Re: [PATCH v5 3/4] mptcp: fix syncookie process if mptcp can not_accept new subflow

[Jianguo Wu <wujianguo106@163.com>]

On 2021/6/21 14:24, Jianguo Wu wrote:
> 
> Hi Mat,
> 
> On 2021/6/19 7:19, Mat Martineau wrote:
>> On Wed, 16 Jun 2021, wujianguo106@163.com wrote:
>>
>>> From: Jianguo Wu <wujianguo@chinatelecom.cn>
>>>
>>> Lots of "TCP: tcp_fin: Impossible, sk->sk_state=7" in client side
>>> when doing stress testing.
>>
>> Hi Jianguo -
>>
>> Like patch 1, do the existing syncookie selftests trigger this sometimes?
>>
>> Or are there selftests that could be added to this series, or packetdrill tests, to do such syncookie stress tests?
>>
> 
> This is triggered by the same test in patch 1,
> I will try selftests too, or add selftest if necessary.
> 
>> Thanks,
>>
>> Mat
>>
>>
>>>
>>> There are at least two cases may trigger this warning:
>>> 1.mptcp is in syncookie, and server recv MP_JOIN SYN request,
>>>  in subflow_check_req(), the mptcp_can_accept_new_subflow()
>>>  return false, so subflow_init_req_cookie_join_save() isn't
>>>  called, i.e. not store the data present in the MP_JOIN syn
>>>  request and the random nonce in hash table - join_entries[],
>>>  but still send synack. When recv 3rd-ack,
>>>  mptcp_token_join_cookie_init_state() will return false, and
>>>  3rd-ack is dropped, then if mptcp conn is closed by client,
>>>  client will send a DATA_FIN and a MPTCP FIN, the DATA_FIN
>>>  doesn't have MP_CAPABLE or MP_JOIN,
>>>  so mptcp_subflow_init_cookie_req() will return 0, and pass
>>>  the cookie check, MP_JOIN request is fallback to normal TCP.
>>>  Server will send a TCP FIN if closed, in client side,
>>>  when process TCP FIN, it will do reset, the code path is:
>>>    tcp_data_queue()->mptcp_incoming_options()
>>>      ->check_fully_established()->mptcp_subflow_reset().
>>>  mptcp_subflow_reset() will set sock state to TCP_CLOSE,
>>>  so tcp_fin will hit TCP_CLOSE, and print the warning.
>>> 2.mptcp is in syncookie, and server recv 3rd-ack, in
>>>  mptcp_subflow_init_cookie_req(), mptcp_can_accept_new_subflow()
>>>  return false, and subflow_req->mp_join is not set to 1,
>>>  so in subflow_syn_recv_sock() will not reset the MP_JOIN
>>>  subflow, but fallback to normal TCP, and then the same thing
>>>  happens when server will send a TCP FIN if closed.
>>>
>>> For case1, subflow_check_req() return -EPERM,
>>> then tcp_conn_request() will drop MP_JOIN SYN.
>>>

Hi Mat,

As MP_JOIN SYN is dropped, no SYN/ACK will be replied, So test case "subflows limited by server w cookies" in mptcp_join.sh should be updated,
like this:

diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 523c7797f30a..37b7da3cd5ca 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -1398,7 +1398,7 @@ syncookies_tests()
        ip netns exec $ns2 ./pm_nl_ctl add 10.0.3.2 flags subflow
        ip netns exec $ns2 ./pm_nl_ctl add 10.0.2.2 flags subflow
        run_tests $ns1 $ns2 10.0.1.1
-       chk_join_nr "subflows limited by server w cookies" 2 2 1
+       chk_join_nr "subflows limited by server w cookies" 2 1 1

        # test signal address with cookies
        reset_with_cookies

Is this OK?

Jianguo,
Thanks

>>> For case2, let subflow_syn_recv_sock() call mptcp_can_accept_new_subflow(),
>>> and do fatal fallback, send reset.
>>>
>>> Fixes: 9466a1ccebbe("mptcp: enable JOIN requests even if cookies are in use")
>>> Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
>>> ---
>>> net/mptcp/subflow.c | 6 +++---
>>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
>>> index 75ed530706c0..6d98e19a20aa 100644
>>> --- a/net/mptcp/subflow.c
>>> +++ b/net/mptcp/subflow.c
>>> @@ -224,6 +224,8 @@ static int subflow_check_req(struct request_sock *req,
>>>         if (unlikely(req->syncookie)) {
>>>             if (mptcp_can_accept_new_subflow(subflow_req->msk))
>>>                 subflow_init_req_cookie_join_save(subflow_req, skb);
>>> +            else
>>> +                return -EPERM;
>>>         }
>>>
>>>         pr_debug("token=%u, remote_nonce=%u msk=%p", subflow_req->token,
>>> @@ -263,9 +265,7 @@ int mptcp_subflow_init_cookie_req(struct request_sock *req,
>>>         if (!mptcp_token_join_cookie_init_state(subflow_req, skb))
>>>             return -EINVAL;
>>>
>>> -        if (mptcp_can_accept_new_subflow(subflow_req->msk))
>>> -            subflow_req->mp_join = 1;
>>> -
>>> +        subflow_req->mp_join = 1;
>>>         subflow_req->ssn_offset = TCP_SKB_CB(skb)->seq - 1;
>>>     }
>>>
>>> -- 
>>> 1.8.3.1
>>>
>>>
>>>
>>
>> -- 
>> Mat Martineau
>> Intel
>