linux-kernel - [PATCH v3 11/12] landlock: Document good practices about filesystem policies

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Date:   Fri,  6 May 2022 18:11:01 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jann Horn <jannh@...gle.com>,
        John Johansen <john.johansen@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        Konstantin Meskhidze <konstantin.meskhidze@...wei.com>,
        Paul Moore <paul@...l-moore.com>,
        Shuah Khan <shuah@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: [PATCH v3 11/12] landlock: Document good practices about filesystem policies

Explain how to set access rights per hierarchy in an efficient and safe
way, especially with the LANDLOCK_ACCESS_FS_REFER side effect (i.e.
partial ordering and constraints for access rights per hierarchy).

Reviewed-by: Paul Moore <paul@...l-moore.com>
Signed-off-by: Mickaël Salaün <mic@...ikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-12-mic@digikod.net
---

Changes since v1:
* Add more explanation in the commit message.

Changes since v1:
* Add Reviewed-by: Paul Moore.
---
 Documentation/userspace-api/landlock.rst | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index ae2aea986aa6..7b4fe6218132 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -156,6 +156,27 @@ 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 don't
+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
 ---------------------------------
 
-- 
2.35.1