On Fri, Mar 15, 2024 at 4:55 PM Johannes Weiner <hannes@xxxxxxxxxxx> 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>
Nice catch! Thanks for the report, Chengming and Jann, and thanks for
the fix, Johannes!
Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
> 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))
> --
> 2.44.0
>
