Re: [PATCH v2] net-fq: Add WARN_ON check for null flow.

[Arend van Spriel <arend.vanspriel@broadcom.com>]

On 6/8/2018 5:17 PM, Ben Greear wrote:

I recalled an email from Michał leaving tieto so adding his alternate 
email he provided back then.

Gr. AvS

> On 06/07/2018 04:59 PM, Cong Wang wrote:
>> On Thu, Jun 7, 2018 at 4:48 PM,  <greearb@candelatech.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
>