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

[Jianguo Wu <wujianguo106@163.com>]

On 2021/6/24 18:02, Paolo Abeni wrote:
> On Thu, 2021-06-24 at 09:57 +0800, Jianguo Wu wrote:
>>
>> 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.
> 
> Even the hook in tcp_rcv_state_process() can be followed by
> tcp_data_queue().
> 
> I *think* it's better ignoring the return value of
> mptcp_incoming_options() only in tcp_reset(), adding there a comment -
> something alike "mptcp can't tell us to ignore reset pkts".
> 
> Cheers,
> 
> Paolo
> 
> p.s. I'm sorry for the long, difficult and somewhat on/off review
> process. This change is indeed tricky. Don't despair, it looks like
> it's near to an happy ending!
> 

Thanks for your review and suggestions! I will send a new version.

>