Re: [PATCH] l2tp: Support several sockets with same IP/port quadruple

[James Chapman <jchapman@katalix.com>]

On 03/05/2024 00:14, Samuel Thibault wrote:
> Some l2tp providers will use 1701 as origin port and open several
> tunnels for the same origin and target. On the Linux side, this
> may mean opening several sockets, but then trafic will go to only
> one of them, losing the trafic for the tunnel of the other socket
> (or leaving it up to userland, consuming a lot of cpu%).
>
> This can also happen when the l2tp provider uses a cluster, and
> load-balancing happens to migrate from one origin IP to another one,
> for which a socket was already established. Managing reassigning
> tunnels from one socket to another would be very hairy for userland.
>
> Lastly, as documented in l2tpconfig(1), as client it may be necessary
> to use 1701 as origin port for odd firewalls reasons, which could
> prevent from establishing several tunnels to a l2tp server, for the
> same reason: trafic would get only on one of the two sockets.
>
> With the V2 protocol it is however easy to route trafic to the proper
> tunnel, by looking up the tunnel number in the network namespace. This
> fixes the three cases altogether.

Hi Samuel,

Thanks for working on this.

I'm currently working on changes that address this for both L2TPv2 and 
L2TPv3 which will avoid separate tunnel and session lookups in the 
datapath. However, my changes aren't ready yet; I hope to post them in a 
week or so.

Please find comments on your patch inline below.

> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> ---
>   net/l2tp/l2tp_core.c | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
>
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 8d21ff25f160..128f1146c135 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -794,6 +794,7 @@ static void l2tp_session_queue_purge(struct l2tp_session *session)
>   static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
>   {
>   	struct l2tp_session *session = NULL;
> +	struct l2tp_tunnel *orig_tunnel = tunnel;
>   	unsigned char *ptr, *optr;
>   	u16 hdrflags;
>   	u32 tunnel_id, session_id;
> @@ -845,6 +846,20 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
>   		/* Extract tunnel and session ID */
>   		tunnel_id = ntohs(*(__be16 *)ptr);
>   		ptr += 2;
> +
> +		if (tunnel_id != tunnel->tunnel_id && tunnel->l2tp_net) {
Can tunnel->l2tp_net be NULL?
> +			/* We are receiving trafic for another tunnel, probably
> +			 * because we have several tunnels between the same
> +			 * IP/port quadruple, look it up.
> +			 */
> +			struct l2tp_tunnel *alt_tunnel;
> +
> +			alt_tunnel = l2tp_tunnel_get(tunnel->l2tp_net, tunnel_id);
This misses a check that alt_tunnel's protocol version matches the 
header. Move the existing header version check to after this fragment?
> +			if (!alt_tunnel)
> +				goto pass;
> +			tunnel = alt_tunnel;
> +		}
> +
>   		session_id = ntohs(*(__be16 *)ptr);
>   		ptr += 2;
>   	} else {
> @@ -875,6 +890,9 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
>   	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
>   	l2tp_session_dec_refcount(session);
>   
> +	if (tunnel != orig_tunnel)
> +		l2tp_tunnel_dec_refcount(tunnel);
> +
>   	return 0;
>   
>   invalid:
> @@ -884,6 +902,9 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
>   	/* Put UDP header back */
>   	__skb_push(skb, sizeof(struct udphdr));
>   
> +	if (tunnel != orig_tunnel)
> +		l2tp_tunnel_dec_refcount(tunnel);
> +
>   	return 1;
>   }
>