linux-kernel - Re: [PATCH v4 1/1] userfaultfd: fix a crash in UFFDIO

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Message-ID: <CAJuCfpF+OFZ3RNFaBVBQWmCG_EKxj9Jrb4QGz-Ladw3HJ9AN8A@mail.gmail.com>
Date: Thu, 7 Aug 2025 20:05:18 +0000
From: Suren Baghdasaryan <surenb@...gle.com>
To: David Hildenbrand <david@...hat.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 Thu, Aug 7, 2025 at 7:48 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Thu, Aug 7, 2025 at 7:42 PM David Hildenbrand <david@...hat.com> wrote:
> >
> > 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 :)
>
> I wouldn't do that to him! :)
> Let me quickly send an updated version instead.

Update posted at
https://lore.kernel.org/all/20250807200418.1963585-1-surenb@google.com/

>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >