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Message-ID: <aEBnjWkghvXqlYZo@x1.local>
Date: Wed, 4 Jun 2025 11:34:37 -0400
From: Peter Xu <peterx@...hat.com>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Barry Song <21cnbao@...il.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
David Hildenbrand <david@...hat.com>,
Lokesh Gidra <lokeshgidra@...gle.com>, stable@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm: userfaultfd: fix race of userfaultfd_move and
swap cache
On Wed, Jun 04, 2025 at 11:10:38PM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.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@...r.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@...cent.com>
> Reviewed-by: Lokesh Gidra <lokeshgidra@...gle.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@...hat.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
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