Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasaryan wrote:
> On Fri, Aug 1, 2025 at 7:16 AM Peter Xu <peterx@redhat.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@syzkaller.appspotmail.com
> > > > Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > Cc: stable@vger.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)) {

[2]

> > > > +                                           struct folio *folio = pmd_folio(*src_pmd);

[3]

> > > > +
> > > > +                                           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.

First for all, thanks for looking into the issue with Lokesh; I am still
catching up with emails after taking weeks off.

I didn't yet read into the syzbot report, but I thought the bug was about
referencing the folio on top of a swap entry after reading your current
patch, which has:

	if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
	    !pmd_none(dst_pmdval)) {
		struct folio *folio = pmd_folio(*src_pmd); <----

Here looks like *src_pmd can be a migration entry. Is my understanding
correct?

> If we drop the check at [1] then the path that takes us to

If my above understanding is correct, IMHO it should be [2] above that
makes sure the reference won't happen on a swap entry, not necessarily [1]?

> split_huge_pmd() will bail out inside split_huge_pmd_locked() with no
> indication that split did not happen. Afterwards we will retry

So we're talking about the case where it's a swap pmd entry, right?  Could
you elaborate why the split would fail?  AFAIU, split_huge_pmd_locked()
should still work for a migration pmd entry.

Thanks,

> 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
> >
> 

-- 
Peter Xu