[PATCH] mm: list_lru: fix UAF for memory cgroup

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

The mem_cgroup_from_slab_obj() is supposed to be called under rcu
lock or cgroup_mutex or others which could prevent returned memcg
from being freed. Fix it by adding missing rcu read lock.

Fixes: 0a97c01cd20bb ("list_lru: allow explicit memcg and NUMA node selection)
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/list_lru.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 3fd64736bc458..225da0778a3be 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -85,6 +85,7 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
 }
 #endif /* CONFIG_MEMCG_KMEM */
 
+/* The caller must ensure the memcg lifetime. */
 bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
 		    struct mem_cgroup *memcg)
 {
@@ -109,14 +110,20 @@ EXPORT_SYMBOL_GPL(list_lru_add);
 
 bool list_lru_add_obj(struct list_lru *lru, struct list_head *item)
 {
+	bool ret;
 	int nid = page_to_nid(virt_to_page(item));
-	struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
-		mem_cgroup_from_slab_obj(item) : NULL;
+	struct mem_cgroup *memcg;
 
-	return list_lru_add(lru, item, nid, memcg);
+	rcu_read_lock();
+	memcg = list_lru_memcg_aware(lru) ? mem_cgroup_from_slab_obj(item) : NULL;
+	ret = list_lru_add(lru, item, nid, memcg);
+	rcu_read_unlock();
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(list_lru_add_obj);
 
+/* The caller must ensure the memcg lifetime. */
 bool list_lru_del(struct list_lru *lru, struct list_head *item, int nid,
 		    struct mem_cgroup *memcg)
 {
@@ -139,11 +146,16 @@ EXPORT_SYMBOL_GPL(list_lru_del);
 
 bool list_lru_del_obj(struct list_lru *lru, struct list_head *item)
 {
+	bool ret;
 	int nid = page_to_nid(virt_to_page(item));
-	struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
-		mem_cgroup_from_slab_obj(item) : NULL;
+	struct mem_cgroup *memcg;
 
-	return list_lru_del(lru, item, nid, memcg);
+	rcu_read_lock();
+	memcg = list_lru_memcg_aware(lru) ? mem_cgroup_from_slab_obj(item) : NULL;
+	ret = list_lru_del(lru, item, nid, memcg);
+	rcu_read_unlock();
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(list_lru_del_obj);
 
-- 
2.20.1