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

[Jianguo Wu <wujianguo106@163.com>]

Hi Paolo,

On 2021/6/9 22:18, Paolo Abeni wrote:
> Hello,
> 
> On Wed, 2021-06-09 at 18:39 +0800, Jianguo Wu wrote:
>> From: Jianguo Wu <wujianguo@chinatelecom.cn>
>>
>> Lots of "TCP: tcp_fin: Impossible, sk->sk_state=7" in client side when doing stress testing.
>> 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.
> 
> Minor nit: each line in the commit message should not be longer than 72
> chars.
> 

Ok, I will be more carefully with this.

>> For case1, subflow_check_req() return -EPERM, then tcp_conn_request() will drop MP_JOIN SYN.
>> For case2, let subflow_syn_recv_sock() do mptcp_can_accept_new_subflow() check, and do fatal fallback, send reset.
> 
> Possibly worthy 2 separate patches.
> 
> pktdrill test cases would help.
> 
> I still have to wrap my mind on the above, still I have a relevant
> comment, see below...
> 
>> And do sanity check in tcp_data_queue().
>>
>> Fixes: 9466a1ccebbe("mptcp: enable JOIN requests even if cookies are in use")
>> Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
>> ---
>>  net/ipv4/tcp_input.c | 7 ++++++-
>>  net/mptcp/subflow.c  | 6 +++---
>>  2 files changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index 7d5e59f..537f24a 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -4941,8 +4941,13 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
>>  	bool fragstolen;
>>  	int eaten;
>>
>> -	if (sk_is_mptcp(sk))
>> +	if (sk_is_mptcp(sk)) {
>>  		mptcp_incoming_options(sk, skb);
>> +		if (sk->sk_state == TCP_CLOSE) {
>> +			__kfree_skb(skb);
>> +			return;
>> +		}
>> +	}
> 
> _if_ I read correctly the commit message, this is needed to avoid
> processing FIN packets on
> when check_fully_established()/check_fully_established() causes a
> subflow reset.
> 

Yes

> I think we can handle the above in the MPTCP code - in
> check_fully_established() -  setting:
> 
> 	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
> 

I will use this.

> so that the following check will drop the pkt:
> 
>>  	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
>>  		__kfree_skb(skb);
>>
> 
> /P
>