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From: "Mickaël Salaün" <>
To: Alejandro Colomar <>,
	Michael Kerrisk <>
Cc: "Mickaël Salaün" <>,
	"Jann Horn" <>,
	"Jonathan Corbet" <>,
	"Kees Cook" <>,
	"Randy Dunlap" <>,
	"Vincent Dagonneau" <>,,,,,
	"Mickaël Salaün" <>
Subject: [PATCH v3 1/4] landlock.7: Add a new page to introduce Landlock
Date: Fri, 30 Jul 2021 16:41:13 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Mickaël Salaün <>

From the user point of view, Landlock is a set of system calls enabling
to build and enforce a set of access-control rules.  A ruleset can be
created with landlock_create_ruleset(2), populated with
landlock_add_rule(2) and enforced with landlock_restrict_self(2).  This
man page gives an overview of the whole mechanism.  Details of these
system calls are documented in their respective man pages.

This is an adaptation of

Signed-off-by: Mickaël Salaün <>

Changes since v2:
* Fix semantic newlines according to Alejandro Colomar.
* Fix minor grammar issues spotted by Alejandro.
* Fix .IR and .BR use as explained by Alejandro.

Changes since v1:
* Replace all ".I" with ".IR", except when used for titles.
* Append punctuation to ".IR" and ".BR" when it makes sense (requested
  by Alejandro Colomar).
* Cut lines according to the semantic newline rules (requested by
  Alejandro Colomar).
* Remove roman style from ".TP" section titles (requested by Alejandro
* Add comma after "i.e." and "e.g.".
* Move the example in a new EXAMPLES section.
* Improve title.
* Explain the LSM acronym at first use.
 man7/landlock.7 | 361 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 361 insertions(+)
 create mode 100644 man7/landlock.7

diff --git a/man7/landlock.7 b/man7/landlock.7
new file mode 100644
index 000000000000..f6a9b02d434a
--- /dev/null
+++ b/man7/landlock.7
@@ -0,0 +1,361 @@
+.\" Copyright © 2017-2020 Mickaël Salaün <>
+.\" Copyright © 2019-2020 ANSSI
+.\" Copyright © 2021 Microsoft Corporation
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein.  The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.TH LANDLOCK 7 2021-06-27 Linux "Linux Programmer's Manual"
+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:
+.IP \(bu 2
+.BR landlock_create_ruleset (2)
+creates a new ruleset;
+.IP \(bu
+.BR landlock_add_rule (2)
+adds a new rule to a ruleset;
+.IP \(bu
+.BR 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.
+.SS 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
+.BR 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.
+.SS 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.
+.BR landlock_add_rule (2)
+.BR landlock_create_ruleset (2)
+for more context.
+A file can only receive these access rights:
+Execute a file.
+Open a file with write access.
+Open a file with read access.
+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:
+Open a directory or list its content.
+the following access rights only apply to the content of a directory,
+not the directory itself:
+Remove an empty directory or rename one.
+Unlink (or rename) a file.
+Create (or rename or link) a character device.
+Create (or rename) a directory.
+Create (or rename or link) a regular file.
+Create (or rename or link) a UNIX domain socket.
+Create (or rename or link) a named pipe.
+Create (or rename or link) a block device.
+Create (or rename or link) a symbolic link.
+.SS 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.).
+.SS Bind mounts and OverlayFS
+Landlock enables restricting access to file hierarchies,
+which means that these access rights can be propagated with bind mounts
+.BR 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 OverlayFS layers and merge hierarchies are standalone and contain
+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.
+.SS Inheritance
+Every new thread resulting from a
+.BR clone (2)
+inherits Landlock domain restrictions from its parent.
+This is similar to the
+.BR seccomp (2)
+inheritance or any other LSM dealing with tasks'
+.BR 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.
+.BR 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.
+.SS 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
+.BR 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.
+Landlock was added in Linux 5.13.
+Landlock is enabled by
+.I lsm=lsm1,...,lsmN
+command line parameter controls the sequence of the initialization of
+Linux Security Modules.
+It must contain the string
+.I landlock
+to enable Landlock.
+If the command line parameter is not specified,
+the initialization falls back to the value of the deprecated
+.I security=
+command line parameter and further to the value of CONFIG_LSM.
+We can check that Landlock is enabled by looking for
+.I landlock: Up and running.
+in kernel logs.
+It is currently not possible to restrict some file-related actions
+accessible through these system call families:
+.BR chdir (2),
+.BR truncate (2),
+.BR stat (2),
+.BR flock (2),
+.BR chmod (2),
+.BR chown (2),
+.BR setxattr (2),
+.BR utime (2),
+.BR ioctl (2),
+.BR fcntl (2),
+.BR 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 kind of actions.
+See below for the description of filesystem actions.
+.PP +4n
+int ruleset_fd;
+struct landlock_ruleset_attr ruleset_attr = {
+    .handled_access_fs =
+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
+.IR /usr .
+Without another rule, write actions would then be denied by the ruleset.
+To add
+.I /usr
+to the ruleset, we open it with the
+flag and fill the
+.I struct landlock_path_beneath_attr
+with this file descriptor.
+.PP +4n
+int err;
+struct landlock_path_beneath_attr path_beneath = {
+    .allowed_access =
+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);
+if (err) {
+    perror("Failed to update ruleset");
+    close(ruleset_fd);
+    return 1;
+We now have a ruleset with one rule allowing read access to
+.I /usr
+while denying all other handled accesses for the filesystem.
+The next step is to restrict the current thread from gaining more
+(e.g., thanks to a set-user-ID binary).
+.PP +4n
+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.
+.PP +4n
+if (landlock_restrict_self(ruleset_fd, 0)) {
+    perror("Failed to enforce ruleset");
+    close(ruleset_fd);
+    return 1;
+If the
+.BR 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
+.BR landlock_create_ruleset (2),
+.BR landlock_add_rule (2),
+.BR landlock_restrict_self (2)

  reply	other threads:[~2021-07-30 14:39 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 14:41 [PATCH v3 0/4] Add Landlock man pages Mickaël Salaün
2021-07-30 14:41 ` Mickaël Salaün [this message]
2021-07-30 14:41 ` [PATCH v3 2/4] landlock_create_ruleset.2: Document new syscall Mickaël Salaün
2021-07-30 14:41 ` [PATCH v3 3/4] landlock_add_rule.2: " Mickaël Salaün
2021-07-30 14:41 ` [PATCH v3 4/4] landlock_restrict_self.2: " Mickaël Salaün

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