Re: [PATCH] mm/gup: check folio->mapping under folio lock in memfd_pin_folios()

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 31 Oct 2026 20:20:03 -0700 Andy Lutomirski <luto@amacapital.net> wrote:

> On Sat, Oct 31, 2026 at 7:40 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Sat, 31 Oct 2026 20:35:40 +0000 Samiullah Khawaja <skhawaja@google.com> wrote:
> >
> > > memfd_pin_folios() pins folios from the page cache without taking the
> > > folio lock. So khugepaged or MADV_COLLAPSE can replace the folio after
> > > its refcount check while memfd_pin_folios() still pins the old folio,
> > > and the caller ends up with a folio that is no longer part of the memfd.
> > >
> > > Lock the folio and recheck folio->mapping before pinning it, like other
> > > page cache users do. If the folio was removed, retry the folio lookup
> > > from the index that was removed.
> >
> > Can we please include a clear description of the userspace-visible
> > runtime effects of the bug?
> 
> It's straight-up data corruption if you manage to trigger it, no?  If
> you manage to race a memfd_pin_folios() against collapse,
> memfd_pin_folios() will pin the wrong folio.
> 
> Fortunately, there are a whopping three callers in the whole tree.
> You can hit it from udmabuf or iommufd or memfd LUO.
> 
> The former two will direct DMA to the wrong folio.  Fortunately it's
> not UAF as the DMA will be directed to a folio that is still pinned,
> but it's the wrong one.  DMA reads will read potentially stale data
> and DMA writes will overwrite that stale data but not update the live
> copy.  (The DMA will not be directed to some other application's
> memory, just to a leftover copy of the calling application's memory.)
> 
> As far as I know, the effect with LUO would be for stale data to
> reappear in the memfd after handover.  With the patchset that started
> this whole discussion applied, I think it might result in misdirected
> DMA to freed memory (I do not stand by that claim at all), but that
> patchset is not applied.

Thanks.  How's this for a changelog?

: memfd_pin_folios() pins folios from the page cache without taking the
: folio lock. khugepaged or MADV_COLLAPSE can therefore replace a folio
: after checking its refcount while memfd_pin_folios() still pins the old
: folio. The caller then receives a folio which is no longer part of the
: memfd.
: 
: For udmabuf and iommufd, this can cause data corruption: DMA accesses
: the detached folio rather than the live memfd contents. DMA reads can
: return stale data, while DMA writes update the detached copy and are
: lost from the live memfd. The detached folio remains pinned, so these
: accesses do not constitute a use-after-free or target another
: application's memory. For memfd live update, the stale contents can
: reappear in the memfd after handover.
: 
: Lock the folio and recheck folio->mapping before pinning it, like other
: page cache users do. If the folio was removed, retry the folio lookup
: from the index that was removed.

Samiullah's patch assumes the presence of Chris Mason's "mm/gup: fix
clearing folios in memfd_pin_folios()" which is in mm-next, so... 
->David