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Message-ID: <43f91e3e-84c5-4fd1-9b63-4e2cb28dab36@redhat.com>
Date: Thu, 7 Aug 2025 21:42:47 +0200
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, peterx@...hat.com, aarcange@...hat.com,
lokeshgidra@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com, stable@...r.kernel.org
Subject: Re: [PATCH v4 1/1] userfaultfd: fix a crash in UFFDIO_MOVE when PMD
is a migration entry
On 07.08.25 17:27, Suren Baghdasaryan wrote:
> On Thu, Aug 7, 2025 at 3:31 AM David Hildenbrand <david@...hat.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@...kaller.appspotmail.com
>>> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
>>> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
>>> Reviewed-by: Peter Xu <peterx@...hat.com>
>>> Cc: stable@...r.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
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