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

[Yunsheng Lin]

On 2021/5/7 16:25, Dongseok Yi wrote:
> On Thu, May 06, 2021 at 09:53:45PM -0400, Willem de Bruijn wrote:
>> On Thu, May 6, 2021 at 9:45 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>>
>>> 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?
>>
>> gro takes care of this. see for instance inet_gro_complete for updates
>> to the ip header.
> 
> I think clearing the gso type will get an error at tcp4_gso_segment
> because netif_needs_gso returns true in validate_xmit_skb.

So the bpf_skb_proto_6_to_4() is called after validate_xmit_skb() and
before tcp4_gso_segment()?
If Yes, clearing the gso type here does not seem to help.

> 
>>
>>> 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
>>
>> Yes. Essentially doing the same calculation as the gso code that is
>> causing the packet to be dropped.
> 
> BPF program is usually out of control. Can we take a general approach?
> The below 2 cases has no issue when mss upgrading.
> 1) skb->data_len > mss + 20
> 2) skb->data_len < mss && skb->data_len > 20
> The corner case is when
> 3) skb->data_len > mss && skb->data_len < mss + 20

As my understanding:

Usually skb_headlen(skb) >= (mac header + ip/ipv6 header + udp/tcp header),
other than that, there is no other guarantee as long as:
skb->len = skb_headlen(skb) + skb->data_len

So the cases should be:
1. skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff
2. skb->len - (mac header + ip/ipv6 header + udp/tcp header) <= mss + len_diff

The corner case is case 2.

> 
> But to cover #3 case, we should check the condition Yunsheng Lin said.
> What if we do mss upgrading for both #1 and #2 cases only?
> 
> +               unsigned short off_len = skb->data_len > shinfo->gso_size ?
> +                       shinfo->gso_size : 0;
> [...]
>                 /* Due to IPv4 header, MSS can be upgraded. */
> -               skb_increase_gso_size(shinfo, len_diff);
> +               if (skb->data_len - off_len > len_diff)
> +                       skb_increase_gso_size(shinfo, 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;
>>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>>
>>>> .
>>>>
>>>
> 
> 
> .
>