l2tp: don't use l2tp_tunnel_find() in l2tp_ip and l2tp_ip6

commit 8f7dc9ae4a7aece9fbc3e6637bdfa38b36bcdf09 upstream.

Using l2tp_tunnel_find() in l2tp_ip_recv() is wrong for two reasons:

  * It doesn't take a reference on the returned tunnel, which makes the
    call racy wrt. concurrent tunnel deletion.

  * The lookup is only based on the tunnel identifier, so it can return
    a tunnel that doesn't match the packet's addresses or protocol.

For example, a packet sent to an L2TPv3 over IPv6 tunnel can be
delivered to an L2TPv2 over UDPv4 tunnel. This is worse than a simple
cross-talk: when delivering the packet to an L2TP over UDP tunnel, the
corresponding socket is UDP, where ->sk_backlog_rcv() is NULL. Calling
sk_receive_skb() will then crash the kernel by trying to execute this
callback.

And l2tp_tunnel_find() isn't even needed here. __l2tp_ip_bind_lookup()
properly checks the socket binding and connection settings. It was used
as a fallback mechanism for finding tunnels that didn't have their data
path registered yet. But it's not limited to this case and can be used
to replace l2tp_tunnel_find() in the general case.

Fix l2tp_ip6 in the same way.

Fixes: 0d76751fad77 ("l2tp: Add L2TPv3 IP encapsulation (no UDP) support")
Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for IPv6")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 15 deletions

diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index 4d4561dd4023..5bb5337e74fc 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -134,6 +134,7 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
 	unsigned char *ptr, *optr;
 	struct l2tp_session *session;
 	struct l2tp_tunnel *tunnel = NULL;
+	struct ipv6hdr *iph;
 	int length;
 
 	if (!pskb_may_pull(skb, 4))
@@ -193,24 +194,16 @@ pass_up:
 		goto discard;
 
 	tunnel_id = ntohl(*(__be32 *) &skb->data[4]);
-	tunnel = l2tp_tunnel_find(net, tunnel_id);
-	if (tunnel) {
-		sk = tunnel->sock;
-		sock_hold(sk);
-	} else {
-		struct ipv6hdr *iph = ipv6_hdr(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);
-			goto discard;
-		}
+	iph = ipv6_hdr(skb);
 
-		sock_hold(sk);
+	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);
+		goto discard;
 	}
+	sock_hold(sk);
+	read_unlock_bh(&l2tp_ip6_lock);
 
 	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
 		goto discard_put;