Re: [PATCH net-next 3/4] net: resolve uncommon partial checksums at ingress

[Willem de Bruijn]

> > > > +
> > > > +     if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
> > > > +                                            inner_proto))
> > > > +             return 0;
> > > > +
> > > > +     if (skb_is_gso(skb))
> > > > +             return -EINVAL;
> >
> > This is a risk the commit message will have to call out.
>
> I tried with this:
>
>         > > > We can't easily "complete the checksum" for GSO we don't recognize - GSO
>         > > > assumes knowledge of the protocol. We could segment ignoring the
>         > > > checksum then do whatever csum_start and csum_offset say, but that
>         > > > seems like an overkill for something that should happen.
>
> I guess I can add a more detailed explanation about what the risk is?

Let's just explicitly mention the consequence: non-standard GSO
packets that previous would be sent may now fail.

>
> > I don't see a simple way around it. We discussed segmenting here too,
> > but that requires intrusive changes to all callers too.
> > Alternatively,
> >
> > 1. call segmentation, but only to see it remove the SKB_GSO_DODGY and
> > thus verify that the TSO/USO packet is valid. Still drop otherwise.
> > This also does not add overhead, because it would be called later
> > anyway.
> >
> > 2. a failsafe off-by-default sysctl + static_branch that skips the
> > drop when set. Messy, but so is the situation we're dealing with.
>
> Tack this on top or do you feel it's needed straight away.

Not urgent. Can even wait until a report appears. But worth taking a
look sooner if either of us gets around to it.