netdev - Re: [PATCH v2] net-fq: Add WARN

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Date:   Fri, 8 Jun 2018 08:17:12 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Michal Kazior <michal.kazior@...to.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH v2] net-fq: Add WARN_ON check for null flow.

On 06/07/2018 04:59 PM, Cong Wang wrote:
> On Thu, Jun 7, 2018 at 4:48 PM,  <greearb@...delatech.com> wrote:
>> diff --git a/include/net/fq_impl.h b/include/net/fq_impl.h
>> index be7c0fa..cb911f0 100644
>> --- a/include/net/fq_impl.h
>> +++ b/include/net/fq_impl.h
>> @@ -78,7 +78,10 @@ static struct sk_buff *fq_tin_dequeue(struct fq *fq,
>>                         return NULL;
>>         }
>>
>> -       flow = list_first_entry(head, struct fq_flow, flowchain);
>> +       flow = list_first_entry_or_null(head, struct fq_flow, flowchain);
>> +
>> +       if (WARN_ON_ONCE(!flow))
>> +               return NULL;
>
> This does not make sense either. list_first_entry_or_null()
> returns NULL only when the list is empty, but we already check
> list_empty() right before this code, and it is protected by fq->lock.
>

Hello Michal,

git blame shows you as the author of the fq_impl.h code.

I saw a crash when debugging funky ath10k firmware in a 4.16 + hacks kernel.  There was an apparent
mostly-null deref in the fq_tin_dequeue method.  According to gdb, it was within
1 line of the dereference of 'flow'.

My hack above is probably not that useful.  Cong thinks maybe the locking is bad.

If you get a chance, please review this thread and see if you have any ideas for
a better fix (or better debugging code).

As always, if you would like me to generate you a buggy firmware that will crash
in the tx path and cause all sorts of mayhem in the ath10k driver and wifi stack,
I will be happy to do so.

https://www.mail-archive.com/netdev@vger.kernel.org/msg239738.html

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com