Landlock: unprivileged access control

Author:

Mickaël Salaün

Date:

October 2022

The goal of Landlock is to enable to restrict ambient rights (e.g. global filesystem access) for a set of processes. Because Landlock is a stackable LSM, it makes 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 or unexpected/malicious behaviors in user space applications. Landlock empowers any process, including unprivileged ones, to securely restrict themselves.

We can quickly make sure that Landlock is enabled in the running system by looking for "landlock: Up and running" in kernel logs (as root): dmesg | grep landlock || journalctl -kg landlock . Developers can also easily check for Landlock support with a related system call. If Landlock is not currently supported, we need to configure the kernel appropriately.

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. A set of rules is aggregated in a ruleset, which can then restrict the thread enforcing it, and its future children.

Defining and enforcing a security policy

We first need to define 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 kind of actions. This is required for backward and forward compatibility (i.e. the kernel and user space may not know each other's supported restrictions), hence the need to be explicit about the denied-by-default access rights.

struct landlock_ruleset_attr ruleset_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,
};

Because we may not know on which kernel version an application will be executed, it is safer to follow a best-effort security approach. Indeed, we should try to protect users as much as possible whatever the kernel they are using. To avoid binary enforcement (i.e. either all security features or none), we can leverage a dedicated Landlock command to get the current version of the Landlock ABI and adapt the handled accesses. Let's check if we should remove the LANDLOCK_ACCESS_FS_REFER or LANDLOCK_ACCESS_FS_TRUNCATE access rights, which are only supported starting with the second and third version of the ABI.

int abi;

abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
if (abi < 0) {
    /* Degrades gracefully if Landlock is not handled. */
    perror("The running kernel does not enable to use Landlock");
    return 0;
}
switch (abi) {
case 1:
    /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
    ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
    __attribute__((fallthrough));
case 2:
    /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
    ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
}

This enables to create an inclusive ruleset that will contain our rules.

int ruleset_fd;

ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
if (ruleset_fd < 0) {
    perror("Failed to create a ruleset");
    return 1;
}

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.

int err;
struct landlock_path_beneath_attr 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 < 0) {
    perror("Failed to open file");
    close(ruleset_fd);
    return 1;
}
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);
    return 1;
}

It may also be required to create rules following the same logic as explained for the ruleset creation, by filtering access rights according to the Landlock ABI version. In this example, this is not required because all of the requested allowed_access rights are already available in ABI 1.

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 SUID binary).

if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
    perror("Failed to restrict privileges");
    close(ruleset_fd);
    return 1;
}

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);
    return 1;
}
close(ruleset_fd);

If the landlock_restrict_self 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 samples/landlock/sandboxer.c.

Good practices

It is recommended setting access rights to file hierarchy leaves as much as possible. For instance, it is better to be able to have ~/doc/ as a read-only hierarchy and ~/tmp/ as a read-write hierarchy, compared to ~/ as a read-only hierarchy and ~/tmp/ as a read-write hierarchy. Following this good practice leads to self-sufficient hierarchies that do not depend on their location (i.e. parent directories). This is particularly relevant when we want to allow linking or renaming. Indeed, having consistent access rights per directory enables to change the location of such directory without relying on the destination directory access rights (except those that are required for this operation, see LANDLOCK_ACCESS_FS_REFER documentation). Having self-sufficient hierarchies also helps to tighten the required access rights to the minimal set of data. This also helps avoid sinkhole directories, i.e. directories where data can be linked to but not linked from. However, this depends on data organization, which might not be controlled by developers. In this case, granting read-write access to ~/tmp/, instead of write-only access, would potentially allow to move ~/tmp/ to a non-readable directory and still keep the ability to list the content of ~/tmp/.

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 stacked 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 to restrict access to file hierarchies, which means that these access rights can be propagated with bind mounts (cf. Shared Subtrees) but not with Overlay Filesystem.

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 reflects on the upper layer. From a Landlock policy point of view, each OverlayFS layers and merge hierarchies are standalone and contains their own set of files and directories, which is different from bind mounts. 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 inheritance (cf. Seccomp BPF (SECure COMPuting with filters)) or any other LSM dealing with task's 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 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 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.

Compatibility

Backward and forward compatibility

Landlock is designed to be compatible with past and future versions of the kernel. This is achieved thanks to the system call attributes and the associated bitflags, particularly the ruleset's handled_access_fs. Making handled access right explicit enables the kernel and user space to have a clear contract with each other. This is required to make sure sandboxing will not get stricter with a system update, which could break applications.

Developers can subscribe to the Landlock mailing list to knowingly update and test their applications with the latest available features. In the interest of users, and because they may use different kernel versions, it is strongly encouraged to follow a best-effort security approach by checking the Landlock ABI version at runtime and only enforcing the supported features.

