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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 18 Jul 2024 16:36:07 +0800 Muchun Song <songmuchun@bytedance.com> 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.

"or others" is rather vague.  What others?

> @@ -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();

We don't need rcu_read_lock() to evaluate NULL.

	memcg = NULL;
	if (list_lru_memcg_aware(lru)) {
		rcu_read_lock();
		memcg = mem_cgroup_from_slab_obj(item);
		rcu_read_unlock();
	}

Seems worthwhile?