Re: [PATCH net] ip_tunnel: drop stale dst from generated PMTU ICMP replies

[Paolo Abeni <pabeni@redhat.com>]

On 6/13/26 3:00 AM, Laika Price via B4 Relay wrote:
> From: Laika Price <laikabcprice@gmail.com>
> 
> iptunnel_pmtud_build_icmp(...) and iptunnel_pmtud_build_icmpv6(...) take
> in an sk_buff, modify it to create a PMTU ICMP error reply, and return it.
> As part of these modifications, the source/destination ethernet and IP
> addresses are swapped around which makes the sk_buff's current dst invalid.
> 
> If the stale dst is left, the packet can skip input routing and be
> forwarded using the original output device. This was observed when sending
> packets to a VXLAN over a WireGuard tunnel - the ICMP reply was generated
> but it was sent over the VXLAN instead of to the WireGuard tunnel.
> 
> Drop the stale dst after building the PMTU reply so that the packet is
> routed using its new headers when it is reinjected.
> 
> Signed-off-by: Laika Price <laikabcprice@gmail.com>
> ---
>  net/ipv4/ip_tunnel_core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
> index d3c677e9b..949150e43 100644
> --- a/net/ipv4/ip_tunnel_core.c
> +++ b/net/ipv4/ip_tunnel_core.c
> @@ -267,6 +267,7 @@ static int iptunnel_pmtud_build_icmp(struct sk_buff *skb, int mtu)
>  
>  	eth_header(skb, skb->dev, ntohs(eh.h_proto), eh.h_source, eh.h_dest, 0);
>  	skb_reset_mac_header(skb);
> +	skb_dst_drop(skb);
>  
>  	return skb->len;
>  }
> @@ -370,6 +371,7 @@ static int iptunnel_pmtud_build_icmpv6(struct sk_buff *skb, int mtu)
>  
>  	eth_header(skb, skb->dev, ntohs(eh.h_proto), eh.h_source, eh.h_dest, 0);
>  	skb_reset_mac_header(skb);
> +	skb_dst_drop(skb);
>  
>  	return skb->len;
>  }
> 
This apparently causes self-tests pmtu failure for the virtio_net device:

# 135.54 [+6.50] TEST: IPv4, bridged vxlan4: PMTU exceptions
            [FAIL]
# 142.17 [+6.63] TEST: IPv4, bridged vxlan4: PMTU exceptions - nexthop
objects       [FAIL]
# 148.71 [+6.53] TEST: IPv6, bridged vxlan4: PMTU exceptions
            [FAIL]
# 155.44 [+6.74] TEST: IPv6, bridged vxlan4: PMTU exceptions - nexthop
objects       [FAIL]
# 161.94 [+6.49] TEST: IPv4, bridged vxlan6: PMTU exceptions
            [FAIL]
# 168.69 [+6.76] TEST: IPv4, bridged vxlan6: PMTU exceptions - nexthop
objects       [FAIL]
# 175.01 [+6.32] TEST: IPv6, bridged vxlan6: PMTU exceptions
            [FAIL]
# 181.83 [+6.82] TEST: IPv6, bridged vxlan6: PMTU exceptions - nexthop
objects       [FAIL]
# 188.42 [+6.59] TEST: IPv4, bridged geneve4: PMTU exceptions
            [FAIL]
# 195.31 [+6.89] TEST: IPv4, bridged geneve4: PMTU exceptions - nexthop
objects      [FAIL]
# 201.99 [+6.68] TEST: IPv6, bridged geneve4: PMTU exceptions
            [FAIL]
# 209.10 [+7.11] TEST: IPv6, bridged geneve4: PMTU exceptions - nexthop
objects      [FAIL]
# 216.12 [+7.02] TEST: IPv4, bridged geneve6: PMTU exceptions
            [FAIL]
# 223.36 [+7.23] TEST: IPv4, bridged geneve6: PMTU exceptions - nexthop
objects      [FAIL]
# 230.26 [+6.90] TEST: IPv6, bridged geneve6: PMTU exceptions
            [FAIL]
# 237.52 [+7.26] TEST: IPv6, bridged geneve6: PMTU exceptions - nexthop
objects      [FAIL]
# 264.58 [+27.06] TEST: IPv4, OVS vxlan4: PMTU exceptions
             [FAIL]
# 293.85 [+29.27] TEST: IPv4, OVS vxlan4: PMTU exceptions - nexthop
objects           [FAIL]
# 322.61 [+28.76] TEST: IPv6, OVS vxlan4: PMTU exceptions
             [FAIL]
# 350.36 [+27.75] TEST: IPv6, OVS vxlan4: PMTU exceptions - nexthop
objects           [FAIL]
# 376.99 [+26.62] TEST: IPv4, OVS vxlan6: PMTU exceptions
             [FAIL]
# 404.00 [+27.02] TEST: IPv4, OVS vxlan6: PMTU exceptions - nexthop
objects           [FAIL]
# 429.71 [+25.71] TEST: IPv6, OVS vxlan6: PMTU exceptions
             [FAIL]
# 455.60 [+25.88] TEST: IPv6, OVS vxlan6: PMTU exceptions - nexthop
objects           [FAIL]
# 481.32 [+25.72] TEST: IPv4, OVS geneve4: PMTU exceptions
             [FAIL]
# 507.38 [+26.06] TEST: IPv4, OVS geneve4: PMTU exceptions - nexthop
objects          [FAIL]
# 533.62 [+26.24] TEST: IPv6, OVS geneve4: PMTU exceptions
             [FAIL]
# 560.92 [+27.30] TEST: IPv6, OVS geneve4: PMTU exceptions - nexthop
objects          [FAIL]
# 588.20 [+27.29] TEST: IPv4, OVS geneve6: PMTU exceptions
             [FAIL]
# 615.22 [+27.02] TEST: IPv4, OVS geneve6: PMTU exceptions - nexthop
objects          [FAIL]
# 641.40 [+26.18] TEST: IPv6, OVS geneve6: PMTU exceptions
             [FAIL]
# 666.47 [+25.07] TEST: IPv6, OVS geneve6: PMTU exceptions - nexthop
objects          [FAIL]

See:
https://github.com/linux-netdev/nipa/wiki/How-to-run-netdev-selftests-CI-style
on how to reproduce that results.

/P