On 29/02/2020 18:23, Randy Dunlap wrote:

Hi,
Here are a few corrections for you to consider.


On 2/24/20 8:02 AM, Mickaël Salaün wrote:

quoted

This documentation can be built with the Sphinx framework.

Another location might be more appropriate, though.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Vincent Dagonneau <redacted>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: James Morris <jmorris@namei.org>
Cc: Kees Cook <redacted>
Cc: Serge E. Hallyn <serge@hallyn.com>
---

Changes since v13:
* Rewrote the documentation according to the major revamp.

Previous version:
https://lore.kernel.org/lkml/20191104172146.30797-8-mic@digikod.net/ (local)
---
 Documentation/security/index.rst           |   1 +
 Documentation/security/landlock/index.rst  |  18 ++
 Documentation/security/landlock/kernel.rst |  44 ++++
 Documentation/security/landlock/user.rst   | 233 +++++++++++++++++++++
 4 files changed, 296 insertions(+)
 create mode 100644 Documentation/security/landlock/index.rst
 create mode 100644 Documentation/security/landlock/kernel.rst
 create mode 100644 Documentation/security/landlock/user.rst

diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst
new file mode 100644
index 000000000000..dbd33b96ce60
--- /dev/null
+++ b/Documentation/security/landlock/index.rst

@@ -0,0 +1,18 @@
+=========================================
+Landlock LSM: unprivileged access control
+=========================================
+
+:Author: Mickaël Salaün
+
+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 empower any

                                                                       empowers

quoted

+process, including unprivileged ones, to securely restrict themselves.
+
+.. toctree::
+
+    user
+    kernel

diff --git a/Documentation/security/landlock/kernel.rst b/Documentation/security/landlock/kernel.rst
new file mode 100644
index 000000000000..b87769909029
--- /dev/null
+++ b/Documentation/security/landlock/kernel.rst

@@ -0,0 +1,44 @@
+==============================
+Landlock: kernel documentation
+==============================
+
+Landlock's goal is to create scoped access-control (i.e. sandboxing).  To
+harden a whole system, this feature should be available to any process,
+including unprivileged ones.  Because such process may be compromised or
+backdoored (i.e. untrusted), Landlock's features must be safe to use from the
+kernel and other processes point of view.  Landlock's interface must therefore
+expose a minimal attack surface.
+
+Landlock is designed to be usable by unprivileged processes while following the
+system security policy enforced by other access control mechanisms (e.g. DAC,
+LSM).  Indeed, a Landlock rule shall not interfere with other access-controls
+enforced on the system, only add more restrictions.
+
+Any user can enforce Landlock rulesets on their processes.  They are merged and
+evaluated according to the inherited ones in a way that ensure that only more

                                                           ensures

quoted

+constraints can be added.
+
+
+Guiding principles for safe access controls
+===========================================
+
+* A Landlock rule shall be focused on access control on kernel objects instead
+  of syscall filtering (i.e. syscall arguments), which is the purpose of
+  seccomp-bpf.
+* To avoid multiple kind of side-channel attacks (e.g. leak of security

                       kinds

quoted

+  policies, CPU-based attacks), Landlock rules shall not be able to
+  programmatically communicate with user space.
+* Kernel access check shall not slow down access request from unsandboxed
+  processes.
+* Computation related to Landlock operations (e.g. enforce a ruleset) shall
+  only impact the processes requesting them.
+
+
+Landlock rulesets and domains
+=============================
+
+A domain is a read-only ruleset tied to a set of subjects (i.e. tasks).  A
+domain can transition to a new one which is the intersection of the constraints
+from the current and a new ruleset.  The definition of a subject is implicit
+for a task sandboxing itself, which makes the reasoning much easier and helps
+avoid pitfalls.

diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst
new file mode 100644
index 000000000000..cbd7f61fca8c
--- /dev/null
+++ b/Documentation/security/landlock/user.rst

@@ -0,0 +1,233 @@
+=================================
+Landlock: userspace documentation
+=================================
+
+Landlock rules
+==============
+
+A Landlock rule enables to describe an action on an object.  An object is
+currently a file hierarchy, and the related filesystem actions are defined in
+`Access rights`_.  A set of rules are aggregated in a ruleset, which can then

                                     is

quoted

+restricts the thread enforcing it, and its future children.

   restrict

quoted

+
+
+Defining and enforcing a security policy
+----------------------------------------
+
+Before defining a security policy, an application should first probe for the
+features supported by the running kernel, which is important to be compatible
+with older kernels.  This can be done thanks to the `landlock` syscall (cf.
+:ref:`syscall`).
+
+.. code-block:: c
+
+    struct landlock_attr_features attr_features;
+
+    if (landlock(LANDLOCK_CMD_GET_FEATURES, LANDLOCK_OPT_GET_FEATURES,
+            sizeof(attr_features), &attr_features)) {
+        perror("Failed to probe the Landlock supported features");
+        return 1;
+    }
+
+Then, we need to create the ruleset that will contains our rules.  For this

