Re: [PATCH] net: check if protocol extracted by virtio_net_hdr_set_proto is correct

[Jason Wang]

On 2021/2/18 11:50 下午, Willem de Bruijn wrote:
> On Thu, Feb 18, 2021 at 10:01 AM Balazs Nemeth <bnemeth@xxxxxxxxxx> 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 by gso.
> >
> > The network header of gso packets starts at 14 bytes, but a specially
> > crafted packet could fool the call to skb_flow_dissect_flow_keys_basic
> > as the network header offset in the skb could be incorrect.
> > Consequently, EINVAL is not returned.
> >
> > There are even packets that can cause an infinite loop. For example, a
> > packet with ethernet type ETH_P_MPLS_UC (which is unnoticed by
> > virtio_net_hdr_to_skb) that is sent to a geneve interface will be
> > handled by geneve_build_skb. In turn, it calls
> > udp_tunnel_handle_offloads which then calls skb_reset_inner_headers.
> > After that, the packet gets passed to mpls_gso_segment. That function
> > calculates the mpls header length by taking the difference between
> > network_header and inner_network_header. Since the two are equal
> > (due to the earlier call to skb_reset_inner_headers), it will calculate
> > a header of length 0, and it will not pull any headers. Then, it will
> > call skb_mac_gso_segment which will again call mpls_gso_segment, etc...
> > This leads to the infinite loop.


I remember kernel will validate dodgy gso packets in gso ops. I wonder 
why not do the check there? The reason is that virtio/TUN is not the 
only source for those packets.

Thanks


> > For that reason, address the root cause of the issue: don't blindly
> > trust the information provided by the virtio net header. Instead,
> > check if the protocol in the packet actually matches the protocol set by
> > virtio_net_hdr_set_proto.
> >
> > Fixes: 9274124f023b ("net: stricter validation of untrusted gso packets")
> > Signed-off-by: Balazs Nemeth <bnemeth@xxxxxxxxxx>
> > ---
> >   include/linux/virtio_net.h | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index e8a924eeea3d..cf2c53563f22 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -79,8 +79,13 @@ 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);
> > +
> Unfortunately, cannot assume that the device type is ARPHRD_ETHER.
>
> The underlying approach is sound: packets that have a gso type set in
> the virtio_net_hdr have to be IP packets.
>
> >                                  virtio_net_hdr_set_proto(skb, hdr);
> > +                               if (skb->protocol != eth->h_proto)
> > +                                       return -EINVAL;
> > +                       }
> >   retry:
> >                          if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
> >                                                                NULL, 0, 0, 0,
> > --
> > 2.29.2