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

[Barry Song <21cnbao@gmail.com>]

On Sat, May 31, 2025 at 7:06 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Sat, May 31, 2025 at 7:00 PM Kairui Song <ryncsn@gmail.com> wrote:
> >
> > On Sat, May 31, 2025 at 2:36 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Sat, May 31, 2025 at 6:25 PM Kairui Song <ryncsn@gmail.com> wrote:
> > > >
> > > > On Sat, May 31, 2025 at 11:39 AM Barry Song <21cnbao@gmail.com> wrote:
> > > > >
> > > > > On Sat, May 31, 2025 at 11:40 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
> > > > > >
> > > > > > On Fri, May 30, 2025 at 1:17 PM 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].
> > > > > >
> > > > > > Thanks for catching and fixing this. Just to clarify a few things
> > > > > > about your reproducer:
> > > > > > 1. Is it necessary for the 'race' mapping to be MAP_SHARED, or
> > > > > > MAP_PRIVATE will work as well?
> > > > > > 2. You mentioned that the 'current dir is on a block device'. Are you
> > > > > > indicating that if we are using zram for swap then it doesn't
> > > > > > reproduce?
> > > > > >
> > > > > > >
> > > > > > > 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>
> > > > > > I guess you mistakenly left it from your reproducer code :)
> > > > > > >  #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));
> > > > > >
> > > > > > Given the non-trivial overhead of filemap_get_folio(), do you think it
> > > > > > will work if filemap_get_filio() was only once after locking src_ptl?
> > > > > > Please correct me if my assumption about the overhead is wrong.
> > > > >
> > > > > not quite sure as we have a folio_lock(src_folio) before move_swap_pte().
> > > > > can we safely folio_move_anon_rmap + src_folio->index while not holding
> > > > > folio lock?
> > > >
> > > > I think no, we can't even make sure the folio is still in the swap
> > > > cache, so it can be a freed folio that does not belong to any VMA
> > > > while not holding the folio lock.
> > >
> > > Right, but will the following be sufficient, given that we don’t really
> > > care about the folio—only whether there’s new cache?
> > >
> > > if (READ_ONCE(si->swap_map[offset]) & SWAP_HAS_CACHE) {
> > >              double_pt_unlock(dst_ptl, src_ptl);
> > >              return -EAGAIN;
> > > }
> >
> > The problem is reading swap_map without locking the cluster map seems
> > unstable, and has strange false positives, a swapin will set this bit
> > first, while not adding the folio to swap cache or even when skipping
> > the swap cache, that seems could make it more complex.
>
> As long as it's a false positive and not a false negative, I think it's
> acceptable—especially if we're concerned about the overhead of
> filemap_get_folio. The probability is extremely low (practically close
> to 0%), but we still need to call filemap_get_folio for every swap PTE.

By the way, if we do want to eliminate the false positives, we could do
the following:

if (READ_ONCE(si->swap_map[offset]) & SWAP_HAS_CACHE) {
              folio = filemap_get_folio()
              if (folio....) {
                   double_pt_unlock(dst_ptl, src_ptl);
                   return -EAGAIN;
              }
}

This might eliminate 99.999999...% of filemap_get_folio calls, though my
gut feeling is that it might not be necessary :-)

Thanks
Barry