                                                 contain

quoted

+example, the ruleset will contains rules which only allow read actions, but

                             contain

quoted

+write actions will be denied.  The ruleset then needs to handle both of these
+kind of actions.  To have a backward compatibility, these actions should be
+ANDed with the supported ones.
+
+.. code-block:: c
+
+    int ruleset_fd;
+    struct landlock_attr_ruleset ruleset = {
+        .handled_access_fs =
+            LANDLOCK_ACCESS_FS_READ |
+            LANDLOCK_ACCESS_FS_READDIR |
+            LANDLOCK_ACCESS_FS_EXECUTE |
+            LANDLOCK_ACCESS_FS_WRITE |
+            LANDLOCK_ACCESS_FS_TRUNCATE |
+            LANDLOCK_ACCESS_FS_CHMOD |
+            LANDLOCK_ACCESS_FS_CHOWN |
+            LANDLOCK_ACCESS_FS_CHGRP |
+            LANDLOCK_ACCESS_FS_LINK_TO |
+            LANDLOCK_ACCESS_FS_RENAME_FROM |
+            LANDLOCK_ACCESS_FS_RENAME_TO |
+            LANDLOCK_ACCESS_FS_RMDIR |
+            LANDLOCK_ACCESS_FS_UNLINK |
+            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,
+    };
+
+    ruleset.handled_access_fs &= attr_features.access_fs;
+    ruleset_fd = landlock(LANDLOCK_CMD_CREATE_RULESET,
+                    LANDLOCK_OPT_CREATE_RULESET, sizeof(ruleset), &ruleset);
+    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 enable to read the
+file hierarchy ``/usr``.  Without other rule, write actions would then be

                             Without other rules,
or
                             Without another rule,

quoted

+denied by the ruleset.  To add ``/usr`` to the ruleset, we open it with the
+``O_PATH`` flag and fill the &struct landlock_attr_path_beneath with this file
+descriptor.
+
+.. code-block:: c
+
+    int err;
+    struct landlock_attr_path_beneath path_beneath = {
+        .ruleset_fd = ruleset_fd,
+        .allowed_access =
+            LANDLOCK_ACCESS_FS_READ |
+            LANDLOCK_ACCESS_FS_READDIR |
+            LANDLOCK_ACCESS_FS_EXECUTE,
+    };
+
+    path_beneath.allowed_access &= attr_features.access_fs;
+    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(LANDLOCK_CMD_ADD_RULE, LANDLOCK_OPT_ADD_RULE_PATH_BENEATH,
+            sizeof(path_beneath), &path_beneath);
+    close(path_beneath.parent_fd);
+    if (err) {
+        perror("Failed to update ruleset");
+        close(ruleset_fd);
+        return 1;
+    }
+
+We now have a ruleset with one rule allowing read access to ``/usr`` while
+denying all accesses featured in ``attr_features.access_fs`` to everything else
+on the filesystem.  The next step is to restrict the current thread from
+gaining more privileges (e.g. thanks to a SUID binary).
+
+.. code-block:: c
+
+    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.
+
+.. code-block:: c
+
+    struct landlock_attr_enforce attr_enforce = {
+        .ruleset_fd = ruleset_fd,
+    };
+
+    if (landlock(LANDLOCK_CMD_ENFORCE_RULESET, LANDLOCK_OPT_ENFORCE_RULESET,
+            sizeof(attr_enforce), &attr_enforce)) {
+        perror("Failed to enforce ruleset");
+        close(ruleset_fd);
+        return 1;
+    }
+    close(ruleset_fd);
+
+If this last system call succeeds, the current thread is now restricted and

   If this last landlock system call succeeds,

[because close() is the last system call]

quoted

+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,

                                                   preferably:         policy;

quoted

+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.
+
+A full working code can be found in `samples/landlock/sandboxer.c`_.

   Full working code

quoted

+
+
+Inheritance
+-----------
+
+Every new thread resulting from a :manpage:`clone(2)` inherits Landlock program
+restrictions from its parent.  This is similar to the seccomp inheritance (cf.
+:doc:`/userspace-api/seccomp_filter`) or any other LSM dealing with task's
+:manpage:`credentials(7)`.  For instance, one process' thread may apply

                                                 process's

quoted

+Landlock rules to itself, but they will not be automatically applied to other
+sibling threads (unlike POSIX thread credential changes, cf.
+:manpage:`nptl(7)`).

[snip]

thanks for the documentation.

Done. Thanks for this attentive review!