[PATCH] mm: userfaultfd: fix race of userfaultfd_move and swap cache - Linux-stable-mirror

30 May 2025

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... [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;
+		}
    }
orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
@@ -1409,6 +1425,16 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
    			folio_lock(src_folio);
    			goto retry;
    		}
+			/*
+			 * 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 (unlikely(!folio_test_swapcache(folio) ||
+				     entry.val != folio->swap.val)) {
+				err = -EAGAIN;
+				goto out;
+			}
    	}
    	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,
-- 
2.49.0



    