Re: [PATCH v4] mm: userfaultfd: fix race of userfaultfd_move and swap cache

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jun 4, 2025 at 8:34 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Jun 04, 2025 at 11:10:38PM +0800, Kairui Song wrote:
> > From: Kairui Song <kasong@tencent.com>
> >
> > On seeing a swap entry PTE, userfaultfd_move does a lockless swap
> > cache lookup, and tries to move the found folio to the faulting vma.
> > Currently, it relies on checking the PTE value to ensure that the moved
> > folio still belongs to the src swap entry and that no new folio has
> > been added to the swap cache, which turns out to be unreliable.
> >
> > While working and reviewing the swap table series with Barry, following
> > existing races are observed and reproduced [1]:
> >
> > In the example below, move_pages_pte is moving src_pte to dst_pte,
> > where src_pte is a swap entry PTE holding swap entry S1, and S1
> > is not in the swap cache:
> >
> > CPU1                               CPU2
> > userfaultfd_move
> >   move_pages_pte()
> >     entry = pte_to_swp_entry(orig_src_pte);
> >     // Here it got entry = S1
> >     ... < interrupted> ...
> >                                    <swapin src_pte, alloc and use folio A>
> >                                    // folio A is a new allocated folio
> >                                    // and get installed into src_pte
> >                                    <frees swap entry S1>
> >                                    // src_pte now points to folio A, S1
> >                                    // has swap count == 0, it can be freed
> >                                    // by folio_swap_swap or swap
> >                                    // allocator's reclaim.
> >                                    <try to swap out another folio B>
> >                                    // folio B is a folio in another VMA.
> >                                    <put folio B to swap cache using S1 >
> >                                    // S1 is freed, folio B can use it
> >                                    // for swap out with no problem.
> >                                    ...
> >     folio = filemap_get_folio(S1)
> >     // Got folio B here !!!
> >     ... < interrupted again> ...
> >                                    <swapin folio B and free S1>
> >                                    // Now S1 is free to be used again.
> >                                    <swapout src_pte & folio A using S1>
> >                                    // Now src_pte is a swap entry PTE
> >                                    // holding S1 again.
> >     folio_trylock(folio)
> >     move_swap_pte
> >       double_pt_lock
> >       is_pte_pages_stable
> >       // Check passed because src_pte == S1
> >       folio_move_anon_rmap(...)
> >       // Moved invalid folio B here !!!
> >
> > The race window is very short and requires multiple collisions of
> > multiple rare events, so it's very unlikely to happen, but with a
> > deliberately constructed reproducer and increased time window, it
> > can be reproduced easily.
> >
> > This can be fixed by checking if the folio returned by filemap is the
> > valid swap cache folio after acquiring the folio lock.
> >
> > Another similar race is possible: filemap_get_folio may return NULL, but
> > folio (A) could be swapped in and then swapped out again using the same
> > swap entry after the lookup. In such a case, folio (A) may remain in the
> > swap cache, so it must be moved too:
> >
> > CPU1                               CPU2
> > userfaultfd_move
> >   move_pages_pte()
> >     entry = pte_to_swp_entry(orig_src_pte);
> >     // Here it got entry = S1, and S1 is not in swap cache
> >     folio = filemap_get_folio(S1)
> >     // Got NULL
> >     ... < interrupted again> ...
> >                                    <swapin folio A and free S1>
> >                                    <swapout folio A re-using S1>
> >     move_swap_pte
> >       double_pt_lock
> >       is_pte_pages_stable
> >       // Check passed because src_pte == S1
> >       folio_move_anon_rmap(...)
> >       // folio A is ignored !!!
> >
> > Fix this by checking the swap cache again after acquiring the src_pte
> > lock. And to avoid the filemap overhead, we check swap_map directly [2].
> >
> > The SWP_SYNCHRONOUS_IO path does make the problem more complex, but so
> > far we don't need to worry about that, since folios can only be exposed
> > to the swap cache in the swap out path, and this is covered in this
> > patch by checking the swap cache again after acquiring the src_pte lock.
> >
> > Testing with a simple C program that allocates and moves several GB of
> > memory did not show any observable performance change.
> >
> > Cc: <stable@vger.kernel.org>
> > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > Closes: https://lore.kernel.org/linux-mm/CAMgjq7B1K=6OOrK2OUZ0-tqCzi+EJt+2_K97TPGoSt=9+JwP7Q@mail.gmail.com/ [1]
> > Link: https://lore.kernel.org/all/CAGsJ_4yJhJBo16XhiC-nUzSheyX-V3-nFE+tAi=8Y560K8eT=A@mail.gmail.com/ [2]
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > Reviewed-by: Lokesh Gidra <lokeshgidra@google.com>

Very interesting races. Thanks for the fix!

