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

[Dongseok Yi]

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.

> 
> > 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

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;
> > >>>>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>
> > >>
> > >
> > > .
> > >
> >