Re: [PATCH] mm: list_lru: fix UAF for memory cgroup

[Vlastimil Babka]

On 7/18/24 10:36 AM, Muchun Song wrote:
> The mem_cgroup_from_slab_obj() is supposed to be called under rcu
> lock or cgroup_mutex or others which could prevent returned memcg
> from being freed. Fix it by adding missing rcu read lock.

Was the UAF ever observed, or is this due to code review?
Should there be some lockdep_assert somwhere?

> Fixes: 0a97c01cd20bb ("list_lru: allow explicit memcg and NUMA node selection)
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  mm/list_lru.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 3fd64736bc458..225da0778a3be 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -85,6 +85,7 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
>  }
>  #endif /* CONFIG_MEMCG_KMEM */
>  
> +/* The caller must ensure the memcg lifetime. */
>  bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
>  		    struct mem_cgroup *memcg)
>  {
> @@ -109,14 +110,20 @@ EXPORT_SYMBOL_GPL(list_lru_add);
>  
>  bool list_lru_add_obj(struct list_lru *lru, struct list_head *item)
>  {
> +	bool ret;
>  	int nid = page_to_nid(virt_to_page(item));
> -	struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
> -		mem_cgroup_from_slab_obj(item) : NULL;
> +	struct mem_cgroup *memcg;
>  
> -	return list_lru_add(lru, item, nid, memcg);
> +	rcu_read_lock();
> +	memcg = list_lru_memcg_aware(lru) ? mem_cgroup_from_slab_obj(item) : NULL;
> +	ret = list_lru_add(lru, item, nid, memcg);
> +	rcu_read_unlock();
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(list_lru_add_obj);
>  
> +/* The caller must ensure the memcg lifetime. */
>  bool list_lru_del(struct list_lru *lru, struct list_head *item, int nid,
>  		    struct mem_cgroup *memcg)
>  {
> @@ -139,11 +146,16 @@ EXPORT_SYMBOL_GPL(list_lru_del);
>  
>  bool list_lru_del_obj(struct list_lru *lru, struct list_head *item)
>  {
> +	bool ret;
>  	int nid = page_to_nid(virt_to_page(item));
> -	struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
> -		mem_cgroup_from_slab_obj(item) : NULL;
> +	struct mem_cgroup *memcg;
>  
> -	return list_lru_del(lru, item, nid, memcg);
> +	rcu_read_lock();
> +	memcg = list_lru_memcg_aware(lru) ? mem_cgroup_from_slab_obj(item) : NULL;
> +	ret = list_lru_del(lru, item, nid, memcg);
> +	rcu_read_unlock();
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(list_lru_del_obj);
>