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 09:41:31AM -0700, Suren Baghdasaryan wrote:
> On Fri, Aug 1, 2025 at 9:23 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > 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?
> 
> 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]?
> 
> Yes, in case of migration entry this is what protects us.
> 
> >
> > > 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?
> 
> Hmm, my understanding is that it's being treated as a swap entry but
> in reality is not. I thought THPs are always split before they get
> swapped, no?

Yes they should be split, afaiu.

> 
> > Could you elaborate why the split would fail?
> 
> Just looking at the code, split_huge_pmd_locked() checks for
> (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)).
> pmd_trans_huge() is false if !pmd_present() and it's not a migration
> entry, so __split_huge_pmd_locked() will be skipped.

Here might be the major part of where confusion came from: I thought it
must be a migration pmd entry to hit the issue, so it's not?

I checked the code just now:

__handle_mm_fault:
		if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
			VM_BUG_ON(thp_migration_supported() &&
					  !is_pmd_migration_entry(vmf.orig_pmd));

So IIUC pmd migration entry is still the only possible way to have a swap
entry.  It doesn't look like we have "real" swap entries for PMD (which can
further points to some swapfiles)?

-- 
Peter Xu