landlock: Clarify documentation for the LANDLOCK_ACCESS_FS_REFER right

Clarify the "refer" documentation by splitting up a big paragraph of
text.

- Call out specifically that the denial by default applies to ABI v1 as
  well.
- Turn the three additional constraints for link/rename operations
  into bullet points, to give it more structure.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20230221165205.4231-1-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index f3223f9..81d09ef 100644 (file)
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -130,21 +130,37 @@ struct landlock_path_beneath_attr {
  * - %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 also the
- *   only access right which is always considered handled by any ruleset in
- *   such a way that reparenting a file hierarchy is always denied by default.
- *   To avoid privilege escalation, it is not enough to add a rule with this
- *   access right.  When linking or renaming a file, the destination directory
- *   hierarchy must also always have the same or a superset of restrictions of
- *   the source hierarchy.  If it is not the case, or if the domain doesn't
- *   handle this access right, such actions are denied by default with errno
- *   set to ``EXDEV``.  Linking also requires a ``LANDLOCK_ACCESS_FS_MAKE_*``
- *   access right on the destination directory, and renaming also requires a
- *   ``LANDLOCK_ACCESS_FS_REMOVE_*`` access right on the source's (file or
- *   directory) parent.  Otherwise, such actions are denied with errno set to
- *   ``EACCES``.  The ``EACCES`` errno prevails over ``EXDEV`` to let user space
- *   efficiently deal with an unrecoverable error.
+ *   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``.
  *
  * .. warning::
  *