Re: [PATCH v5 4/4] mptcp: avoid processing packet if a subflow reset

[Jianguo Wu <wujianguo106@163.com>]

On 2021/6/23 17:48, Paolo Abeni wrote:
> On Tue, 2021-06-22 at 17:00 -0700, Mat Martineau wrote:
>> On Mon, 21 Jun 2021, Jianguo Wu wrote:
>>
>>> Hi Mat,
>>>
>>> On 2021/6/19 8:19, Mat Martineau wrote:
>>>> On Wed, 16 Jun 2021, wujianguo106@163.com wrote:
>>>>
>>>>> From: Jianguo Wu <wujianguo@chinatelecom.cn>
>>>>>
>>>>> If check_fully_established() causes a subflow reset, it should not
>>>>> continue to process the packet in tcp_data_queue().
>>>>>
>>>>> setting:
>>>>>     TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
>>>>>
>>>>> so that the following check will drop the pkt in
>>>>> tcp_data_queue():
>>>>>  if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
>>>>>     __kfree_skb(skb);
>>>>>     return;
>>>>>  }
>>>>>
>>>>> Fixes: d582484726c4 ("mptcp: fix fallback for MP_JOIN subflows")
>>>>> Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
>>>>> ---
>>>>> net/mptcp/options.c | 6 ++++++
>>>>> 1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/net/mptcp/options.c b/net/mptcp/options.c
>>>>> index 1aec01686c1a..be435c5421cd 100644
>>>>> --- a/net/mptcp/options.c
>>>>> +++ b/net/mptcp/options.c
>>>>> @@ -926,6 +926,12 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
>>>>>     return true;
>>>>>
>>>>> reset:
>>>>> +    /* If a subflow is reset, the packet should not continue to be
>>>>> +     * processed in tcp_data_queue(), so setting: end_seq = seq,
>>>>> +     * then tcp_data_queue() will drop the packet.
>>>>> +     */
>>>>> +    TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
>>>>> +
>>>>
>>>> This does have the desired effect when mptcp_incoming_options() is 
>>>> called from tcp_data_queue(), but mptcp_incoming_options() is also 
>>>> called from tcp_reset() and tcp_rcv_state_process(). The other callers 
>>>> appear to tolerate the sequence number modification.
>>>>
>>>> I think it would be clearer to either add a return value or output 
>>>> parameter to mptcp_incoming_options() to explicitly tell the caller 
>>>> that a reset has been sent and tcp_done() called. Then it would be 
>>>> clearer in tcp_data_queue() that the packet is being discarded due to 
>>>> mptcp header content.
>>>>
>>>
>>> If a reset has been sent and tcp_done() called in 
>>> check_fully_established(), the sk_state will be TCP_CLOSE, how about 
>>> just do (sk_state == TCP_CLOSE) check in tcp_data_queue() as it did in 
>>> the V1 of this patch?
>>
>> Oh, I see now that Paolo suggested the the end_seq assignment in order to 
>> only modify MPTCP code.
>>
>> I still think it's better to make it clear that we're discarding a packet 
>> due to the mptcp headers - using the existing sequence check (intended to 
>> detect acks) in tcp_data_queue() seems sneaky to me.
>>
>> Something like
>>
>> if (sk_is_mptcp(sk) && !mptcp_incoming_options(sk, skb)) {
>>  	__kfree_skb(skb);
>>  	return;
>> }
>>
>> seems both compact and clear. Does that seem ok Paolo?
> 
> Uhmmm... we need to touch every mptcp_incoming_options() call site, and
> in tcp_reset() the above chunk looks a bit strange to me. Probably we
> could just ignore the mptcp_incoming_options() return value there. 
> 
> Otherwise no big objections - not sure about upstream ;)
> 

Hi Mat and Paolo,

If you both agree, I will send a new version that mptcp_incoming_options() add a return value, and only check return value in tcp_data_queue(),
ignore the return value in other call site.

> Cheers,
> 
> Paolo
>