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From:   Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>
To:     Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Xiyu Yang <xiyuyang19@fudan.edu.cn>,
        Xin Tan <tanxin.ctf@gmail.com>,
        Xin Xiong <xiongx18@fudan.edu.cn>,
        Tom Parkin <tparkin@katalix.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH 1/1] l2tp: fix possible use-after-free
Date:   Fri, 10 Jun 2022 13:16:33 +0800
Message-Id: <20220610051633.8582-1-xiaohuizhang@ruc.edu.cn>
Precedence: bulk

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@ruc.edu.cn>
---
 net/l2tp/l2tp_core.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 7499c51b1850..96e8966f900d 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;
@@ -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;
@@ -294,8 +294,8 @@ struct l2tp_session *l2tp_session_get_nth(struct l2tp_tunnel *tunnel, int nth)
 	rcu_read_lock_bh();
 	for (hash = 0; hash < L2TP_HASH_SIZE; hash++) {
 		hlist_for_each_entry_rcu(session, &tunnel->session_hlist[hash], hlist) {
-			if (++count > nth) {
-				l2tp_session_inc_refcount(session);
+			if (++count > nth &&
+			    refcount_inc_not_zero(&session->ref_count)) {
 				rcu_read_unlock_bh();
 				return session;
 			}
@@ -321,8 +321,8 @@ struct l2tp_session *l2tp_session_get_by_ifname(const struct net *net,
 	rcu_read_lock_bh();
 	for (hash = 0; hash < L2TP_HASH_SIZE_2; hash++) {
 		hlist_for_each_entry_rcu(session, &pn->l2tp_session_hlist[hash], global_hlist) {
-			if (!strcmp(session->ifname, ifname)) {
-				l2tp_session_inc_refcount(session);
+			if (!strcmp(session->ifname, ifname) &&
+			    refcount_inc_not_zero(&session->ref_count)) {
 				rcu_read_unlock_bh();
 
 				return session;
-- 
2.17.1