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NAME | DESCRIPTION | VERSIONS | NOTES | CAVEATS | EXAMPLES | SEE ALSO |
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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.
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.
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.
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),
ioctl(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;
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/⟩
Linux man-pages (unreleased) (date) 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 2023-12-22 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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