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Message-ID: <20250905160215.6ca5d764@elisabeth>
Date: Fri, 5 Sep 2025 16:02:15 +0200
From: Stefano Brivio <sbrivio@...hat.com>
To: Antoine Tenart <atenart@...nel.org>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
edumazet@...gle.com, dsahern@...nel.org, netdev@...r.kernel.org, Adrian
Moreno <amorenoz@...hat.com>
Subject: Re: [PATCH net] tunnels: reset the GSO metadata before reusing the
skb
On Thu, 4 Sep 2025 14:53:50 +0200
Antoine Tenart <atenart@...nel.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@...hat.com>
> Signed-off-by: Antoine Tenart <atenart@...nel.org>
Thanks for fixing this!
Reviewed-by: Stefano Brivio <sbrivio@...hat.com>
--
Stefano
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