linux-kernel - [PATCH 1/2] l2tp: fix possible use-after-free

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Date:   Wed,  8 Jun 2022 00:16:13 +0800
From:   Xiaohui Zhang <xiaohuizhang@....edu.cn>
To:     Xiaohui Zhang <xiaohuizhang@....edu.cn>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Xin Tan <tanxin.ctf@...il.com>,
        Xin Xiong <xiongx18@...an.edu.cn>,
        Tom Parkin <tparkin@...alix.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] l2tp: fix possible use-after-free

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: Xiaohui Zhang <xiaohuizhang@....edu.cn>
---
 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 7499c51b1850..b759fbd09b65 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -252,8 +252,8 @@ struct l2tp_session *l2tp_tunnel_get_session(struct l2tp_tunnel *tunnel,
 
 	rcu_read_lock_bh();
 	hlist_for_each_entry_rcu(session, session_list, 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