netdev - Re: [PATCH net 4/4] udp: prevent local UDP tunnel packets from being GROed

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Date: Mon, 18 Mar 2024 09:43:57 +0100
From: Antoine Tenart <atenart@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc: steffen.klassert@...unet.com, netdev@...r.kernel.org
Subject: Re: [PATCH net 4/4] udp: prevent local UDP tunnel packets from being GROed

Hello,

Quoting Willem de Bruijn (2024-03-16 16:43:26)
> Antoine Tenart wrote:
> > GRO has a fundamental issue with UDP tunnel packets as it can't detect
> > those in a foolproof way and GRO could happen before they reach the
> > tunnel endpoint. Previous commits have fixed issues when UDP tunnel
> > packets come from a remote host, but if those packets are issued locally
> > they could run into checksum issues.
> > 
> > If the inner packet has a partial checksum the information will be lost
> > in the GRO logic, either in udp4/6_gro_complete or in
> > udp_gro_complete_segment and packets will have an invalid checksum when
> > leaving the host.
> 
> Before the previous patch, the tunnel code would convert ip_summed to
> CHECKSUM_UNNECESSARY. After that patch CHECKSUM_PARTIAL is preserved.
> Are the tunneled packets still corrupted once forwarded to the egress
> path? In principle CHECKSUM partial with tunnel and GSO should work,
> whether built as such or arrived at through GRO.

Previous patch removed the partial -> unnecessary conversion for
fraglist only; but packets GROed by rx-udp-gro-forwarding can hit
udp_gro_complete_segment and the partial info would be overwritten
there in case of an UDP tunnel packet GROed at the UDP level with the
inner csum being partial.

Thanks!
Antoine