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

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>

net/l2tp/l2tp_ip.c

@@ -183,13 +183,14 @@ 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);
-	}
-
-	if (sk == NULL)
 			goto discard;
+		}
 
 		sock_hold(sk);
+		read_unlock_bh(&l2tp_ip_lock);
+	}
 
 	if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_put;

net/l2tp/l2tp_ip6.c

@@ -198,13 +198,14 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
 		read_lock_bh(&l2tp_ip6_lock);
 		sk = __l2tp_ip6_bind_lookup(net, &iph->daddr,
 					    0, tunnel_id);
+		if (!sk) {
 			read_unlock_bh(&l2tp_ip6_lock);
-	}
-
-	if (sk == NULL)
 			goto discard;
+		}
 
 		sock_hold(sk);
+		read_unlock_bh(&l2tp_ip6_lock);
+	}
 
 	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_put;