[PATCH 2/2] l2tp: fix possible use-after-free

[Xiaohui Zhang]

From: xiaohuizhang98 <ruc_zhangxiaohui@xxxxxxx>

We detected a suspected bug with our code clone detection tool.

Similar to the handling of l2tp_tunnel_get in commit a622b40035d1
("l2ip: fix possible use-after-free"), we thought a patch might
be needed here as well.

Before taking a refcount on a rcu protected structure,
we need to make sure the refcount is not zero.

Signed-off-by: xiaohuizhang98 <ruc_zhangxiaohui@xxxxxxx>
---
 net/l2tp/l2tp_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index b759fbd09b65..c5de6d4e0818 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -273,8 +273,8 @@ struct l2tp_session *l2tp_session_get(const struct net *net, u32 session_id)
 
 	rcu_read_lock_bh();
 	hlist_for_each_entry_rcu(session, session_list, global_hlist)
-		if (session->session_id == session_id) {
-			l2tp_session_inc_refcount(session);
+		if (session->session_id == session_id &&
+		    refcount_inc_not_zero(&session->ref_count)) {
 			rcu_read_unlock_bh();
 
 			return session;
-- 
2.17.1