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Subject: Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     jirislaby@kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-kernel@vger.kernel.org,
        syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
References: <20210807190513.3810821-1-phind.uet@gmail.com>
 <YRYgSZwivcPPMhrS@kroah.com>
From:   Phi Nguyen <phind.uet@gmail.com>
Message-ID: <7323d825-f596-223b-a063-7539a6a57107@gmail.com>
Date:   Sat, 14 Aug 2021 02:35:53 +0800
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On 8/13/2021 3:33 PM, Greg KH wrote:
>>
>> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
>> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
>> ---
>>   drivers/tty/tty_io.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>> index e8532006e960..746fe13a2054 100644
>> --- a/drivers/tty/tty_io.c
>> +++ b/drivers/tty/tty_io.c
>> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>>   	ld = tty_ldisc_ref_wait(tty);
>>   	if (!ld)
>>   		return -EIO;
>> +	tty_buffer_lock_exclusive(tty->port);
>>   	if (ld->ops->receive_buf)
>>   		ld->ops->receive_buf(tty, &ch, &mbz, 1);
>> +	tty_buffer_unlock_exclusive(tty->port);
> 
> Did this fix the syzbot reported issue?
> 
> thanks,
> 
> greg k-h
> Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the 
callers to receive_buff() other than flush_to_ldisc() need to exclude 
the kworker from concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

BR,
Phi.