Re: [PATCH net v2] net: tighten bad gso csum offset check in virtio_net_hdr

[Sudeep Holla <sudeep.holla@arm.com>]

On Tue, Sep 10, 2024 at 05:35:35PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> The referenced commit drops bad input, but has false positives.
> Tighten the check to avoid these.
> 
> The check detects illegal checksum offload requests, which produce
> csum_start/csum_off beyond end of packet after segmentation.
> 
> But it is based on two incorrect assumptions:
> 
> 1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO.
> True in callers that inject into the tx path, such as tap.
> But false in callers that inject into rx, like virtio-net.
> Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or
> CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal.
> 
> 2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL.
> False, as tcp[46]_gso_segment will fix up csum_start and offset for
> all other ip_summed by calling __tcp_v4_send_check.
> 
> Because of 2, we can limit the scope of the fix to virtio_net_hdr
> that do try to set these fields, with a bogus value.
>

I see it is already queued and extremely sorry for not testing and getting
back earlier. Good news: it does fix the issue in my setup(same as reported
at [1])

So, FWIW,

Tested-by: Sudeep Holla <sudeep.holla@arm.com>

-- 
Regards,
Sudeep

[1] https://lore.kernel.org/netdev/ZtsTGp9FounnxZaN@arm.com/