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>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 322d18d..014a4f1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3270,9 +3270,6 @@
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 @@
*/
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 @@
if (is_root_cache(s))
return;
- memcg = s->memcg_params->memcg;
- id = memcg_cache_id(memcg);
-
root = s->memcg_params->root_cache;
- root->memcg_params->memcg_caches[id] = NULL;
+ memcg = s->memcg_params->memcg;
+ id = memcg_cache_id(memcg);
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);
}