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> >
> > ---
> >
> > V1: https://lore.kernel.org/linux-mm/20250530201710.81365-1-ryncsn@gmail.com/
> > Changes:
> > - Check swap_map instead of doing a filemap lookup after acquiring the
> >   PTE lock to minimize critical section overhead [ Barry Song, Lokesh Gidra ]
> >
> > V2: https://lore.kernel.org/linux-mm/20250601200108.23186-1-ryncsn@gmail.com/
> > Changes:
> > - Move the folio and swap check inside move_swap_pte to avoid skipping
> >   the check and potential overhead [ Lokesh Gidra ]
> > - Add a READ_ONCE for the swap_map read to ensure it reads a up to dated
> >   value.
> >
> > V3: https://lore.kernel.org/all/20250602181419.20478-1-ryncsn@gmail.com/
> > Changes:
> > - Add more comments and more context in commit message.
> >
> >  mm/userfaultfd.c | 33 +++++++++++++++++++++++++++++++--
> >  1 file changed, 31 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index bc473ad21202..8253978ee0fb 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -1084,8 +1084,18 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> >                        pte_t orig_dst_pte, pte_t orig_src_pte,
> >                        pmd_t *dst_pmd, pmd_t dst_pmdval,
> >                        spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > -                      struct folio *src_folio)
> > +                      struct folio *src_folio,
> > +                      struct swap_info_struct *si, swp_entry_t entry)
> >  {
> > +     /*
> > +      * Check if the folio still belongs to the target swap entry after
> > +      * acquiring the lock. Folio can be freed in the swap cache while
> > +      * not locked.
> > +      */
> > +     if (src_folio && unlikely(!folio_test_swapcache(src_folio) ||
> > +                               entry.val != src_folio->swap.val))
> > +             return -EAGAIN;
> > +
> >       double_pt_lock(dst_ptl, src_ptl);
> >
> >       if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > @@ -1102,6 +1112,25 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> >       if (src_folio) {
> >               folio_move_anon_rmap(src_folio, dst_vma);
> >               src_folio->index = linear_page_index(dst_vma, dst_addr);
> > +     } else {
> > +             /*
> > +              * Check if the swap entry is cached after acquiring the src_pte
> > +              * lock. Otherwise, we might miss a newly loaded swap cache folio.
> > +              *
> > +              * Check swap_map directly to minimize overhead, READ_ONCE is sufficient.
> > +              * We are trying to catch newly added swap cache, the only possible case is
> > +              * when a folio is swapped in and out again staying in swap cache, using the
> > +              * same entry before the PTE check above. The PTL is acquired and released
> > +              * twice, each time after updating the swap_map's flag. So holding
> > +              * the PTL here ensures we see the updated value. False positive is possible,
> > +              * e.g. SWP_SYNCHRONOUS_IO swapin may set the flag without touching the
> > +              * cache, or during the tiny synchronization window between swap cache and
> > +              * swap_map, but it will be gone very quickly, worst result is retry jitters.
> > +              */
>
> The comment above may not be the best I can think of, but I think I'm
> already too harsh. :)  That's good enough to me.  It's also great to
> mention the 2nd race too as Barry suggested in the commit log.
>
> Thank you!
>
> Acked-by: Peter Xu <peterx@redhat.com>
>
> > +             if (READ_ONCE(si->swap_map[swp_offset(entry)]) & SWAP_HAS_CACHE) {
> > +                     double_pt_unlock(dst_ptl, src_ptl);
> > +                     return -EAGAIN;
> > +             }
> >       }
> >
> >       orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
> > @@ -1412,7 +1441,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >               }
> >               err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
> >                               orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
> > -                             dst_ptl, src_ptl, src_folio);
> > +                             dst_ptl, src_ptl, src_folio, si, entry);
> >       }
> >
> >  out:
> > --
> > 2.49.0
> >
>
> --
> Peter Xu
>