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

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

[Johannes Weiner]

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);
+	}
 	zswap_entry_cache_free(entry);
 	atomic_dec(&zswap_stored_pages);
 	zswap_update_total_size();
-- 
2.43.0

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

[Yosry Ahmed]

On Mon, Jan 29, 2024 at 08:34:38PM -0500, Johannes Weiner 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")

Lots of hotfixes for zswap in v6.8 these couple of days :)

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Yosry Ahmed <yosryahmed@google.com>

Thanks!

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

[Chengming Zhou]

On 2024/1/30 09:34, Johannes Weiner 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>

LGTM, thanks!

Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>

> ---
>  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);
> +	}
>  	zswap_entry_cache_free(entry);
>  	atomic_dec(&zswap_stored_pages);
>  	zswap_update_total_size();

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

[Nhat Pham]

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
>