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

[Vlastimil Babka (SUSE)]

On 7/24/24 4:23 AM, Muchun Song wrote:
> 
> 
>> On Jul 24, 2024, at 08:45, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> 
>> On Thu, 18 Jul 2024 16:36:07 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> 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.

At such point, it's probably best to just:

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

?

> 
>> 	}
>> 
>> Seems worthwhile?
>> 
>> 
> 
>