netdev - Re: [PATCH net] net: drop bad gso csum_start and offset in virtio_net

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Message-ID: <CAF=yD-J57z=iUZChLJR4YXq-3X-qPc+N93jvpCy5HE89B7-Tdw@mail.gmail.com>
Date: Fri, 26 Jul 2024 09:52:01 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	edumazet@...gle.com, mst@...hat.com, jasowang@...hat.com, arefev@...mel.ru, 
	alexander.duyck@...il.com, Willem de Bruijn <willemb@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH net] net: drop bad gso csum_start and offset in virtio_net_hdr

On Fri, Jul 26, 2024 at 4:23 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 7/26/24 04:32, Willem de Bruijn wrot> @@ -182,6 +171,11 @@ static
> inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >                       if (gso_type != SKB_GSO_UDP_L4)
> >                               return -EINVAL;
> >                       break;
> > +             case SKB_GSO_TCPV4:
> > +             case SKB_GSO_TCPV6:
>
> I think we need to add here an additional check:
>
>                         if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
>                                 return -EINVAL;
>

Historically this interface has been able to request
VIRTIO_NET_HDR_GSO_* without VIRTIO_NET_HDR_F_NEEDS_CSUM.

I agree that that makes little sense. Until now we have been
accommodating it, however. See the else branch if that checksum
offload flag is not set.

I would love to clamp down on this, as those packets are essentially
illegal. But we should probably leave that discussion for a separate
patch?

> > +                     if (skb->csum_offset != offsetof(struct tcphdr, check))
> > +                             return -EINVAL;
> > +                     break;
> >               }
> >
> >               /* Kernel has a special handling for GSO_BY_FRAGS. */
> > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> > index 4b791e74529e1..9e49ffcc77071 100644
> > --- a/net/ipv4/tcp_offload.c
> > +++ b/net/ipv4/tcp_offload.c
> > @@ -140,6 +140,9 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
> >       if (thlen < sizeof(*th))
> >               goto out;
> >
> > +     if (unlikely(skb->csum_start != skb->transport_header))
> > +             goto out;
>
> Given that for packet injected from user-space, the transport offset is
> set to csum_start by skb_partial_csum_set(), do we need the above check?
> If so, why don't we need another similar one for csum_offset even here?

Same point. Sadly it is not set if checksum offload is not requested.