directory. If this is attempted, the operation results in an EXDEV error.

• When linking or renaming, the LANDLOCK_ACCESS_FS_MAKE_* right for the respective file type must be granted for the destination directory. Otherwise, the operation results in an EACCES error.

• When renaming, the LANDLOCK_ACCESS_FS_REMOVE_* right for the respective file type must be granted for the source directory. Otherwise, the operation results in an EACCES error.

If multiple requirements are not met, the EACCES error code takes precedence over EXDEV.

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 run-time 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.

Truncating files

The operations covered by LANDLOCK_ACCESS_FS_WRITE_FILE and LANDLOCK_ACCESS_FS_TRUNCATE both change the contents of a file and sometimes overlap in non-intuitive ways. It is recommended to always specify both of these together.

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.

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">

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

https://landlock.io/">