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

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Message-ID: <1097619b-9e03-cba8-e40e-b0c00a53b84c@codeaurora.org>
Date:   Sat, 6 Jan 2018 01:44:58 +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

Hi Alan,


On 1/5/2018 7:45 PM, Alan Cox wrote:
>> 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.
> So you need to move the lock up one line to protect the assignment to
> tty->port->itty. We can do that.
>
> At that point your flush_to_ldisc should see either port->itty = NULL or a
> valid initialized ldisc.
>
>

Yes , with little modification this should work.

+retval =  tty_ldisc_lock(tty, 5 * HZ);
+if (retval)
+         goto err_release_lock;
tty->port->itty = tty;
/*
* Structures all installed ... call the ldisc open routines.
* If we fail here just call release_tty to clean up.  No need
* to decrement the use counts, as release_tty doesn't care.
*/
retval = tty_ldisc_setup(tty, tty->link);
         if (retval)
             goto err_release_tty;
tty_ldisc_unlock(tty);
err_release_tty:
tty_unlock(tty);
tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
     retval, idx);
     release_tty(tty, idx);
     return ERR_PTR(retval);

+err_release_lock;
+tty_ldisc_unlock(tty);
+return ERR_PTR(retval);

Please let me know if above modification seems fine , then we can upload 
this and try reproduction of Bug.

Regards
Gaurav

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