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

[David Hildenbrand (Arm)]

On 7/7/26 19:05, Kiryl Shutsemau wrote:
> 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?

I'd just do it in this patch -- removing unnecessary code instead of fixing it.

However, if there are strong opinions, I don't care that much.

-- 
Cheers,

David