memcg: fix possible NULL deref while traversing memcg_slab_caches list

All caches of the same memory cgroup are linked in the memcg_slab_caches
list via kmem_cache::memcg_params::list.  This list is traversed, for
example, when we read memory.kmem.slabinfo.

Since the list actually consists of memcg_cache_params objects, we have
to convert an element of the list to a kmem_cache object using
memcg_params_to_cache(), which obtains the pointer to the cache from the
memcg_params::memcg_caches array of the corresponding root cache.  That
said the pointer to a kmem_cache in its parent's memcg_params must be
initialized before adding the cache to the list, and cleared only after
it has been unlinked.  Currently it is vice-versa, which can result in a
NULL ptr dereference while traversing the memcg_slab_caches list.  This
patch restores the correct order.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3270,9 +3270,6 @@ void memcg_register_cache(struct kmem_cache *s)
 
 	css_get(&memcg->css);
 
-	mutex_lock(&memcg->slab_caches_mutex);
-	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
-	mutex_unlock(&memcg->slab_caches_mutex);
 
 	/*
 	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
@@ -3281,7 +3278,16 @@ void memcg_register_cache(struct kmem_cache *s)
 	 */
 	smp_wmb();
 
+	/*
+	 * Initialize the pointer to this cache in its parent's memcg_params
+	 * before adding it to the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
 	root->memcg_params->memcg_caches[id] = s;
+
+	mutex_lock(&memcg->slab_caches_mutex);
+	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
+	mutex_unlock(&memcg->slab_caches_mutex);
 }
 
 void memcg_unregister_cache(struct kmem_cache *s)
@@ -3293,16 +3299,21 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	if (is_root_cache(s))
 		return;
 
+	root = s->memcg_params->root_cache;
 	memcg = s->memcg_params->memcg;
 	id = memcg_cache_id(memcg);
 
-	root = s->memcg_params->root_cache;
-	root->memcg_params->memcg_caches[id] = NULL;
-
 	mutex_lock(&memcg->slab_caches_mutex);
 	list_del(&s->memcg_params->list);
 	mutex_unlock(&memcg->slab_caches_mutex);
 
+	/*
+	 * Clear the pointer to this cache in its parent's memcg_params only
+	 * after removing it from the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
+	root->memcg_params->memcg_caches[id] = NULL;
+
 	css_put(&memcg->css);
 }