6.17-stable review patch. If anyone has any objections, please let me know.
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From: Kairui Song kasong@tencent.com
commit 1c2a936edd71e133f2806e68324ec81a4eb07588 upstream.
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.
Link: https://lkml.kernel.org/r/20251111-swap-fix-vma-uaf-v1-1-41c660e58562@tencen... 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 Acked-by: Chris Li chrisl@kernel.org Cc: Baoquan He bhe@redhat.com Cc: Barry Song baohua@kernel.org Cc: Kemeng Shi shikemeng@huaweicloud.com Cc: Nhat Pham nphamcs@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/swap_state.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
--- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -746,6 +746,8 @@ static struct folio *swap_vma_readahead(
blk_start_plug(&plug); for (addr = start; addr < end; ilx++, addr += PAGE_SIZE) { + struct swap_info_struct *si = NULL; + if (!pte++) { pte = pte_offset_map(vmf->pmd, addr); if (!pte) @@ -759,8 +761,19 @@ static struct folio *swap_vma_readahead( 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) {