[PATCH] mm, swap: fix potential UAF issue for VMA readahead

[Kairui Song]

From: Kairui Song <kasong@xxxxxxxxxxx>

Since commit 78524b05f1a3 ("mm, swap: avoid redundant swap device
pinning"), the common helper for allocating and preparing a folio in the
swap cache layer no longer tries to get a swap device reference
internally, because all callers of __read_swap_cache_async are already
holding a swap entry reference. The repeated swap device pinning isn't
needed on the same swap device.

Caller of VMA readahead is also holding a reference to the target
entry's swap device, but VMA readahead walks the page table, so it might
encounter swap entries from other devices, and call
__read_swap_cache_async on another device without holding a reference to
it.

So it is possible to cause a UAF when swapoff of device A raced with
swapin on device B, and VMA readahead tries to read swap entries from
device A. It's not easy to trigger, but in theory, it could cause real
issues.

Make VMA readahead try to get the device reference first if the swap
device is a different one from the target entry.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning")
Suggested-by: Huang Ying <ying.huang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
Sending as a new patch instead of V2 because the approach is very
different.

Previous patch:
https://lore.kernel.org/linux-mm/20251110-revert-78524b05f1a3-v1-1-88313f2b9b20@xxxxxxxxxxx/
---
 mm/swap_state.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 0cf9853a9232..da0481e163a4 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -745,6 +745,7 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 
 	blk_start_plug(&plug);
 	for (addr = start; addr < end; ilx++, addr += PAGE_SIZE) {
+		struct swap_info_struct *si = NULL;
 		softleaf_t entry;
 
 		if (!pte++) {
@@ -759,8 +760,19 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 			continue;
 		pte_unmap(pte);
 		pte = NULL;
+		/*
+		 * Readahead entry may come from a device that we are not
+		 * holding a reference to, try to grab a reference, or skip.
+		 */
+		if (swp_type(entry) != swp_type(targ_entry)) {
+			si = get_swap_device(entry);
+			if (!si)
+				continue;
+		}
 		folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
 						&page_allocated, false);
+		if (si)
+			put_swap_device(si);
 		if (!folio)
 			continue;
 		if (page_allocated) {

---
base-commit: 565d240810a6c9689817a9f3d08f80adf488ca59
change-id: 20251111-swap-fix-vma-uaf-bec70969250f

Best regards,
-- 
Kairui Song <kasong@xxxxxxxxxxx>