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

[Muchun Song]

> On Jul 18, 2024, at 18:30, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> 
> 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?

Just code review.

Thanks.

> Should there be some lockdep_assert somwhere?
> 

It’s a good option to improve this. Maybe mem_cgroup_from_slab_obj() is a good place.

>> 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);
>> 
>