From: Kairui Song kasong@tencent.com
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@vger.kernel.org Fixes: 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning") Suggested-by: Huang Ying ying.huang@linux.alibaba.com Signed-off-by: Kairui Song kasong@tencent.com --- 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-v1-1-88313f2b9... --- 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,