Re: general protection fault in skb_segment

[Willem de Bruijn]

On Tue, Jan 2, 2018 at 12:23 PM, Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>> Actually, changes just to inet_gso_segment and ipv6_gso_segment
>> will suffice:
>>
>>        bool udpfrag = false, fixedid = false, gso_partial, encap;
>>         struct sk_buff *segs = ERR_PTR(-EINVAL);
>> +       unsigned int offset = 0, gso_type;
>>         const struct net_offload *ops;
>> -       unsigned int offset = 0;
>>         struct iphdr *iph;
>>         int proto, tot_len;
>>         int nhoff;
>> @@ -1258,6 +1258,22 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
>>
>>         skb_reset_transport_header(skb);
>>
>> +       gso_type = skb_shinfo(skb)->gso_type;
>> +       if (gso_type & SKB_GSO_DODGY) {
>> +               switch (gso_type & (SKB_GSO_TCPV4 | SKB_GSO_UDP)) {
>> +               case SKB_GSO_TCPV4:
>> +                       if (proto != IPPROTO_TCP)
>> +                               goto out;
>> +                       break;
>> +               case SKB_GSO_UDP:
>> +                       if (proto != IPPROTO_UDP)
>> +                               goto out;
>> +                       break;
>> +               default:
>> +                       goto out;
>> +               }
>> +       }
>>
>> and analogous for IPv6. For a real patch I would deduplicate this
>> logic between them and move it to a separate helper function
>> (in a header file, then).
>
> This approach would also need an skb->protocol check either in
> virtio_net_hdr_to_skb or skb_mac_gso_segment.

It turns out that all paths already call skb_probe_transport_header,
which also does flow dissection, so has the necessary preconditions
for dissection checked: skb->protocol and network header have been
initialized.

I posted http://patchwork.ozlabs.org/patch/861874/ which adds
gso type and protocol tests based on that flow dissector pass.

Flow dissection was short circuited when csum offload already
set the transport header, so this does add a flow dissector call
to gso packets in practice, even if the datapath is not new.