Landlock ABI versions

The Landlock ABI version can be read with the sys_landlock_create_ruleset() system call:

int abi;

abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
if (abi < 0) {
    switch (errno) {
    case ENOSYS:
        printf("Landlock is not supported by the current kernel.\n");
        break;
    case EOPNOTSUPP:
        printf("Landlock is currently disabled.\n");
        break;
    }
    return 0;
}
if (abi >= 2) {
    printf("Landlock supports LANDLOCK_ACCESS_FS_REFER.\n");
}

The following kernel interfaces are implicitly supported by the first ABI version. Features only supported from a specific version are explicitly marked as such.

Kernel interface

Access rights

A set of actions on kernel objects may be defined by an attribute (e.g. struct landlock_path_beneath_attr) including a bitmask of access.

Filesystem flags

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.

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. Note that you might additionally need the LANDLOCK_ACCESS_FS_TRUNCATE right in order to overwrite files with open(2) using O_TRUNC or 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.

Warning

It is currently not possible to restrict some file-related actions accessible through these syscall 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.

Creating a new ruleset

long sys_landlock_create_ruleset(const struct landlock_ruleset_attr __user *const attr, const size_t size, const __u32 flags)

Create a new ruleset

Parameters

const struct landlock_ruleset_attr __user *const attr

Pointer to a struct landlock_ruleset_attr identifying the scope of the new ruleset.

const size_t size

Size of the pointed struct landlock_ruleset_attr (needed for backward and forward compatibility).

const __u32 flags

Supported value: LANDLOCK_CREATE_RULESET_VERSION.

Description

This system call enables to create a new Landlock ruleset, and returns the related file descriptor on success.

If flags is LANDLOCK_CREATE_RULESET_VERSION and attr is NULL and size is 0, then the returned value is the highest supported Landlock ABI version (starting at 1).

Possible returned errors are:

• EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;

• EINVAL: unknown flags, or unknown access, or too small size;

• E2BIG or EFAULT: attr or size inconsistencies;

• ENOMSG: empty landlock_ruleset_attr.handled_access_fs.

struct landlock_ruleset_attr

Ruleset definition

Definition:

struct landlock_ruleset_attr {
    __u64 handled_access_fs;
};

Members

handled_access_fs

Bitmask of actions (cf. Filesystem flags) that is handled by this ruleset and should then be forbidden if no rule explicitly allow them: it is a deny-by-default list that should contain as much Landlock access rights as possible. Indeed, all Landlock filesystem access rights that are not part of handled_access_fs are allowed. This is needed for backward compatibility reasons. One exception is the LANDLOCK_ACCESS_FS_REFER access right, which is always implicitly handled, but must still be explicitly handled to add new rules with this access right.

Description

Argument of sys_landlock_create_ruleset(). This structure can grow in future versions.

Extending a ruleset

long sys_landlock_add_rule(const int ruleset_fd, const enum landlock_rule_type rule_type, const void __user *const rule_attr, const __u32 flags)

Add a new rule to a ruleset

Parameters

const int ruleset_fd

File descriptor tied to the ruleset that should be extended with the new rule.

const enum landlock_rule_type rule_type

Identify the structure type pointed to by rule_attr (only LANDLOCK_RULE_PATH_BENEATH for now).

const void __user *const rule_attr

Pointer to a rule (only of type struct landlock_path_beneath_attr for now).

const __u32 flags

Must be 0.

Description

This system call enables to define a new rule and add it to an existing ruleset.

Possible returned errors are:

• EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;

• EINVAL: flags is not 0, or inconsistent access in the rule (i.e. landlock_path_beneath_attr.allowed_access is not a subset of the ruleset handled accesses);

• ENOMSG: Empty accesses (e.g. landlock_path_beneath_attr.allowed_access);

• EBADF: ruleset_fd is not a file descriptor for the current thread, or a member of rule_attr is not a file descriptor as expected;

• EBADFD: ruleset_fd is not a ruleset file descriptor, or a member of rule_attr is not the expected file descriptor type;

• EPERM: ruleset_fd has no write access to the underlying ruleset;

• EFAULT: rule_attr inconsistency.

enum landlock_rule_type

Landlock rule type

Constants

LANDLOCK_RULE_PATH_BENEATH

Type of a struct landlock_path_beneath_attr .

Description

Argument of sys_landlock_add_rule().

struct landlock_path_beneath_attr

Path hierarchy definition

Definition:

struct landlock_path_beneath_attr {
    __u64 allowed_access;
    __s32 parent_fd;
};

Members

allowed_access

Bitmask of allowed actions for this file hierarchy (cf. Filesystem flags).

parent_fd

File descriptor, preferably opened with O_PATH, which identifies the parent directory of a file hierarchy, or just a file.

Description

Argument of sys_landlock_add_rule().

Enforcing a ruleset

long sys_landlock_restrict_self(const int ruleset_fd, const __u32 flags)

Enforce a ruleset on the calling thread

Parameters

const int ruleset_fd

File descriptor tied to the ruleset to merge with the target.

const __u32 flags

Must be 0.

Description

This system call enables to enforce a Landlock ruleset on the current thread. Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its namespace or is running with no_new_privs. This avoids scenarios where unprivileged tasks can affect the behavior of privileged children.

Possible returned errors are:

• EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;

• EINVAL: flags is not 0.

• EBADF: ruleset_fd is not a file descriptor for the current thread;

• EBADFD: ruleset_fd is not a ruleset file descriptor;

• EPERM: ruleset_fd has no read access to the underlying ruleset, or the current thread is not running with no_new_privs, or it doesn't have CAP_SYS_ADMIN in its namespace.

• E2BIG: The maximum number of stacked rulesets is reached for the current thread.

Current limitations

Filesystem topology modification

As for file renaming and linking, a sandboxed thread cannot modify its filesystem topology, whether via mount(2) or pivot_root(2). However, chroot(2) calls are not denied.

Special filesystems

Access to regular files and directories can be restricted by Landlock, according to the handled accesses of a ruleset. However, files that do not come from a user-visible filesystem (e.g. pipe, socket), but can still be accessed through /proc/<pid>/fd/*, cannot currently be explicitly restricted. Likewise, some special kernel filesystems such as nsfs, which can be accessed through /proc/<pid>/ns/*, cannot currently be explicitly restricted. However, thanks to the ptrace restrictions, access to such sensitive /proc files are automatically restricted according to domain hierarchies. Future Landlock evolutions could still enable to explicitly restrict such paths with dedicated ruleset flags.

Ruleset layers

There is a limit of 16 layers of stacked rulesets. This can be an issue for a task willing to enforce a new ruleset in complement to its 16 inherited rulesets. Once this limit is reached, sys_landlock_restrict_self() returns E2BIG. It is then strongly suggested to carefully build rulesets once in the life of a thread, especially for applications able to launch other applications that may also want to sandbox themselves (e.g. shells, container managers, etc.).

Memory usage

Kernel memory allocated to create rulesets is accounted and can be restricted by the Memory Resource Controller.

Previous limitations

File renaming and linking (ABI < 2)

Because Landlock targets unprivileged access controls, it needs to properly handle composition of rules. Such property also implies rules nesting. Properly handling multiple layers of rulesets, each one of them able to restrict access to files, also implies inheritance of the ruleset restrictions from a parent to its hierarchy. Because files are identified and restricted by their hierarchy, moving or linking a file from one directory to another implies propagation of the hierarchy constraints, or restriction of these actions according to the potentially lost constraints. To protect against privilege escalations through renaming or linking, and for the sake of simplicity, Landlock previously limited linking and renaming to the same directory. Starting with the Landlock ABI version 2, it is now possible to securely control renaming and linking thanks to the new LANDLOCK_ACCESS_FS_REFER access right.

File truncation (ABI < 3)

File truncation could not be denied before the third Landlock ABI, so it is always allowed when using a kernel that only supports the first or second ABI.

Starting with the Landlock ABI version 3, it is now possible to securely control truncation thanks to the new LANDLOCK_ACCESS_FS_TRUNCATE access right.

Kernel support

Landlock was first introduced in Linux 5.13 but it must be configured at build time with CONFIG_SECURITY_LANDLOCK=y. Landlock must also be enabled at boot time as the other security modules. The list of security modules enabled by default is set with CONFIG_LSM. The kernel configuration should then contains CONFIG_LSM=landlock,[...] with [...] as the list of other potentially useful security modules for the running system (see the CONFIG_LSM help).

If the running kernel does not have landlock in CONFIG_LSM, then we can still enable it by adding lsm=landlock,[...] to The kernel's command-line parameters thanks to the bootloader configuration.

Questions and answers

What about user space sandbox managers?

Using user space process to enforce restrictions on kernel resources can lead to race conditions or inconsistent evaluations (i.e. Incorrect mirroring of the OS code and state).

What about namespaces and containers?

Namespaces can help create sandboxes but they are not designed for access-control and then miss useful features for such use case (e.g. no fine-grained restrictions). Moreover, their complexity can lead to security issues, especially when untrusted processes can manipulate them (cf. Controlling access to user namespaces).

Additional documentation

• Landlock LSM: kernel documentation

• https://landlock.io