netdev - Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv

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Date: Wed, 27 Sep 2023 10:14:50 +0200
From: Alexandra Winter <wintera@...ux.ibm.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>, kgraul@...ux.ibm.com,
        jaka@...ux.ibm.com
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv_sock() while
 closing listen socket



On 26.09.23 11:06, D. Wythe wrote:
>> Considering the const, maybe
>>> we need to do :
>>>
>>> 1. hold a refcnt of smc_sock for syn_recv_sock to keep smc sock valid during life time of clc sock
>>> 2. put the refcnt of smc_sock in sk_destruct in tcp_sock to release the very smc sock .
>>>
>>> In that way, we can always make sure the valid of smc sock during the life time of clc sock. Then we can use READ_ONCE rather
>>> than lock.  What do you think ?
>> I am not sure I fully understand the details what you propose to do. And it is not only syn_recv_sock(), right?
>> You need to consider all relations between smc socks and tcp socks; fallback to tcp, initial creation, children of listen sockets, variants of shutdown, ... Preferrably a single simple mechanism covers all situations. Maybe there is such a mechanism already today?
>> (I don't think clcsock->sk->sk_user_data or sk_callback_lock provide this general coverage)
>> If we really have a gap, a general refcnt'ing on smc sock could be a solution, but needs to be designed carefully.
> 
> You are right , we need designed it with care, we will try the referenced solutions internally first, and I will also send some RFCs so that everyone can track the latest progress
> and make it can be all agreed.
>> Many thanks to you and the team to help make smc more stable and robust.
> 
> Our pleasure 😁.  The stability of smc is important to us too.
> 
> Best wishes,
> D. Wythe


Just one more thought: I noticed that 
9744d2bf1976 ("smc: Fix use-after-free in tcp_write_timer_handler().")
states that unlike MPTCP, smc_clcsock_release() does not call __tcp_close().
(which matches your explanation). 
Maybe we something similar to the MPTCP approach could also solve this issue?