linux-kernel - Re: [PATCH net] tcp: Fix error ts_recent time during three-way handshake

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Message-ID: <5fa8fc14-b67b-4da1-ac8e-339fd3e536c2@huawei.com>
Date: Wed, 19 Feb 2025 21:11:19 +0800
From: Wang Hai <wanghai38@...wei.com>
To: Jason Xing <kerneljasonxing@...il.com>
CC: Eric Dumazet <edumazet@...gle.com>, <ncardwell@...gle.com>,
	<kuniyu@...zon.com>, <davem@...emloft.net>, <dsahern@...nel.org>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <horms@...nel.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Fix error ts_recent time during three-way
 handshake



On 2025/2/19 11:31, Jason Xing wrote:
> On Wed, Feb 19, 2025 at 10:16 AM Wang Hai <wanghai38@...wei.com> wrote:
>>
>>
>>
>> On 2025/2/18 21:35, Eric Dumazet wrote:
>>> On Tue, Feb 18, 2025 at 12:00 PM Wang Hai <wanghai38@...wei.com> wrote:
>>>>
>>>> If two ack packets from a connection enter tcp_check_req at the same time
>>>> through different cpu, it may happen that req->ts_recent is updated with
>>>> with a more recent time and the skb with an older time creates a new sock,
>>>> which will cause the tcp_validate_incoming check to fail.
>>>>
>>>> cpu1                                cpu2
>>>> tcp_check_req
>>>>                                       tcp_check_req
>>>> req->ts_recent = tmp_opt.rcv_tsval = t1
>>>>                                       req->ts_recent = tmp_opt.rcv_tsval = t2
>>>>
>>>> newsk->ts_recent = req->ts_recent = t2 // t1 < t2
>>>> tcp_child_process
>>>> tcp_rcv_state_process
>>>> tcp_validate_incoming
>>>> tcp_paws_check
>>>> if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win) // failed
>>>>
>>>> In tcp_check_req, restore ts_recent to this skb's to fix this bug.
>>>>
>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>> Signed-off-by: Wang Hai <wanghai38@...wei.com>
>>>> ---
>>>>    net/ipv4/tcp_minisocks.c | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
>>>> index b089b08e9617..0208455f9eb8 100644
>>>> --- a/net/ipv4/tcp_minisocks.c
>>>> +++ b/net/ipv4/tcp_minisocks.c
>>>> @@ -878,6 +878,10 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
>>>>           sock_rps_save_rxhash(child, skb);
>>>>           tcp_synack_rtt_meas(child, req);
>>>>           *req_stolen = !own_req;
>>>> +       if (own_req && tcp_sk(child)->rx_opt.tstamp_ok &&
>>>> +           unlikely(tcp_sk(child)->rx_opt.ts_recent != tmp_opt.rcv_tsval))
>>>> +               tcp_sk(child)->rx_opt.ts_recent = tmp_opt.rcv_tsval;
>>>> +
>>>>           return inet_csk_complete_hashdance(sk, child, req, own_req);
>>>
>>> Have you seen the comment at line 818 ?
>>>
>>> /* TODO: We probably should defer ts_recent change once
>>>    * we take ownership of @req.
>>>    */
>>>
>>> Plan was clear and explained. Why implement something else (and buggy) ?
>>>
>> Hi Eric,
>>
>> Currently we have a real problem, so we want to solve it. This bug
>> causes the upper layers to be unable to be notified to call accept after
>> the successful three-way handshake.
>>
>> Skb from cpu1 that fails at tcp_paws_check (which it could have
>> succeeded) will not be able to enter the TCP_ESTABLISHED state, and
>> therefore parent->sk_data_ready(parent) will not be triggered, and skb
>> from cpu2 can complete the three-way handshake, but there is also no way
>> to call parent->sk_data_ready(parent) to notify the upper layer, which
>> will result
>> in the upper layer not being able to sense and call accept to obtain the
>> nsk.
>>
>> cpu1                                cpu2
>> tcp_check_req
>>                                       tcp_check_req
>> req->ts_recent = tmp_opt.rcv_tsval = t1
>>                                       req->ts_recent=tmp_opt.rcv_tsval= t2
>>
>> newsk->ts_recent = req->ts_recent = t2 // t1 < t2
>> tcp_child_process
>>    tcp_rcv_state_process
>>     tcp_validate_incoming
>>      tcp_paws_check // failed
>>    parent->sk_data_ready(parent); // will not be called
>>                                       tcp_v4_do_rcv
>>                                       tcp_rcv_state_process // Complete the three-way handshake
>>                                                                                                          // missing parent->sk_data_ready(parent);
> 
> IIUC, the ack received from cpu1 triggered calling
> inet_csk_complete_hashdance() so its state transited from
> TCP_NEW_SYN_RECV to TCP_SYN_RECV, right? If so, the reason why not
> call sk_data_ready() if the skb entered into tcp_child_process() is
> that its state failed to transit to TCP_ESTABLISHED?
> 
Yes, because it didn't switch to TCP_ESTABLISHED
> Here is another question. How did the skb on the right side enter into
> tcp_v4_do_rcv() after entering tcp_check_req() if the state of sk
> which the skb belongs to is TCP_NEW_SYN_RECV? Could you elaborate more
> on this point?
Since cpu1 successfully created the child sock, cpu2 will return
null in tcp_check_req and req_stolen is set to true, so that it will
subsequently go to 'goto lookup' to re-process the packet, and at
this point, sk->sk_state is already in TCP_SYN_RECV state, and then
then tcp_v4_do_rcv is called.
> 
> Thanks,
> Jason