Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4

[Yunsheng Lin]

On 2021/5/7 9:25, Willem de Bruijn wrote:
>>>> head_skb's data_len is the sum of skb_gro_len for each skb of the frags.
>>>> data_len could be 8 if server sent a small size packet and it is GROed
>>>> to head_skb.
>>>>
>>>> Please let me know if I am missing something.
>>>
>>> This is my understanding of the data path. This is a forwarding path
>>> for TCP traffic.
>>>
>>> GRO is enabled and will coalesce multiple segments into a single large
>>> packet. In bad cases, the coalesced packet payload is > MSS, but < MSS
>>> + 20.
>>>
>>> Somewhere between GRO and GSO you have a BPF program that converts the
>>> IPv6 address to IPv4.
>>
>> Your understanding is right. The data path is GRO -> BPF 6 to 4 ->
>> GSO.
>>
>>>
>>> There is no concept of head_skb at the time of this BPF program. It is
>>> a single SKB, with an skb linear part and multiple data items in the
>>> frags (no frag_list).
>>
>> Sorry for the confusion. head_skb what I mentioned was a skb linear
>> part. I'm considering a single SKB with frags too.
>>
>>>
>>> When entering the GSO stack, this single skb now has a payload length
>>> < MSS. So it would just make a valid TCP packet on its own?
>>>
>>> skb_gro_len is only relevant inside the GRO stack. It internally casts
>>> the skb->cb[] to NAPI_GRO_CB. This field is a scratch area that may be
>>> reused for other purposes later by other layers of the datapath. It is
>>> not safe to read this inside bpf_skb_proto_6_to_4.
>>
>> The condition what I made uses skb->data_len not skb_gro_len. Does
>> skb->data_len have a different meaning on each layer? As I know,
>> data_len indicates the amount of frags or frag_list. skb->data_len
>> should be > 20 in the sample case because the payload size of the skb
>> linear part is the same with mss.
> 
> Ah, got it.
> 
> data_len is the length of the skb minus the length in the skb linear
> section (as seen in skb_headlen).
> 
> So this gso skb consists of two segments, the first one entirely
> linear, the payload of the second is in skb_shinfo(skb)->frags[0].
> 
> It is not guaranteed that gso skbs built from two individual skbs end
> up looking like that. Only protocol headers in the linear segment and
> the payload of both in frags is common.
> 
>> We can modify netif_needs_gso as another option to hit
>> skb_needs_linearize in validate_xmit_skb. But I think we should compare
>> skb->gso_size and skb->data_len too to check if mss exceed a payload
>> size.
> 
> The rest of the stack does not build such gso packets with payload len
> < mss, so we should not have to add workarounds in the gso hot path
> for this.
> 
> Also no need to linearize this skb. I think that if the bpf program
> would just clear the gso type, the packet would be sent correctly.
> Unless I'm missing something.

Does the checksum/len field in ip and tcp/udp header need adjusting
before clearing gso type as the packet has became bigger?

Also, instead of testing skb->data_len, may test the skb->len?

skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff

> 
> But I don't mean to argue that it should do that in production.
> Instead, not playing mss games would solve this and stay close to the
> original datapath if no bpf program had been present. Including
> maintaining the GSO invariant of sending out the same chain of packets
> as received (bar the IPv6 to IPv4 change).
> 
> This could be achieved by adding support for the flag
> BPF_F_ADJ_ROOM_FIXED_GSO in the flags field of bpf_skb_change_proto.
> And similar to bpf_skb_net_shrink:
> 
>                 /* Due to header shrink, MSS can be upgraded. */
>                 if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
>                         skb_increase_gso_size(shinfo, len_diff);
> 
> The other case, from IPv4 to IPv6 is more difficult to address, as not
> reducing the MSS will result in packets exceeding MTU. That calls for
> workarounds like MSS clamping. Anyway, that is out of scope here.
> 
> 
> 
>>>
>>>
>>>>>
>>>>> One simple solution if this packet no longer needs to be segmented
>>>>> might be to reset the gso_type completely.
>>>>
>>>> I am not sure gso_type can be cleared even when GSO is needed.
>>>>
>>>>>
>>>>> In general, I would advocate using BPF_F_ADJ_ROOM_FIXED_GSO. When
>>>>> converting from IPv6 to IPv4, fixed gso will end up building packets
>>>>> that are slightly below the MTU. That opportunity cost is negligible
>>>>> (especially with TSO). Unfortunately, I see that that flag is
>>>>> available for bpf_skb_adjust_room but not for bpf_skb_proto_6_to_4.
>>>>>
>>>>>
>>>>>>>> would increse the gso_size to 1392. tcp_gso_segment will get an error
>>>>>>>> with 1380 <= 1392.
>>>>>>>>
>>>>>>>> Check for the size of GROed payload if it is really bigger than target
>>>>>>>> mss when increase mss.
>>>>>>>>
>>>>>>>> Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper)
>>>>>>>> Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
>>>>>>>> ---
>>>>>>>>   net/core/filter.c | 4 +++-
>>>>>>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>>>>>> index 9323d34..3f79e3c 100644
>>>>>>>> --- a/net/core/filter.c
>>>>>>>> +++ b/net/core/filter.c
>>>>>>>> @@ -3308,7 +3308,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
>>>>>>>>             }
>>>>>>>>
>>>>>>>>             /* Due to IPv4 header, MSS can be upgraded. */
>>>>>>>> -           skb_increase_gso_size(shinfo, len_diff);
>>>>>>>> +           if (skb->data_len > len_diff)
>>>>>>>
>>>>>>> Could you elaborate some more on what this has to do with data_len specifically
>>>>>>> here? I'm not sure I follow exactly your above commit description. Are you saying
>>>>>>> that you're hitting in tcp_gso_segment():
>>>>>>>
>>>>>>>          [...]
>>>>>>>          mss = skb_shinfo(skb)->gso_size;
>>>>>>>          if (unlikely(skb->len <= mss))
>>>>>>>                  goto out;
>>>>>>>          [...]
>>>>>>
>>>>>> Yes, right
>>>>>>
>>>>>>>
>>>>>>> Please provide more context on the bug, thanks!
>>>>>>
>>>>>> tcp_gso_segment():
>>>>>>         [...]
>>>>>>         __skb_pull(skb, thlen);
>>>>>>
>>>>>>         mss = skb_shinfo(skb)->gso_size;
>>>>>>         if (unlikely(skb->len <= mss))
>>>>>>         [...]
>>>>>>
>>>>>> skb->len will have total GROed TCP payload size after __skb_pull.
>>>>>> skb->len <= mss will not be happened in a normal GROed situation. But
>>>>>> bpf_skb_proto_6_to_4 would upgrade MSS by increasing gso_size, it can
>>>>>> hit an error condition.
>>>>>>
>>>>>> We should ensure the following condition.
>>>>>> total GROed TCP payload > the original mss + (IPv6 size - IPv4 size)
>>>>>>
>>>>>> Due to
>>>>>> total GROed TCP payload = the original mss + skb->data_len
>>>>>> IPv6 size - IPv4 size = len_diff
>>>>>>
>>>>>> Finally, we can get the condition.
>>>>>> skb->data_len > len_diff
>>>>>>
>>>>>>>
>>>>>>>> +                   skb_increase_gso_size(shinfo, len_diff);
>>>>>>>> +
>>>>>>>>             /* Header must be checked, and gso_segs recomputed. */
>>>>>>>>             shinfo->gso_type |= SKB_GSO_DODGY;
>>>>>>>>             shinfo->gso_segs = 0;
>>>>>>>>
>>>>>>
>>>>>>
>>>>
>>
> 
> .
>