linux-kernel - Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO

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Message-ID: <CAJuCfpGGGJfnvzzdhOEwsXRWPm1nJoPcm2FcrYnkcJtc9W96gA@mail.gmail.com>
Date: Wed, 6 Aug 2025 10:09:30 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: akpm@...ux-foundation.org, david@...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 v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some
 non-present PMDs

On Wed, Aug 6, 2025 at 9:56 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
>
> The migration entry can appear with/without ALLOW_SRC_HOLES, right?  Maybe
> drop this line?

Yes, you are right. I'll update.

>
> If we need another repost, the subject can further be tailored to mention
> migration entry too rather than non-present.  IMHO that's clearer on
> explaining the issue this patch is fixing (e.g. a valid transhuge THP can
> also have present bit cleared).
>
> > encounters a non-present PMD (migration entry), it proceeds with folio
> > access even though the folio is not present. Add the missing check and
>
> IMHO "... even though folio is not present" is pretty vague.  Maybe
> "... even though it's a swap entry"?  Fundamentally it's because of the
> different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
> not be used on top of swap entries.

Well, technically a migration entry is a non_swap_entry(), so calling
migration entries "swap entries" is confusing to me. Any better
wording we can use or do you think that's ok?

>
> > 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>
> > Cc: stable@...r.kernel.org
> > ---
> > Changes since v2 [1]
> > - Updated the title and changelog, per David Hildenbrand
> > - Removed extra checks for non-present not-migration PMD entries,
> > per Peter Xu
> >
> > [1] https://lore.kernel.org/all/20250731154442.319568-1-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 || (!is_huge_zero_folio(folio) &&
> > +                                                    !PageAnonExclusive(&folio->page))) {
> > +                                             spin_unlock(ptl);
> > +                                             err = -EBUSY;
> > +                                             break;
> > +                                     }
> >                               }
>
> The change itself looks all correct, thanks.  If you agree with above
> commit message / subject updates, feel free to take this after some
> amendment of the commit message:
>
> Reviewed-by: Peter Xu <peterx@...hat.com>
>
> >
> >                               spin_unlock(ptl);
> >
> > base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
> > --
> > 2.50.1.565.gc32cd1483b-goog
> >
>
> --
> Peter Xu
>