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

[Chris Li]

On Tue, Nov 11, 2025 at 5:56 PM Huang, Ying
<ying.huang@xxxxxxxxxxxxxxxxx> wrote:
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > 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) {
>
> Personally, I prefer to call put_swap_device() after all swap operations
> on the swap entry, that is, after possible swap_read_folio() and
> folio_put() in the loop to make it easier to follow the
> get/put_swap_device() rule.  But I understand that it will make
>
> if (!folio)
>         continue;
>
> to use 'goto' and introduce more change.  So, it's up to you to decide
> whether to do that.

Personally I prefer it to keep the put_swap_device() in the current
location, closer to the matching get_swap_device(). To me that is
simpler, I don't need to reason about other branch out conditions.
Those error handling branch conditions are very error prone, I have
made enough mistakes on those goto branch handling in my past
experience. The si reference is only needed for the
__read_swap_cache_async() anyway.

To it to the end also works, just take more brain power to reason it.

> Otherwise, LGTM, Thanks for doing this!  Feel free to add my
>
> Reviewed-by: Huang Ying <ying.huang@xxxxxxxxxxxxxxxxx>

Thank you for the review.

Chris