Re: [PATCH net] tunnels: reset the GSO metadata before reusing the skb

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  4 Sep 2025 14:53:50 +0200
Antoine Tenart <atenart@kernel.org> wrote:

> If a GSO skb is sent through a Geneve tunnel and if Geneve options are
> added, the split GSO skb might not fit in the MTU anymore and an ICMP
> frag needed packet can be generated. In such case the ICMP packet might
> go through the segmentation logic (and dropped) later if it reaches a
> path were the GSO status is checked and segmentation is required.
> 
> This is especially true when an OvS bridge is used with a Geneve tunnel
> attached to it. The following set of actions could lead to the ICMP
> packet being wrongfully segmented:
> 
> 1. An skb is constructed by the TCP layer (e.g. gso_type SKB_GSO_TCPV4,
>    segs >= 2).
> 
> 2. The skb hits the OvS bridge where Geneve options are added by an OvS
>    action before being sent through the tunnel.
> 
> 3. When the skb is xmited in the tunnel, the split skb does not fit
>    anymore in the MTU and iptunnel_pmtud_build_icmp is called to
>    generate an ICMP fragmentation needed packet. This is done by reusing
>    the original (GSO!) skb. The GSO metadata is not cleared.
> 
> 4. The ICMP packet being sent back hits the OvS bridge again and because
>    skb_is_gso returns true, it goes through queue_gso_packets...
> 
> 5. ...where __skb_gso_segment is called. The skb is then dropped.
> 
> 6. Note that in the above example on re-transmission the skb won't be a
>    GSO one as it would be segmented (len > MSS) and the ICMP packet
>    should go through.
> 
> Fix this by resetting the GSO information before reusing an skb in
> iptunnel_pmtud_build_icmp and iptunnel_pmtud_build_icmpv6.
> 
> Fixes: 4cb47a8644cc ("tunnels: PMTU discovery support for directly bridged IP packets")
> Reported-by: Adrian Moreno <amorenoz@redhat.com>
> Signed-off-by: Antoine Tenart <atenart@kernel.org>

Thanks for fixing this!

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

-- 
Stefano