memcg: fix possible NULL deref while traversing memcg_slab_caches list (96403da2) · Commits · 戴 / test

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

+18 −7

Original line number	Diff line number	Diff line
		@@ -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);
		}

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