Re: [PATCH v2 1/2] net: check if protocol extracted by virtio_net_hdr_set_proto is correct

[Willem de Bruijn]

On Tue, Mar 9, 2021 at 6:26 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Mar 08, 2021 at 11:31:25AM +0100, Balazs Nemeth wrote:
> > For gso packets, virtio_net_hdr_set_proto sets the protocol (if it isn't
> > set) based on the type in the virtio net hdr, but the skb could contain
> > anything since it could come from packet_snd through a raw socket. If
> > there is a mismatch between what virtio_net_hdr_set_proto sets and
> > the actual protocol, then the skb could be handled incorrectly later
> > on.
> >
> > An example where this poses an issue is with the subsequent call to
> > skb_flow_dissect_flow_keys_basic which relies on skb->protocol being set
> > correctly. A specially crafted packet could fool
> > skb_flow_dissect_flow_keys_basic preventing EINVAL to be returned.
> >
> > Avoid blindly trusting the information provided by the virtio net header
> > by checking that the protocol in the packet actually matches the
> > protocol set by virtio_net_hdr_set_proto. Note that since the protocol
> > is only checked if skb->dev implements header_ops->parse_protocol,
> > packets from devices without the implementation are not checked at this
> > stage.
> >
> > Fixes: 9274124f023b ("net: stricter validation of untrusted gso packets")
> > Signed-off-by: Balazs Nemeth <bnemeth@xxxxxxxxxx>
> > ---
> >  include/linux/virtio_net.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index e8a924eeea3d..6c478eee0452 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -79,8 +79,14 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >               if (gso_type && skb->network_header) {
> >                       struct flow_keys_basic keys;
> >
> > -                     if (!skb->protocol)
> > +                     if (!skb->protocol) {
> > +                             const struct ethhdr *eth = skb_eth_hdr(skb);
> > +                             __be16 etype = dev_parse_header_protocol(skb);
> > +
> >                               virtio_net_hdr_set_proto(skb, hdr);
> > +                             if (etype && etype != skb->protocol)
> > +                                     return -EINVAL;
> > +                     }
>
>
> Well the protocol in the header is an attempt at an optimization to
> remove need to parse the packet ... any data on whether this
> affecs performance?

This adds a branch and reading a cacheline that is inevitably read not
much later. It shouldn't be significant.

And this branch is only taken if skb->protocol is not set. So the cost
can easily be avoided by passing the information.

But you raise a good point, because TUNTAP does set it, but only after
the call to virtio_net_hdr_to_skb.

That should perhaps be inverted (in a separate net-next patch).