linux-kernel - Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO

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Message-ID: <CAJuCfpHkxe1Sb3jL6hK02+zQVFw7yOYte3BR5XDnHRvx7aTjNg@mail.gmail.com>
Date: Fri, 1 Aug 2025 08:11:54 -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 v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles
 a THP hole

On Fri, Aug 1, 2025 at 7:21 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 31.07.25 17:44, Suren Baghdasaryan wrote:
>
> Hi!
>
> Did you mean in you patch description:
>
> "userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs"
>
> Talking about THP holes is very very confusing.

Hi David,
Yes, "hole" is not a technical term, so I'll change as you suggested.

>
> > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> > encounters a non-present THP, it fails to properly recognize an unmapped
>
> You mean a "non-present PMD that is not a migration entry".

Yes, will fix.


>
> > hole and tries to access a non-existent folio, resulting in
> > a crash. Add a check to skip non-present THPs.
>
> That makes sense. The code we have after this patch is rather
> complicated and hard to read.
>
> >
> > 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 v1 [1]
> > - Fixed step size calculation, per Lokesh Gidra
> > - Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra
> >
> > [1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/
> >
> >   mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++----------------
> >   1 file changed, 29 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index cbed91b09640..b5af31c22731 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> >
> >               ptl = pmd_trans_huge_lock(src_pmd, src_vma);
> >               if (ptl) {
> > -                     /* 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 (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) {
> > +                             /* 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)) {
> > +                                     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;
> > +                                             }
> > +                                     }
>
> ... in particular that. Is there some way to make this code simpler /
> easier to read? Like moving that whole last folio-check thingy into a
> helper?

Do you mean refactor the section after "if (ptf)" into a separate function?
I was trying to minimize the code changes to simplify backporting but
since additional indentation changes this whole block, I think it does
not make much difference. Please let me know if I understood you
correctly and I'll move the code into a separate function.
Thanks,
Suren.

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