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

[Willem de Bruijn]

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.
>
> 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
>