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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>
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1 changed files with 31 additions and 15 deletions
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@ -130,21 +130,37 @@ struct landlock_path_beneath_attr {
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* - %LANDLOCK_ACCESS_FS_MAKE_BLOCK: Create (or rename or link) a block device.
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* - %LANDLOCK_ACCESS_FS_MAKE_SYM: Create (or rename or link) a symbolic link.
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* - %LANDLOCK_ACCESS_FS_REFER: Link or rename a file from or to a different
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* directory (i.e. reparent a file hierarchy). This access right is
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* available since the second version of the Landlock ABI. This is also the
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* only access right which is always considered handled by any ruleset in
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* such a way that reparenting a file hierarchy is always denied by default.
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* To avoid privilege escalation, it is not enough to add a rule with this
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* access right. When linking or renaming a file, the destination directory
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* hierarchy must also always have the same or a superset of restrictions of
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* the source hierarchy. If it is not the case, or if the domain doesn't
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* handle this access right, such actions are denied by default with errno
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* set to ``EXDEV``. Linking also requires a ``LANDLOCK_ACCESS_FS_MAKE_*``
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* access right on the destination directory, and renaming also requires a
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* ``LANDLOCK_ACCESS_FS_REMOVE_*`` access right on the source's (file or
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* directory) parent. Otherwise, such actions are denied with errno set to
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* ``EACCES``. The ``EACCES`` errno prevails over ``EXDEV`` to let user space
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* efficiently deal with an unrecoverable error.
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* directory (i.e. reparent a file hierarchy).
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*
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* This access right is available since the second version of the Landlock
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* ABI.
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*
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* This is the only access right which is denied by default by any ruleset,
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* even if the right is not specified as handled at ruleset creation time.
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* The only way to make a ruleset grant this right is to explicitly allow it
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* for a specific directory by adding a matching rule to the ruleset.
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*
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* In particular, when using the first Landlock ABI version, Landlock will
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* always deny attempts to reparent files between different directories.
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*
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* In addition to the source and destination directories having the
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* %LANDLOCK_ACCESS_FS_REFER access right, the attempted link or rename
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* operation must meet the following constraints:
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*
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* * The reparented file may not gain more access rights in the destination
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* directory than it previously had in the source directory. If this is
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* attempted, the operation results in an ``EXDEV`` error.
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*
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* * When linking or renaming, the ``LANDLOCK_ACCESS_FS_MAKE_*`` right for the
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* respective file type must be granted for the destination directory.
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* Otherwise, the operation results in an ``EACCES`` error.
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*
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* * When renaming, the ``LANDLOCK_ACCESS_FS_REMOVE_*`` right for the
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* respective file type must be granted for the source directory. Otherwise,
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* the operation results in an ``EACCES`` error.
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*
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* If multiple requirements are not met, the ``EACCES`` error code takes
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* precedence over ``EXDEV``.
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*
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* .. warning::
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*
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