[PATCH 3.2 75/94] l2tp: don't use l2tp_tunnel_find() in l2tp_ip and l2tp_ip6

From: Ben Hutchings
Date: Thu Dec 28 2017 - 13:15:47 EST

3.2.97-rc1 review patch. If anyone has any objections, please let me know.


From: Guillaume Nault <g.nault@xxxxxxxxxxxx>

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

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@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.2:
- Always look up in init_net
- Drop changes in l2tp_ip6.c
- Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -127,6 +127,7 @@ static int l2tp_ip_recv(struct sk_buff *
unsigned char *ptr, *optr;
struct l2tp_session *session;
struct l2tp_tunnel *tunnel = NULL;
+ struct iphdr *iph;
int length;
int offset;

@@ -189,23 +190,16 @@ pass_up:
goto discard;

tunnel_id = ntohl(*(__be32 *) &skb->data[4]);
- tunnel = l2tp_tunnel_find(&init_net, tunnel_id);
- if (tunnel) {
- sk = tunnel->sock;
- sock_hold(sk);
- } else {
- struct iphdr *iph = (struct iphdr *) skb_network_header(skb);
- read_lock_bh(&l2tp_ip_lock);
- sk = __l2tp_ip_bind_lookup(&init_net, iph->daddr, 0, tunnel_id);
- if (!sk) {
- read_unlock_bh(&l2tp_ip_lock);
- goto discard;
- }
+ iph = (struct iphdr *)skb_network_header(skb);

- sock_hold(sk);
+ read_lock_bh(&l2tp_ip_lock);
+ sk = __l2tp_ip_bind_lookup(&init_net, iph->daddr, 0, tunnel_id);
+ if (!sk) {
+ goto discard;
+ sock_hold(sk);
+ read_unlock_bh(&l2tp_ip_lock);

if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
goto discard_put;