netdev - Re: [PATCH net 2/2] net/smc: fix kernel panic caused by race of smc

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Message-ID: <444f15e6-db60-3e86-fe20-32f24928844c@linux.ibm.com>
Date:   Wed, 29 Dec 2021 13:33:20 +0100
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     Dust Li <dust.li@...ux.alibaba.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        Wen Gu <guwen@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>
Subject: Re: [PATCH net 2/2] net/smc: fix kernel panic caused by race of
 smc_sock

On 28/12/2021 10:03, Dust Li wrote:
> A crash occurs when smc_cdc_tx_handler() tries to access smc_sock
> but smc_release() has already freed it.

I am not sure about what happened here. 
Your patch removes the whole dismisser concept that was introduced to
solve exactly the problem you describe. And you implemented a different approach.

In theory, when smc_cdc_tx_handler() is called but the connection is already
freed than the connection should have gone through smc_cdc_tx_dismiss_slots(),
called by smc_conn_kill() or smc_conn_free(). If that happened there would be no
access to an already freed address in smc_cdc_tx_handler().

Can you explain why the code reached smc_cdc_tx_handler() with cdcpend->conn
pointing to a connection that is already freed? I think if there is a bug it should
be fixed instead of replacing the code by a new construct.

Thoughts?