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... [1] Link: https://lore.kernel.org/all/CAGsJ_4yJhJBo16XhiC-nUzSheyX-V3-nFE+tAi=8Y560K8e... [2] Signed-off-by: Kairui Song kasong@tencent.com Reviewed-by: Lokesh Gidra lokeshgidra@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