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Commit 96403da2 authored by Vladimir Davydov's avatar Vladimir Davydov Committed by Linus Torvalds
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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: default avatarVladimir 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 959c8963

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);

}