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

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>

2 files changed, 18 insertions, 30 deletions

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index 4d322c1b7233..e4280b6568b4 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -123,6 +123,7 @@ static int l2tp_ip_recv(struct sk_buff *skb)
         unsigned char *ptr, *optr;
         struct l2tp_session *session;
         struct l2tp_tunnel *tunnel = NULL;
+        struct iphdr *iph;
         int length;
 
         if (!pskb_may_pull(skb, 4))
@@ -178,24 +179,17 @@ 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 iphdr *iph = (struct iphdr *) skb_network_header(skb);
-
-                read_lock_bh(&l2tp_ip_lock);
-                sk = __l2tp_ip_bind_lookup(net, iph->daddr, iph->saddr,
-                                           inet_iif(skb), 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(net, iph->daddr, iph->saddr, inet_iif(skb),
+                                   tunnel_id);
+        if (!sk) {
                 read_unlock_bh(&l2tp_ip_lock);
+                goto discard;
         }
+        sock_hold(sk);
+        read_unlock_bh(&l2tp_ip_lock);
 
         if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
                 goto discard_put;
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index 88b397c30d86..8bcaa975b432 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -136,6 +136,7 @@ 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))
@@ -192,24 +193,17 @@ 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, &iph->saddr,
-                                            inet6_iif(skb), 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, &iph->saddr,
+                                    inet6_iif(skb), 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;