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

[Nhat Pham]

On Tue, Nov 11, 2025 at 5:36 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> 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);

Shouldn't we reset si to NULL here?

Otherwise, suppose we're swapping in a readahead window. One of the
swap entries in the window is on a different swapfile from the target
entry. We look up and get a reference to that different swapfile,
setting it to si.

We do the swapping in work, then we release the recently acquired reference.

In the next iteration in the for loop, we will still see si != NULL,
and we put_swap_device() it again, i.e double releasing reference to
that swap device.

Or am I missing something?

>                 if (!folio)
>                         continue;
>                 if (page_allocated) {
>