Subject: Re: [PATCH net-next] netfilter: bridge: reset skb->pkt_type after NF_INET_POST_ROUTING traversal

From: Florian Westphal <[email-masked]>
Date: Mon, 23 Nov 2020 19:32:53 +0100

Antoine Tenart <[email-masked]> wrote:
> Netfilter changes PACKET_OTHERHOST to PACKET_HOST before invoking the
> hooks as, while it's an expected value for a bridge, routing expects
> PACKET_HOST. The change is undone later on after hook traversal. This
> can be seen with pairs of functions updating skb>pkt_type and then
> reverting it to its original value:
> 
> For hook NF_INET_PRE_ROUTING:
>   setup_pre_routing / br_nf_pre_routing_finish
> 
> For hook NF_INET_FORWARD:
>   br_nf_forward_ip / br_nf_forward_finish
> 
> But the third case where netfilter does this, for hook
> NF_INET_POST_ROUTING, the packet type is changed in br_nf_post_routing
> but never reverted. A comment says:
> 
>   /* We assume any code from br_dev_queue_push_xmit onwards doesn't care
>    * about the value of skb->pkt_type. */

[..]
> But when having a tunnel (say vxlan) attached to a bridge we have the
> following call trace:

> In this specific case, this creates issues such as when an ICMPv6 PTB
> should be sent back. When CONFIG_BRIDGE_NETFILTER is enabled, the PTB
> isn't sent (as skb_tunnel_check_pmtu checks if pkt_type is PACKET_HOST
> and returns early).
> 
> If the comment is right and no one cares about the value of
> skb->pkt_type after br_dev_queue_push_xmit (which isn't true), resetting
> it to its original value should be safe.

That comment is 18 years old, safe bet noone thought of
ipv6-in-tunnel-interface-added-as-bridge-port back then.

Reviewed-by: Florian Westphal <[email-masked]>