Re: [PATCH] mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()

[Kiryl Shutsemau]

On Tue, Jul 07, 2026 at 05:05:49PM +0200, David Hildenbrand (Arm) wrote:
> On 7/3/26 18:18, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> > 
> > copy_hugetlb_page_range() clears the uffd-wp bit of hwpoison and
> > migration entries with huge_pte_clear_uffd_wp(), which operates on the
> > present-PTE bit position. Swap entries keep the uffd-wp state elsewhere
> > -- the same branches read and set it with pte_swp_uffd_wp() and
> > pte_swp_mkuffd_wp() -- and the present-PTE position falls into the swap
> > payload. On x86-64 it lands in the inverted swap offset, where a
> > naturally-aligned hugetlb PFN always has the affected bit set, so the
> > clear advances the encoded PFN by two pages.
> > 
> > No userfaultfd needs to be involved: the clear is guarded only by the
> > child VMA not being uffd-wp registered, so a plain fork() with an
> > in-flight hugetlb migration entry (or a poisoned hugetlb page) corrupts
> > the entry copied into the child. Instrumenting the hwpoison branch and
> > forking after MADV_HWPOISON on a 2MB anon hugetlb page shows:
> > 
> >   offset before=120e00
> >   offset after =120e02
> > 
> > The fallout is mostly latent: rmap walks match migration entries by
> > folio range and remove_migration_pte() rebuilds the PTE from the folio,
> > so a within-folio PFN skew heals once migration completes. But any path
> > that re-encodes the corrupted offset -- e.g. hugetlb_change_protection()
> > rewriting a writable migration entry via
> > make_readable_migration_entry(swp_offset(entry)) -- propagates it, and
> > an hwpoison entry misidentifies which page is poisoned.
> > 
> > Use pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and
> > move_huge_pte().
> > 
> > Reported-by: Sashiko AI review <sashiko-bot@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@xxxxxxxxxxxxxxx/
> > Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> > Assisted-by: Claude:claude-fable-5
> > ---
> >  mm/hugetlb.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 571212b80835..a4e6dd3a82f4 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4918,7 +4918,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> >  		softleaf = softleaf_from_pte(entry);
> >  		if (unlikely(softleaf_is_hwpoison(softleaf))) {
> >  			if (!userfaultfd_wp(dst_vma))
> > -				entry = huge_pte_clear_uffd_wp(entry);
> > +				entry = pte_swp_clear_uffd_wp(entry);
> 
> I think installing a hwpoison pte will actually drop the uffd marker.
> 
> hugetlb_change_protection() does nothing on hwpoison entrues.
> 
> So how could be possibly get a hwpoison entry with an uffd-wp bit set here?
> 
> If we indeed can't, Id assume there is nothing to clear here at all.

You are right.

I am inclined to remove it in a separate cleanup patch. Any objections?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov