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

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Nov 11, 2025 at 11:48 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Tue, Nov 11, 2025 at 5:36 AM Kairui Song <ryncsn@gmail.com> wrote:
> >
> > 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-v1-1-88313f2b9b20@tencent.com/
> > ---
> >  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?

Nvm - Andrew pointed out to me that si was NULLED at the beginning of
the loop. Looks like I was blind :)

Anyway:
Acked-by: Nhat Pham <nphamcs@gmail.com>