linux-kernel - Re: [PATCH] tty: fix data race in n_tty_receive_buf

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Date:   Fri, 5 Jan 2018 19:26:39 +0530
From:   "Kohli, Gaurav" <gkohli@...eaurora.org>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     jslaby@...e.com, gregkh@...uxfoundation.org, mikey@...ling.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] tty: fix data race in n_tty_receive_buf_common



On 1/5/2018 7:06 PM, Alan Cox wrote:
> On Fri, 5 Jan 2018 13:15:45 +0530
> "Kohli, Gaurav" <gkohli@...eaurora.org> wrote:
>
>> Hi Alan,
>>>>> Can you make that code available otherwise it's impossible to see
>>>>> what the problem might be.
>>>   
>>    https://source.codeaurora.org/quic/la/kernel/msm-4.9/tree/drivers/tty/serial?h=msm-4.9
>>    As discussed , there not seems a problem as we are getting print
>> request even when port seems to closed.
>>
>>
>> tty_ldisc_lock(tty, 5 * HZ);
>>    tty_ldisc_setup(tty);
>>    tty_ldisc_unlock(tty)
>>
>> But in above lock,  there is a chance when flush_to_ldisc will occur
>> first and acquired a lock in
>> tty_ldisc_ref itself.
> Which is fine.
>
> If the flush_to_ldisc gets there first then it will find there is a NULL
> ldisc and do nothing. When it finishes the tty_init_dev will run and will
> be protected from a further re-entry.
>
> If the init_dev gets there first it will complete the init before the
> flush_to_ldisc is permitted to proceed.
>
> In other words we restore the intended invariant that ldisc's do not get
> entered while their setup routine is running.
>
>

But in above case , there we can hit another race, if we have a sequence 
like this
tty_init_dev->alloc_tty_struct -> tty_ldisc_init -> this will initialize 
ldisc ,
but at this moment disc_data is still NULL

And if flush_to_ldisc comes in between, it will take ldisc reference and 
proceeds receive buffer.


Regards
Gaurav


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