Re: [PATCH net] l2tp: don't use l2tp_tunnel_find() in l2tp_ip and l2tp_ip6

[David Miller]

From: Guillaume Nault 
Date: Fri, 3 Nov 2017 16:49:00 +0100

> 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 

Applied, thanks.

[PATCH net] l2tp: don't use l2tp_tunnel_find() in l2tp_ip and l2tp_ip6

[Guillaume Nault]

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 
---
 net/l2tp/l2tp_ip.c  | 24 +---
 net/l2tp/l2tp_ip6.c | 24 +---
 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 @@ static int l2tp_ip_recv(struct sk_buff *skb)
goto discard;
 
tunnel_id = ntohl(*(__be32 *) >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(_ip_lock);
-   sk = __l2tp_ip_bind_lookup(net, iph->daddr, iph->saddr,
-  inet_iif(skb), tunnel_id);
-   if (!sk) {
-   read_unlock_bh(_ip_lock);
-   goto discard;
-   }
+   iph = (struct iphdr *)skb_network_header(skb);
 
-   sock_hold(sk);
+   read_lock_bh(_ip_lock);
+   sk = __l2tp_ip_bind_lookup(net, iph->daddr, iph->saddr, inet_iif(skb),
+  tunnel_id);
+   if (!sk) {
read_unlock_bh(_ip_lock);
+   goto discard;
}
+   sock_hold(sk);
+   read_unlock_bh(_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 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
goto discard;
 
tunnel_id = ntohl(*(__be32 *) >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(_ip6_lock);
-   sk = __l2tp_ip6_bind_lookup(net, >daddr, >saddr,
-   inet6_iif(skb), tunnel_id);
-   if (!sk) {
-   read_unlock_bh(_ip6_lock);
-   goto discard;
-   }
+   iph = ipv6_hdr(skb);
 
-   sock_hold(sk);
+   read_lock_bh(_ip6_lock);
+   sk = __l2tp_ip6_bind_lookup(net, >daddr, >saddr,
+   inet6_iif(skb), tunnel_id);
+   if (!sk) {
read_unlock_bh(_ip6_lock);
+   goto discard;
}
+   sock_hold(sk);
+   read_unlock_bh(_ip6_lock);
 
if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
goto discard_put;
-- 
2.15.0