Re: [PATCH v2] mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork()

[David Hildenbrand (Arm)]

On 7/8/26 11:01, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> 
> copy_hugetlb_page_range() clears the uffd-wp bit of migration and
> hwpoison entries with huge_pte_clear_uffd_wp(), which operates on the
> present-PTE bit position. Swap entries keep the uffd-wp state elsewhere
> -- the migration branch reads and sets 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 clear 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.
> 
> Migration entries legitimately carry uffd-wp, so clear it with
> pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and
> move_huge_pte().
> 
> A hwpoison entry, on the other hand, never carries the uffd-wp bit: it
> is installed fresh by make_hwpoison_entry() (try_to_unmap_one() does not
> preserve uffd-wp on the hwpoison path) and hugetlb_change_protection()
> leaves hwpoison entries untouched. There was nothing to clear there,
> only the corruption, so drop the clear entirely.
> 
> Reported-by: Sashiko AI review <sashiko-bot@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@xxxxxxxxxxxxxxx/
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> 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
> ---

Thanks!

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David