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NAME | DESCRIPTION | VERSIONS | NOTES | CAVEATS | EXAMPLES | SEE ALSO | COLOPHON |
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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 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.
The following access rights apply only to files:
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.
This access right is available since the third version of
the Landlock ABI.
Whether an opened file can be truncated with ftruncate(2) or used
with ioctl(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.
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.
The following access right applies to both files and directories:
LANDLOCK_ACCESS_FS_IOCTL_DEV
Invoke ioctl(2) commands on an opened character or block
device.
This access right applies to all ioctl(2) commands
implemented by device drivers. However, the following
common IOCTL commands continue to be invokable independent
of the LANDLOCK_ACCESS_FS_IOCTL_DEV right:
• IOCTL commands targeting file descriptors (FIOCLEX,
FIONCLEX),
• IOCTL commands targeting file descriptions (FIONBIO,
FIOASYNC),
• IOCTL commands targeting file systems (FIFREEZE, FITHAW,
FIGETBSZ, FS_IOC_GETFSUUID, FS_IOC_GETFSSYSFSPATH)
• Some IOCTL commands which do not make sense when used
with devices, but whose implementations are safe and
return the right error codes (FS_IOC_FIEMAP, FICLONE,
FICLONERANGE, FIDEDUPERANGE)
This access right is available since the fifth version of
the Landlock ABI.
Network flags
These flags enable to restrict a sandboxed process to a set of
network actions. This is supported since the Landlock ABI version
4.
The following access rights apply to TCP port numbers:
LANDLOCK_ACCESS_NET_BIND_TCP
Bind a TCP socket to a local port.
LANDLOCK_ACCESS_NET_CONNECT_TCP
Connect an active TCP socket to a remote port.
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.
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 │
├─────┼────────┼─────────────────────────────────────────────────┤
│ 4 │ 6.7 │ LANDLOCK_ACCESS_NET_BIND_TCP │
│ │ │ LANDLOCK_ACCESS_NET_CONNECT_TCP │
├─────┼────────┼─────────────────────────────────────────────────┤
│ 5 │ 6.10 │ LANDLOCK_ACCESS_FS_IOCTL_DEV │
└─────┴────────┴─────────────────────────────────────────────────┘
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.
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), stat(2),
flock(2), chmod(2), chown(2), setxattr(2), utime(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 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 |
LANDLOCK_ACCESS_FS_IOCTL_DEV;
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" */
(LANDLOCK_ACCESS_FS_TRUNCATE << 1) - 1, /* v4: TCP support */
(LANDLOCK_ACCESS_FS_IOCTL_DEV << 1) - 1, /* v5: add "ioctl_dev" */
};
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/⟩
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.10 2024-08-21 Landlock(7)
Pages that refer to this page: setpriv(1), landlock_add_rule(2), landlock_create_ruleset(2), landlock_restrict_self(2)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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