Re: [PATCH] mm: cachestat: fix two shmem bugs

[Chengming Zhou]

On 2024/3/15 17:55, Johannes Weiner wrote:
> When cachestat on shmem races with swapping and invalidation, there
> are two possible bugs:
> 
> 1) A swapin error can have resulted in a poisoned swap entry in the
>    shmem inode's xarray. Calling get_shadow_from_swap_cache() on it
>    will result in an out-of-bounds access to swapper_spaces[].
> 
>    Validate the entry with non_swap_entry() before going further.
> 
> 2) When we find a valid swap entry in the shmem's inode, the shadow
>    entry in the swapcache might not exist yet: swap IO is still in
>    progress and we're before __remove_mapping; swapin, invalidation,
>    or swapoff have removed the shadow from swapcache after we saw the
>    shmem swap entry.
> 
>    This will send a NULL to workingset_test_recent(). The latter
>    purely operates on pointer bits, so it won't crash - node 0, memcg
>    ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a
>    bogus test. In theory that could result in a false "recently
>    evicted" count.
> 
>    Such a false positive wouldn't be the end of the world. But for
>    code clarity and (future) robustness, be explicit about this case.
> 
>    Bail on get_shadow_from_swap_cache() returning NULL.
> 
> Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
> Cc: stable@xxxxxxxxxxxxxxx				[v6.5+]
> Reported-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>	[Bug #1]
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>		[Bug #2]
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Looks good to me.

Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>

Thanks.

> ---
>  mm/filemap.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 222adac7c9c5..0aa91bf6c1f7 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -4198,7 +4198,23 @@ static void filemap_cachestat(struct address_space *mapping,
>  				/* shmem file - in swap cache */
>  				swp_entry_t swp = radix_to_swp_entry(folio);
>  
> +				/* swapin error results in poisoned entry */
> +				if (non_swap_entry(swp))
> +					goto resched;
> +
> +				/*
> +				 * Getting a swap entry from the shmem
> +				 * inode means we beat
> +				 * shmem_unuse(). rcu_read_lock()
> +				 * ensures swapoff waits for us before
> +				 * freeing the swapper space. However,
> +				 * we can race with swapping and
> +				 * invalidation, so there might not be
> +				 * a shadow in the swapcache (yet).
> +				 */
>  				shadow = get_shadow_from_swap_cache(swp);
> +				if (!shadow)
> +					goto resched;
>  			}
>  #endif
>  			if (workingset_test_recent(shadow, true, &workingset))