Landlock
Section: Environments, Tables, and Troff Macros (7)
Updated: 202-0-08
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NAME
Landlock - unprivileged acces-control
DESCRIPTION
Landlock is an acces-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 syste-wide acces-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 rea-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:
- [bu]
-
landlock_create_ruleset(2)
creates a new ruleset;
- [bu]
-
landlock_add_rule(2)
adds a new rule to a ruleset;
- [bu]
-
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
which the process intends to perform.
A set of rules is aggregated in a ruleset,
which can then restrict the thread enforcing it,
and its future children.
The two existing types of rules are:
- Filesystem rules
-
For these rules, the object is a file hierarchy,
and the related filesystem actions are defined with
filesystem access rights.
- Network rules (since ABI v4)
-
For these rules, the object is a TCP port,
and the related actions are defined with
network access rights.
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.
- 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:
-
- [bu]
-
IOCTL commands targeting file descriptors
(FIOCLEX,
FIONCLEX),
- [bu]
-
IOCTL commands targeting file descriptions
(FIONBIO,
FIOASYNC),
- [bu]
-
IOCTL commands targeting file systems
(FIFREEZE,
FITHAW,
FIGETBSZ,
FS_IOC_GETFSUUID,
FS_IOC_GETFSSYSFSPATH)
- [bu]
-
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.
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:
-
- [bu]
-
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.
- [bu]
-
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.
- [bu]
-
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.
Network flags
These flags enable to restrict a sandboxed process
to a set of network actions.
This is supported since 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.
Scope flags
These flags enable isolating a sandboxed process from a set of IPC actions.
Setting a flag for a ruleset will isolate the Landlock domain
to forbid connections to resources outside the domain.
This is supported since Landlock ABI version 6.
The following scopes exist:
- LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET
-
Restrict a sandboxed process from connecting to an abstract UNIX socket
created by a process outside the related Landlock domain
(e.g., a parent domain or a no-sandboxed process).
- LANDLOCK_SCOPE_SIGNAL
-
Restrict a sandboxed process from sending a signal
to another process outside the domain.
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 ru-time parent policies.
Ptrace restrictions
A sandboxed process has less privileges than a no-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 su-domain of the tracer.
IPC scoping
Similar to the implicit
Ptrace restrictions,
we may want to further restrict interactions between sandboxes.
Therefore, at ruleset creation time,
each Landlock domain can restrict the scope for certain operations,
so that these operations can only reach out to processes
within the same Landlock domain or in a nested Landlock domain (the "scope").
The operations which can be scoped are:
- LANDLOCK_SCOPE_SIGNAL
-
This limits the sending of signals to target processes
which run within the same or a nested Landlock domain.
- LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET
-
This limits the set of abstract
unix(7)
sockets to which we can
connect(2)
to socket addresses which were created
by a process in the same or a nested Landlock domain.
-
A
sendto(2)
on a no-connected datagram socket is treated as if it were doing an implicit
connect(2)
and will be blocked if the remote end does not stem
from the same or a nested Landlock domain.
-
A
sendto(2)
on a socket which was previously connected will not be restricted.
This works for both datagram and stream sockets.
IPC scoping does not support exceptions via
landlock_add_rule(2).
If an operation is scoped within a domain,
no rules can be added to allow access to
resources or processes outside of the scope.
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
no-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
|
|
|
|
| 4 | 6.7 | LANDLOCK_ACCESS_NET_BIND_TCP
|
| LANDLOCK_ACCESS_NET_CONNECT_TCP |
|
|
|
| 5 | 6.10 | LANDLOCK_ACCESS_FS_IOCTL_DEV
|
|
|
|
| 6 | 6.12 | LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET
|
| LANDLOCK_SCOPE_SIGNAL |
|
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 fil-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.
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 |
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 se-use-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
SEE ALSO
landlock_create_ruleset(2),
landlock_add_rule(2),
landlock_restrict_self(2)
Index
- NAME
-
- DESCRIPTION
-
- Landlock rules
-
- Filesystem actions
-
- Network flags
-
- Scope flags
-
- Layers of file path access rights
-
- Bind mounts and OverlayFS
-
- Inheritance
-
- Ptrace restrictions
-
- IPC scoping
-
- Truncating files
-
- VERSIONS
-
- NOTES
-
- CAVEATS
-
- EXAMPLES
-
- SEE ALSO
-
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