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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.
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.
VERSIONS
Landlock was introduced in Linux 5.13.
To determine which Landlock features are available, users should query the Landlock ABI
version:
┌────┬────────┬───────────────────────────────────────────────────────────────────────────┐
│ABI │ Kernel │ Newly introduced access rights │
├────┼────────┼───────────────────────────────────────────────────────────────────────────┤
│ 1 │ 5.13 │ 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 │
├────┼────────┼───────────────────────────────────────────────────────────────────────────┤
│ 2 │ 5.19 │ LANDLOCK_ACCESS_FS_REFER │
├────┼────────┼───────────────────────────────────────────────────────────────────────────┤
│ 3 │ 6.2 │ LANDLOCK_ACCESS_FS_TRUNCATE │
└────┴────────┴───────────────────────────────────────────────────────────────────────────┘
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/⟩