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

[Muchun Song <muchun.song@linux.dev>]

> On Jul 24, 2024, at 08:45, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> 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?

Like objcg_lock. I have added this one into obj_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();
> 
> 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();

Actually, the access to memcg is in list_lru_add(), so the rcu lock should
also cover this function rather than only mem_cgroup_from_slab_obj().
Something like:

memcg = NULL;
if (list_lru_memcg_aware(lru)) {
	rcu_read_lock();
	memcg = mem_cgroup_from_slab_obj(item);
}
ret = list_lru_add(lru, item, nid, memcg);
if (list_lru_memcg_aware(lru))
	rcu_read_unlock();

Not concise. I don't know if this is good.

> 	}
> 
> Seems worthwhile?
> 
>