[PATCH v4 2/5] landlock: Require LANDLOCK_ACCESS_FS_MAKE_REG for whiteout creation

[Mickaël Salaün]

On Fri, Jul 31, 2026 at 04:21:14PM +0200, Günther Noack wrote:
> On Fri, Jul 31, 2026 at 01:07:57PM +0200, Mickaël Salaün wrote:
> > On Fri, Jul 24, 2026 at 06:10:01PM +0200, Günther Noack wrote:
> > > +
> > > +/**
> > > + * DOC: erratum_4
> > > + *
> > > + * Erratum 4: Creation of whiteout objects
> > > + * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > + *
> > > + * This fix addresses an issue through which it was possible to create whiteout
> > > + * objects, even when all file creation is restricted using Landlock.
> > > + *
> > > + * With this fix, the creation of whiteout objects is now guarded using
> > > + * ``LANDLOCK_ACCESS_FS_MAKE_REG``, both when it is done through
> > > + * :manpage:`renameat2(2)` with `RENAME_WHITEOUT`, and when it is done through
> > > + * :manpage:`mknod(2)` with ``S_IFCHR`` and ``makedev(0, 0)`` (which previously
> > > + * required ``LANDLOCK_ACCESS_FS_MAKE_CHAR``).
> > > + *
> > > + * Whiteout objects are special file types used in OverlayFS to mark the absence
> > > + * of a file in an upper file system, even when the lower (often read-only) file
> > > + * system does have a file with the same name.
> > > + *
> > > + * Impact:
> > > + *
> > > + * Without this fix, it was possible to create whiteout files from userspace
> > > + * using :manpage:`renameat2(2)` with the ``RENAME_WHITEOUT`` flag.
> > 
> > The errata should focus on the change of access rights which are needed
> > (and could potentially break some use cases), not to talk about the
> > RENAME_WHITEOUT (bypass) fix.  Most fixes don't get a Landlock errata
> > bit.  The impact should then be explicit that this is for sandboxed
> > programs such as fuse-overlayfs.
> > 
> > This patch does two things:
> > - fix the RENAME_WHITEOUT creating a whitout without being controlled
> >   (no errata, just a fix),
> > - and repurpose the MAKE_REG to control whitetout creation instead of
> >   relying on MAKE_CHAR (which needs an errata because it could break
> >   legitimate use cases/policies).
> 
> FYI, I'm thinking to change the phrasing to this:
> 
> /**
>  * DOC: erratum_4
>  *
>  * Erratum 4: Creation of whiteout objects
>  * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  *
>  * This fix changes the access rights required for the creation of whiteout
>  * objects through :manpage:`mknod(2)` with ``S_IFCHR`` and ``makedev(0, 0)``.

Adding renameat2 + RENAME_WHITEOUT would be better too.

>  * This way of creating whiteout objects is now guarded by
>  * ``LANDLOCK_ACCESS_FS_MAKE_REG`` instead of ``LANDLOCK_ACCESS_FS_MAKE_CHAR``.
>  *
>  * Whiteout objects are used in OverlayFS to mark the absence of a file in an
>  * upper file system.  Despite being created with ``S_IFCHR``, whiteout objects
>  * do not count as character devices.
>  *
>  * Impact:
>  *
>  * Sandboxed programs that create OverlayFS whiteouts (such as fuse-overlayfs)
>  * now require ``LANDLOCK_ACCESS_FS_MAKE_REG`` instead of
>  * ``LANDLOCK_ACCESS_FS_MAKE_CHAR``.
>  */
> LANDLOCK_ERRATUM(4)
> 
> Please let me know whether that sounds better.

This looks good, thanks!