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.
Testing with a simple C program to allocate and move 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... [1] Signed-off-by: Kairui Song kasong@tencent.com
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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.
mm/userfaultfd.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index bc473ad21202..5dc05346e360 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 +1112,15 @@ 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. Or we might miss a new loaded swap cache folio. + */ + 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 +1431,7 @@ 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: