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

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Message-ID: <CAJuCfpExxYOtsWZo6r0FncA0TMeuhpe3SdhLbF+udtbqQ+B_Qg@mail.gmail.com>
Date: Thu, 7 Aug 2025 08:27:07 -0700
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 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?

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