# landlock

## NAME



           Landlock - unprivileged access-control
    


## DESCRIPTION



           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 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.
    
           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 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 non-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 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.
    
       **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 non-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 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.
    


## 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 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.
    


## 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 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⟩
    


* * *