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

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.com>
> ---
>  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?

>  retry:
>  			if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
>  							      NULL, 0, 0, 0,
> -- 
> 2.29.2