Re: [PATCH v4 1/1] userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

[David Hildenbrand <david@redhat.com>]

On 07.08.25 17:27, Suren Baghdasaryan wrote:
> On Thu, Aug 7, 2025 at 3:31 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 07.08.25 00:00, Suren Baghdasaryan wrote:
>>> When UFFDIO_MOVE encounters a migration PMD entry, it proceeds with
>>> obtaining a folio and accessing it even though the entry is swp_entry_t.
>>> Add the missing check and let split_huge_pmd() handle migration entries.
>>>
>>> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
>>> Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
>>> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>> Reviewed-by: Peter Xu <peterx@redhat.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>> Changes since v3 [1]
>>> - Updated the title and changelog, per Peter Xu
>>> - Added Reviewed-by: per Peter Xu
>>>
>>> [1] https://lore.kernel.org/all/20250806154015.769024-1-surenb@google.com/
>>>
>>>    mm/userfaultfd.c | 17 ++++++++++-------
>>>    1 file changed, 10 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>>> index 5431c9dd7fd7..116481606be8 100644
>>> --- a/mm/userfaultfd.c
>>> +++ b/mm/userfaultfd.c
>>> @@ -1826,13 +1826,16 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>>>                        /* Check if we can move the pmd without splitting it. */
>>>                        if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
>>>                            !pmd_none(dst_pmdval)) {
>>> -                             struct folio *folio = pmd_folio(*src_pmd);
>>> -
>>> -                             if (!folio || (!is_huge_zero_folio(folio) &&
>>> -                                            !PageAnonExclusive(&folio->page))) {
>>> -                                     spin_unlock(ptl);
>>> -                                     err = -EBUSY;
>>> -                                     break;
>>> +                             /* Can be a migration entry */
>>> +                             if (pmd_present(*src_pmd)) {
>>> +                                     struct folio *folio = pmd_folio(*src_pmd);
>>> +
>>> +                                     if (!folio
>>
>>
>> How could you get !folio here? That only makes sense when calling
>> vm_normal_folio_pmd(), no?
> 
> Yes, I think you are right, this check is not needed. I can fold it
> into this fix or post a separate cleanup patch. I'm guessing a
> separate patch would be better?

I think you can just post a fixup inline here and ask Andrew to squash 
it. He will shout if he wants a completely new version :)

-- 
Cheers,

David / dhildenb