[PATCH] mm: list_lru: Fix NULL pointer dereference in list_lru_add()

[Youling Tang]

From: Youling Tang <tangyouling@xxxxxxxxxx>

Note that list_lru_from_memcg_idx() may return NULL, so it is necessary
to error handle the return value to avoid triggering NULL pointer
dereference BUG.

The issue was triggered for discussion [1],
Link [1]: https://lore.kernel.org/linux-bcachefs/84de6cb1-57bd-42f7-8029-4203820ef0b4@xxxxxxxxx/T/#m901bb26cdb1d9d4bacebf0d034f0a5a712cc93a6

Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxx>
---
 mm/list_lru.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 3fd64736bc45..ee7424c3879d 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -94,6 +94,9 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
 	spin_lock(&nlru->lock);
 	if (list_empty(item)) {
 		l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
+		if (!l)
+			goto out;
+
 		list_add_tail(item, &l->list);
 		/* Set shrinker bit if the first element was added */
 		if (!l->nr_items++)
@@ -102,6 +105,7 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item, int nid,
 		spin_unlock(&nlru->lock);
 		return true;
 	}
+out:
 	spin_unlock(&nlru->lock);
 	return false;
 }
-- 
2.34.1