Re: [PATCH] mm: zswap: fix objcg use-after-free in entry destruction

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Jan 29, 2024 at 5:34 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> In the per-memcg LRU universe, LRU removal uses entry->objcg to
> determine which list count needs to be decreased. Drop the objcg
> reference after updating the LRU, to fix a possible use-after-free.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/zswap.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index de68a5928527..7f88b3a77e4a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -522,10 +522,6 @@ static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
>   */
>  static void zswap_free_entry(struct zswap_entry *entry)
>  {
> -       if (entry->objcg) {
> -               obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
> -               obj_cgroup_put(entry->objcg);
> -       }
>         if (!entry->length)
>                 atomic_dec(&zswap_same_filled_pages);
>         else {
> @@ -534,6 +530,10 @@ static void zswap_free_entry(struct zswap_entry *entry)
>                 atomic_dec(&entry->pool->nr_stored);
>                 zswap_pool_put(entry->pool);
>         }
> +       if (entry->objcg) {
> +               obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
> +               obj_cgroup_put(entry->objcg);
> +       }

Nice catch!
Reviewed-by: Nhat Pham <nphamcs@gmail.com>

>         zswap_entry_cache_free(entry);
>         atomic_dec(&zswap_stored_pages);
>         zswap_update_total_size();
> --
> 2.43.0
>