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Landlock(7) Miscellaneous Information Manual Landlock(7)
NAME
Landlock - unprivileged access-control
DESCRIPTION
Landlock is an access-control system that enables any processes to se-
curely 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 ex-
pected 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:
o landlock_create_ruleset(2) creates a new ruleset;
o landlock_add_rule(2) adds a new rule to a ruleset;
o 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 cur-
rently a file hierarchy, and the related filesystem actions are defined
with access rights (see landlock_add_rule(2)). A set of rules is ag-
gregated 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 sand-
boxing 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 di-
rectory, 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 rule-
set 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, Land-
lock will always deny attempts to reparent files between differ-
ent directories.
In addition to the source and destination directories having the
LANDLOCK_ACCESS_FS_REFER access right, the attempted link or re-
name operation must meet the following constraints:
o 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.
o When linking or renaming, the LANDLOCK_ACCESS_FS_MAKE_* right
for the respective file type must be granted for the destina-
tion directory. Otherwise, the operation results in an EAC-
CES error.
o When renaming, the LANDLOCK_ACCESS_FS_REMOVE_* right for the
respective file type must be granted for the source direc-
tory. 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 Land-
lock domain with a new layer of policy. Indeed, this complementary
policy is composed with the potentially other rulesets already re-
stricting 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., filesys-
tem 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 destina-
tion path. These rules restrict access when they are encountered on a
path, which means that they can restrict access to multiple file hier-
archies 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 Over-
layFS 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 re-
strictions from its parent. This is similar to the seccomp(2) inheri-
tance or any other LSM dealing with tasks' credentials(7). For in-
stance, 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 ap-
plication, which will automatically be composed between themselves ac-
cording 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.
Truncating files
The operations covered by LANDLOCK_ACCESS_FS_WRITE_FILE and LAND-
LOCK_ACCESS_FS_TRUNCATE both change the contents of a file and some-
times overlap in non-intuitive ways. It is recommended to always spec-
ify 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. How-
ever, 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_TRUN-
CATE 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 ver-
sion 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 num-
bers 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_AC-
CESS_FS_REFER access right: In ABI v1, linking and moving of files be-
tween 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 initial-
ization 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 ac-
cessible through these system call families: chdir(2), stat(2),
flock(2), chmod(2), chown(2), setxattr(2), utime(2), ioctl(2), fc-
ntl(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 ac-
tions, 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 |
LANDLOCK_ACCESS_FS_REFER |
LANDLOCK_ACCESS_FS_TRUNCATE;
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 re-
strictions 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_re-
strict_self(2)
<https://landlock.io/>
Linux man-pages 6.04 2023-04-02 Landlock(7)
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