netdev - Re: [PATCH net] net/smc: Avoid setting clcsock options after clcsock released

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Date:   Wed, 12 Jan 2022 00:34:16 +0800
From:   Wen Gu <guwen@...ux.alibaba.com>
To:     Karsten Graul <kgraul@...ux.ibm.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net/smc: Avoid setting clcsock options after clcsock
 released

Thanks for your reply.

On 2022/1/11 6:03 pm, Karsten Graul wrote:
> On 10/01/2022 10:38, Wen Gu wrote:
>> We encountered a crash in smc_setsockopt() and it is caused by
>> accessing smc->clcsock after clcsock was released.
>>
> 
> In the switch() the function smc_switch_to_fallback() might be called which also
> accesses smc->clcsock without further checking. This should also be protected then?
> Also from all callers of smc_switch_to_fallback() ?
> 
> There are more uses of smc->clcsock (e.g. smc_bind(), ...), so why does this problem
> happen in setsockopt() for you only? I suspect it depends on the test case.
> 

Yes, it depends on the test case. The crash described here only happens one time when
I run a stress test of nginx/wrk, accompanied with frequent RNIC up/down operations.

Considering accessing smc->clcsock after its release is an uncommon, low probability
issue and only happens in setsockopt() in my test, I choce an simple way to fix it, using
the existing clcsock_release_lock, and only check in smc_setsockopt() and smc_getsockopt().

> I wonder if it makes sense to check and protect smc->clcsock at all places in the code where
> it is used... as of now we had no such races like you encountered. But I see that in theory
> this problem could also happen in other code areas.
> 

But inspired by your questions, I think maybe we should treat the race as a general problem?
Do you think it is necessary to find all the potential race related to the clcsock release and
fix them in a unified approach? like define smc->clcsock as RCU pointer, hold rcu read lock
before accessing smc->clcsock and call synchronize_rcu() before resetting smc->clcsock? just a rough idea :)

Or we should decide it later, do some more tests to see the probability of recurrence of this problem?

Thanks,
Wen Gu