[PATCH 05/11] memcg: fix possible NULL deref while traversing memcg_slab_caches list

[Vladimir Davydov]

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@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Glauber Costa <glommer@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 mm/memcontrol.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 999e7d4..d918626 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3234,9 +3234,6 @@ 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
@@ -3245,7 +3242,16 @@ 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)
@@ -3257,16 +3263,21 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	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);
 }
 
-- 
1.7.10.4

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