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@xxxxxxxxxxxxxx> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20220506161102.525323-12-mic@xxxxxxxxxxx --- 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
