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

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Message-ID: <CAJuCfpEqOUj8VPybstQjoJvCzyZtG6Q5Vr4WT0Lx_r3LFVS7og@mail.gmail.com>
Date: Fri, 1 Aug 2025 08:28:38 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org, 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 7:16 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hildenbrand 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.
> >
> > > 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".
> >
> > > 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)) {
>
> [1]
>
> > > +                           /* 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?
>
> One question might be relevant is, whether the check above [1] can be
> dropped.
>
> The thing is __pmd_trans_huge_lock() does double check the pmd to be !none
> before returning the ptl.  I didn't follow closely on the recent changes on
> mm side on possible new pmd swap entries, if migration is the only possible
> one then it looks like [1] can be avoided.

Hi Peter,
is_swap_pmd() check in __pmd_trans_huge_lock() allows for (!pmd_none()
&& !pmd_present()) PMD to pass and that's when this crash is hit.
If we drop the check at [1] then the path that takes us to
split_huge_pmd() will bail out inside split_huge_pmd_locked() with no
indication that split did not happen. Afterwards we will retry
thinking that PMD got split and leaving further remapping to
move_pages_pte() (see the comment before "continue"). I think in this
case it will end up in the same path again instead (infinite loop). I
didn't test this but from the code I think that's what would happen.
Does that make sense?

>
> And it also looks applicable to also drop the "else" later, because in "if
> (ptl)" it cannot hit pmd_none().
>
> Thanks,
>
> --
> Peter Xu
>