l2tp: hold socket before dropping lock in l2tp_ip{, 6}_recv()

BugLink: https://bugs.launchpad.net/bugs/1810807

[ Upstream commit a3c18422 ]

Socket must be held while under the protection of the l2tp lock; there
is no guarantee that sk remains valid after the read_unlock_bh() call.

Same issue for l2tp_ip and l2tp_ip6.
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/l2tp/l2tp_ip.c

@@ -184,14 +184,15 @@ static int l2tp_ip_recv(struct sk_buff *skb)
 
 		read_lock_bh(&l2tp_ip_lock);
 		sk = __l2tp_ip_bind_lookup(net, iph->daddr, 0, tunnel_id);
+		if (!sk) {
+			read_unlock_bh(&l2tp_ip_lock);
+			goto discard;
+		}
+
+		sock_hold(sk);
 		read_unlock_bh(&l2tp_ip_lock);
 	}
 
-	if (sk == NULL)
-		goto discard;
-
-	sock_hold(sk);
-
 	if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_put;
 

net/l2tp/l2tp_ip6.c

@@ -196,14 +196,15 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
 		read_lock_bh(&l2tp_ip6_lock);
 		sk = __l2tp_ip6_bind_lookup(&init_net, &iph->daddr,
 					    0, tunnel_id);
+		if (!sk) {
+			read_unlock_bh(&l2tp_ip6_lock);
+			goto discard;
+		}
+
+		sock_hold(sk);
 		read_unlock_bh(&l2tp_ip6_lock);
 	}
 
-	if (sk == NULL)
-		goto discard;
-
-	sock_hold(sk);
-
 	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_put;