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

[Kairui Song <ryncsn@gmail.com>]

On Sat, May 31, 2025 at 2:10 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> On Fri, May 30, 2025 at 9:42 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Sat, May 31, 2025 at 4:04 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Sat, May 31, 2025 at 8:17 AM Kairui Song <ryncsn@gmail.com> wrote:
> > > >
> > > > From: Kairui Song <kasong@tencent.com>
> > > >
> > > > On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache
> > > > lookup, and try to move the found folio to the faulting vma when.
> > > > Currently, it relies on the PTE value check to ensure the moved folio
> > > > still belongs to the src swap entry, which turns out is not reliable.
> > > >
> > > > While working and reviewing the swap table series with Barry, following
> > > > existing race is observed and reproduced [1]:
> > > >
> > > > ( move_pages_pte is moving src_pte to dst_pte, where src_pte is a
> > > >  swap entry PTE holding swap entry S1, and S1 isn't in the swap cache.)
> > > >
> > > > CPU1                               CPU2
> > > > userfaultfd_move
> > > >   move_pages_pte()
> > > >     entry = pte_to_swp_entry(orig_src_pte);
> > > >     // Here it got entry = S1
> > > >     ... < Somehow interrupted> ...
> > > >                                    <swapin src_pte, alloc and use folio A>
> > > >                                    // folio A is just 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 could use it
> > > >                                    // for swap out with no problem.
> > > >                                    ...
> > > >     folio = filemap_get_folio(S1)
> > > >     // Got folio B here !!!
> > > >     ... < Somehow 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 [1].
> > > >
> > > > It's also possible that folio (A) is swapped in, and swapped out again
> > > > after the filemap_get_folio lookup, in such case folio (A) may stay in
> > > > swap cache so it needs to be moved too. In this case we should also try
> > > > again so kernel won't miss a folio move.
> > > >
> > > > Fix this by checking if the folio is the valid swap cache folio after
> > > > acquiring the folio lock, and checking the swap cache again after
> > > > acquiring the src_pte lock.
> > > >
> > > > SWP_SYNCRHONIZE_IO path does make the problem more complex, but so far
> > > > we don't need to worry about that since folios only might get exposed to
> > > > swap cache in the swap out path, and it's covered in this patch too by
> > > > checking the swap cache again after acquiring src_pte lock.
> > > >
> > > > 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]
> > > > Signed-off-by: Kairui Song <kasong@tencent.com>
> > > > ---
> > > >  mm/userfaultfd.c | 26 ++++++++++++++++++++++++++
> > > >  1 file changed, 26 insertions(+)
> > > >
> > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > > index bc473ad21202..a1564d205dfb 100644
> > > > --- a/mm/userfaultfd.c
> > > > +++ b/mm/userfaultfd.c
> > > > @@ -15,6 +15,7 @@
> > > >  #include <linux/mmu_notifier.h>
> > > >  #include <linux/hugetlb.h>
> > > >  #include <linux/shmem_fs.h>
> > > > +#include <linux/delay.h>
> > > >  #include <asm/tlbflush.h>
> > > >  #include <asm/tlb.h>
> > > >  #include "internal.h"
> > > > @@ -1086,6 +1087,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> > > >                          spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > > >                          struct folio *src_folio)
> > > >  {
> > > > +       swp_entry_t entry;
> > > > +
> > > >         double_pt_lock(dst_ptl, src_ptl);
> > > >
> > > >         if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > > > @@ -1102,6 +1105,19 @@ 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 again after acquiring the src_pte lock. Or we might
> > > > +                * miss a new loaded swap cache folio.
> > > > +                */
> > > > +               entry = pte_to_swp_entry(orig_src_pte);
> > > > +               src_folio = filemap_get_folio(swap_address_space(entry),
> > > > +                                             swap_cache_index(entry));
> > > > +               if (!IS_ERR_OR_NULL(src_folio)) {
> > > > +                       double_pt_unlock(dst_ptl, src_ptl);
> > > > +                       folio_put(src_folio);
> > > > +                       return -EAGAIN;
> > > > +               }
> > > >         }
> > >
> > > step 1: src pte points to a swap entry without swapcache
> > > step 2: we call move_swap_pte()
> > > step 3: someone swap-in src_pte by swap_readhead() and make src_pte's swap entry
> > > have swapcache again - for non-sync/non-zRAM swap device;
> > > step 4: move_swap_pte() gets ptl, move src_pte to dst_pte and *clear* src_pte;
> > > step 5: do_swap_page() for src_pte holds the ptl and found pte has
> > > been cleared in
> > >             step 4; pte_same() returns false;
> > > step 6: do_swap_page() won't map src_pte to the new swapcache got from step 3;
> > >             if the swapcache folio is dropped, it seems everything is fine.
> > >
> > > So the real issue is that do_swap_page() doesn’t drop the new swapcache
> > > even when pte_same() returns false? That means the dst_pte swap-in
> > > can still hit the swap cache entry brought in by the src_pte's swap-in?
> >
> > It seems also possible for the sync zRAM device.
> >
> >  step 1: src pte points to a swap entry S without swapcache
> >  step 2: we call move_swap_pte()
> >  step 3: someone swap-in src_pte by sync path, no swapcache; swap slot
> > S is freed.
> >              -- for zRAM;
> >  step 4: someone swap-out src_pte, get the exactly same swap slot S as step 1,
> >              adds folio to swapcache due to swapout;
> >  step 5: move_swap_pte() gets ptl and finds page tables are stable
> > since swap-out
> >              happens to have the same swap slot as step1;
> >  step 6: we clear src_pte, move src_pte to dst_pte; but miss to move the folio.
> >
> > Yep, we really need to re-check pte for swapcache after holding PTL.
> >
> Any idea what is the overhead of filemap_get_folio()? In particular,
> when no folio exists for the given entry, how costly is it?
>
> Given how rare it is, unless filemap_get_folio() is cheap for 'no
> folio' case, if there is no way to avoid calling it after holding PTL,
> then we should do it only once at that point. If a folio is returned,
> then like in the pte_present() case, we attempt folio_trylock() with
> PTL held, otherwise do the retry dance.

Yeah I think filemap_get_folio is cheap, each swap cache space is at
most 64M big, so it just walks at most three xa_nodes and returns, not
involving any synchronization or write.

The swap cache lookup will be even cheaper in the future to be just
checking one plain array element.

I can try to fix this with the folio_trylock inside the PTL lock
approach, maybe the code will be cleaner that way.