netdev - Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net

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Message-ID: <66e05a2259919_9de00294f9@willemb.c.googlers.com.notmuch>
Date: Tue, 10 Sep 2024 10:39:30 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Wang <jasowang@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, 
 davem@...emloft.net, 
 kuba@...nel.org, 
 edumazet@...gle.com, 
 pabeni@...hat.com, 
 stable@...r.kernel.org, 
 nsz@...t70.net, 
 mst@...hat.com, 
 yury.khrustalev@....com, 
 broonie@...nel.org, 
 sudeep.holla@....com, 
 Willem de Bruijn <willemb@...gle.com>, 
 stable@...r.kernel.net
Subject: Re: [PATCH net] net: tighten bad gso csum offset check in
 virtio_net_hdr

Jason Wang wrote:
> On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > From: Willem de Bruijn <willemb@...gle.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.
> >
> > Link: https://lore.kernel.org/netdev/20240909094527.GA3048202@port70.net/
> > Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > Cc: <stable@...r.kernel.net>
> >
> > ---
> >
> > Verified that the syzbot repro is still caught.
> >
> > An equivalent alternative would be to move the check for csum_offset
> > to where the csum_start check is in segmentation:
> >
> > -    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb)))
> > +    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb) ||
> > +                 skb->csum_offset != offsetof(struct tcphdr, check)))
> >
> > Cleaner, but messier stable backport.
> >
> > We'll need an equivalent patch to this for VIRTIO_NET_HDR_GSO_UDP_L4.
> > But that csum_offset test was in a different commit, so different
> 
> Not for this patch, but I see this in UDP_L4:
> 
>                        if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
>                                return -EINVAL;
> 
> This seems to forbid VIRTIO_NET_HDR_F_DATA_VALID. I wonder what's the
> reason for doing this.

It tests &, not == ?

> > Fixes tag.
> > ---
> >  include/linux/virtio_net.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index 6c395a2600e8d..276ca543ef44d 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >                         break;
> >                 case SKB_GSO_TCPV4:
> >                 case SKB_GSO_TCPV6:
> > -                       if (skb->csum_offset != offsetof(struct tcphdr, check))
> > +                       if (skb->ip_summed == CHECKSUM_PARTIAL &&
> > +                           skb->csum_offset != offsetof(struct tcphdr, check))
> >                                 return -EINVAL;
> >                         break;
> >                 }
> > --
> > 2.46.0.598.g6f2099f65c-goog
> >
> 
> Acked-by: Jason Wang <jasowang@...hat.com>

Thanks for reviewing