Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> writes:

> There was previously a theoretical window where swapoff() could run and
> teardown a swap_info_struct while a call to free_swap_and_cache() was
> running in another thread. This could cause, amongst other bad
> possibilities, swap_page_trans_huge_swapped() (called by
> free_swap_and_cache()) to access the freed memory for swap_map.
>
> This is a theoretical problem and I haven't been able to provoke it from
> a test case. But there has been agreement based on code review that this
> is possible (see link below).
>
> Fix it by using get_swap_device()/put_swap_device(), which will stall
> swapoff(). There was an extra check in _swap_info_get() to confirm that
> the swap entry was valid. This wasn't present in get_swap_device() so
> I've added it. I couldn't find any existing get_swap_device() call sites
> where this extra check would cause any false alarms.
>
> Details of how to provoke one possible issue (thanks to David Hilenbrand
> for deriving this):
>
> --8<-----
>
> __swap_entry_free() might be the last user and result in
> "count == SWAP_HAS_CACHE".
>
> swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.
>
> So the question is: could someone reclaim the folio and turn
> si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().
>
> Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are
> still references by swap entries.
>
> Process 1 still references subpage 0 via swap entry.
> Process 2 still references subpage 1 via swap entry.
>
> Process 1 quits. Calls free_swap_and_cache().
> -> count == SWAP_HAS_CACHE
> [then, preempted in the hypervisor etc.]
>
> Process 2 quits. Calls free_swap_and_cache().
> -> count == SWAP_HAS_CACHE
>
> Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
> __try_to_reclaim_swap().
>
> __try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->
> put_swap_folio()->free_swap_slot()->swapcache_free_entries()->
> swap_entry_free()->swap_range_free()->
> ...
> WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
>
> What stops swapoff to succeed after process 2 reclaimed the swap cache
> but before process1 finished its call to swap_page_trans_huge_swapped()?
>
> --8<-----

I think that this can be simplified.  Even for a 4K folio, this could
happen.

CPU0                                     CPU1
----                                     ----

zap_pte_range
  free_swap_and_cache
  __swap_entry_free
  /* swap count become 0 */
                                         swapoff
                                           try_to_unuse
                                             filemap_get_folio
                                             folio_free_swap
                                             /* remove swap cache */
                                           /* free si->swap_map[] */

  swap_page_trans_huge_swapped <-- access freed si->swap_map !!!

> Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch")
> Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@xxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>
> Applies on top of v6.8-rc6 and mm-unstable (b38c34939fe4).
>
> Thanks,
> Ryan
>
>  mm/swapfile.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2b3a2d85e350..f580e6abc674 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1281,7 +1281,9 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>  	smp_rmb();
>  	offset = swp_offset(entry);
>  	if (offset >= si->max)
> -		goto put_out;
> +		goto bad_offset;
> +	if (data_race(!si->swap_map[swp_offset(entry)]))
> +		goto bad_free;
>
>  	return si;
>  bad_nofile:
> @@ -1289,9 +1291,14 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>  out:
>  	return NULL;
>  put_out:
> -	pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val);
>  	percpu_ref_put(&si->users);
>  	return NULL;
> +bad_offset:
> +	pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val);
> +	goto put_out;
> +bad_free:
> +	pr_err("%s: %s%08lx\n", __func__, Unused_offset, entry.val);
> +	goto put_out;
>  }

I don't think that it's a good idea to warn for bad free entries.
get_swap_device() could be called without enough lock to prevent
parallel swap operations on entry.  So, false positive is possible
there.  I think that it's good to add more checks in general, for
example, in free_swap_and_cache(), we can check more because we are sure
the swap entry will not be freed by parallel swap operations.  Just
don't put the check in general get_swap_device().  We can add another
helper to check that.

I found that there are other checks in get_swap_device() already.  I
think that we may need to revert,

commit 23b230ba8ac3 ("mm/swap: print bad swap offset entry in get_swap_device")

which introduces it.  And add check in appropriate places.

--
Best Regards,
Huang, Ying

>  static unsigned char __swap_entry_free(struct swap_info_struct *p,
> @@ -1609,13 +1616,14 @@ int free_swap_and_cache(swp_entry_t entry)
>  	if (non_swap_entry(entry))
>  		return 1;
>
> -	p = _swap_info_get(entry);
> +	p = get_swap_device(entry);
>  	if (p) {
>  		count = __swap_entry_free(p, entry);
>  		if (count == SWAP_HAS_CACHE &&
>  		    !swap_page_trans_huge_swapped(p, entry))
>  			__try_to_reclaim_swap(p, swp_offset(entry),
>  					      TTRS_UNMAPPED | TTRS_FULL);
> +		put_swap_device(p);
>  	}
>  	return p != NULL;
>  }
> --
> 2.25.1