Re: [PATCH -v3] mm: swap: fix race between free_swap_and_cache() and swapoff()

[Ryan Roberts <ryan.roberts@arm.com>]

On 11/03/2024 08:44, Huang Ying wrote:
> From: Ryan Roberts <ryan.roberts@arm.com>
> 
> 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 not free.  This isn't present in get_swap_device()
> because it doesn't make sense in general due to the race between getting
> the reference and swapoff.  So I've added an equivalent check directly in
> free_swap_and_cache().
> 
> Details of how to provoke one possible issue:
> 
> --8<-----
> 
> CPU0                               CPU1
> ----                               ----
> shmem_undo_range
>   shmem_free_swap
>     xa_cmpxchg_irq
>     free_swap_and_cache
>       __swap_entry_free
>       /* swap_count() become 0 */
>                                    swapoff
>                                      try_to_unuse
>                                        shmem_unuse /* cannot find swap entry */
>                                        find_next_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 !!!
> 
> --8<-----
> 
> Link: https://lkml.kernel.org/r/20240306140356.3974886-1-ryan.roberts@arm.com
> Closes: https://lore.kernel.org/linux-mm/8734t27awd.fsf@yhuang6-desk2.ccr.corp.intel.com/
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com> [patch description]
> Cc: David Hildenbrand <david@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> Hi, Andrew,
> 
> If it's not too late.  Please replace v2 of this patch in mm-stable
> with this version.

Thanks for sorting this out, Huang, Ying! I saw your note asking if I could do
it, and it was on my list, but I've been busy debugging other urgent issues in
mm-stable. That should be solved now so unblocks me finishing the testing on my
large folios swap-out v4 series. Hopefully that will be incomming in the next
couple of days.

You did previously suggest you wanted some comments around synchronise_rcu() in
swapoff(), but I don't see those here. I don't think that should hold this up
though.

Thanks,
Ryan


> 
> Changes since v2:
> 
>  - Remove comments for get_swap_device() because it's not correct.
>  - Revised patch description about the race condition description.
> 
> Changes since v1:
> 
>  - Added comments for get_swap_device() as suggested by David
>  - Moved check that swap entry is not free from get_swap_device() to
>    free_swap_and_cache() since there are some paths that legitimately call with
>    a free offset.
> 
> Best Regards,
> Huang, Ying
> 
>  mm/swapfile.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2b3a2d85e350..9e0691276f5e 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1609,13 +1609,19 @@ 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) {
> +		if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
> +			put_swap_device(p);
> +			return 0;
> +		}
> +
>  		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;
>  }