mm: list_lru: fix UAF for memory cgroup

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.

Found by code inspection.

[songmuchun@bytedance.com: only grab rcu lock when necessary, per Vlastimil]
  Link: https://lkml.kernel.org/r/20240801024603.1865-1-songmuchun@bytedance.com
Link: https://lkml.kernel.org/r/20240718083607.42068-1-songmuchun@bytedance.com
Fixes: 0a97c01 ("list_lru: allow explicit memcg and NUMA node selection")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/list_lru.c

@@ -85,6 +85,7 @@ list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
 }
 #endif /* CONFIG_MEMCG */
 
+/* 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 +110,22 @@ 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;
 
-	return list_lru_add(lru, item, nid, memcg);
+	if (list_lru_memcg_aware(lru)) {
+		rcu_read_lock();
+		ret = list_lru_add(lru, item, nid, mem_cgroup_from_slab_obj(item));
+		rcu_read_unlock();
+	} else {
+		ret = list_lru_add(lru, item, nid, NULL);
+	}
+
+	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 +148,18 @@ 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;
 
-	return list_lru_del(lru, item, nid, memcg);
+	if (list_lru_memcg_aware(lru)) {
+		rcu_read_lock();
+		ret = list_lru_del(lru, item, nid, mem_cgroup_from_slab_obj(item));
+		rcu_read_unlock();
+	} else {
+		ret = list_lru_del(lru, item, nid, NULL);
+	}
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(list_lru_del_obj);