Re: [PATCH 1/3] landlock: Fix layer composition of LANDLOCK_ACCESS_FS_REFER

["Günther Noack" <gnoack3000@gmail.com>]

On Sun, Nov 01, 2026 at 07:47:50PM +0700, Dwi Siswanto wrote:
> I saw the docs still describe the old one. include/uapi/linux/landlock.h
> says:
> 
>    * For historical reasons, the %LANDLOCK_ACCESS_FS_REFER right is always denied
>    * by default, even when its bit is not set in @handled_access_fs.
> 
> and, for LANDLOCK_ACCESS_FS_REFER itself:
> 
>    *   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.
> 
> So ruleset that handles no filesystem access right never denies
> LANDLOCK_ACCESS_FS_REFER -- alone or layered.
> 
> Also, the comment above _LANDLOCK_ACCESS_FS_INITIALLY_DENIED in access.h
> has the same problem:
> 
>    * All access rights that are denied by default whether they are handled or not
>    * by a ruleset/layer.  This must be ORed with all domain->handled_masks[]
>    * entries when we need to get the absolute handled access masks, see
>    * landlock_upgrade_handled_access_masks().
> 
> After this patch, the mask only applies to layers that handle filesystem
> access, and after patch 3, nothing ORs it in at read time. *Someone* who
> follows the comment could bring the bug back.

Thanks for pointing this out, the documentation is indeed not up to date.

All this documentation likely stems from a time when *only* access_fs
but none of the other fields existed in struct landlock_ruleset_attr.

It is not very obvious from looking at this patch set alone, but the
intended semantics are clearer when you look at merge_ruleset() in
domain.c.

This in turn calls landlock_upgrade_handled_access_masks() in
access.h to calculcate the updated access masks with the same logic:

	if (access_masks.fs)
  		access_masks.fs |= _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;

	return access_masks;

The behaviour that we have for non-layered domains is the intended one
here -- we only add the REFER right when either one of the other
filesystem access rights is handled, but we do not set it when only
other access right types are handled.

I'll send a documentation update for this, either in the next patch
set revision, or in a follow-up if needed.


> > +	access_mask_t handled = domain->handled_masks[layer_level].fs;
> > +
> > +	/*
> > +	 * Handles all initially denied by default access rights, but only for
> > +	 * layers handling filesystem accesses.
> > +	 */
> > +	if (handled)
> > +		handled |= _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
> > +
> > +	return handled;
> 
> nit: I believe landlock_upgrade_handled_access_masks() already has the
> same "only if fs is non-zero" rule, so this could be
> 
> 	return landlock_upgrade_handled_access_masks(
> 		domain->handled_masks[layer_level]).fs;
> 
> Which keep the rule in one place. It matters little if patch 3 goes in
> on top, and 6.12 doesn't have the helper. What do you think?

Good point, I agree, this function already implements this.

As you are saying, the first commit is mainly useful to backporting,
and the backport to 6.12 will be easier to compare with the original
for stable maintainers if we spell it out instead of using the helper
function.

Not feeling strongly about it though; I'd let Mickaël decide what he
prefers.  In either case, the backporting will require some manual
work because the affected code moved around between files.

–Günther