Re: [PATCH] mm/userfaultfd: Allow hugetlb change protection upon poison entry

[Axel Rasmussen <axelrasmussen@google.com>]

Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

Thanks for the fix, Peter!


On Mon, Apr 8, 2024 at 11:59 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.04.24 01:19, peterx@redhat.com wrote:
> > From: Peter Xu <peterx@redhat.com>
> >
> > After UFFDIO_POISON, there can be two kinds of hugetlb pte markers, either
> > the POISON one or UFFD_WP one.
> >
> > Allow change protection to run on a poisoned marker just like !hugetlb
> > cases, ignoring the marker irrelevant of the permission.
> >
> > Here the two bits are mutual exclusive. For example, when install a
> > poisoned entry it must not be UFFD_WP already (by checking pte_none()
> > before such install).  And it also means if UFFD_WP is set there must have
> > no POISON bit set.  It makes sense because UFFD_WP is a bit to reflect
> > permission, and permissions do not apply if the pte is poisoned and
> > destined to sigbus.
> >
> > So here we simply check uffd_wp bit set first, do nothing otherwise.
> >
> > Attach the Fixes to UFFDIO_POISON work, as before that it should not be
> > possible to have poison entry for hugetlb (e.g., hugetlb doesn't do swap,
> > so no chance of swapin errors).
> >
> > Cc: Axel Rasmussen <axelrasmussen@google.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: linux-stable <stable@vger.kernel.org> # 6.6+
> > Link: https://lore.kernel.org/r/000000000000920d5e0615602dd1@google.com
> > Reported-by: syzbot+b07c8ac8eee3d4d8440f@syzkaller.appspotmail.com
> > Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   mm/hugetlb.c | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 8267e221ca5d..ba7162441adf 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6960,9 +6960,13 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
> >                       if (!pte_same(pte, newpte))
> >                               set_huge_pte_at(mm, address, ptep, newpte, psize);
> >               } else if (unlikely(is_pte_marker(pte))) {
> > -                     /* No other markers apply for now. */
> > -                     WARN_ON_ONCE(!pte_marker_uffd_wp(pte));
> > -                     if (uffd_wp_resolve)
> > +                     /*
> > +                      * Do nothing on a poison marker; page is
> > +                      * corrupted, permissons do not apply.  Here
> > +                      * pte_marker_uffd_wp()==true implies !poison
> > +                      * because they're mutual exclusive.
> > +                      */
> > +                     if (pte_marker_uffd_wp(pte) && uffd_wp_resolve)
> >                               /* Safe to modify directly (non-present->none). */
> >                               huge_pte_clear(mm, address, ptep, psize);
> >               } else if (!huge_pte_none(pte)) {
>
> Reviewed-by: David Hildenbrand <david@redhat.com>
>
> --
> Cheers,
>
> David / dhildenb
>