Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Subject: Re: [BUG] tty: serial: possible deadlock in uart_remove_one_port()
 and uart_hangup()
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     jirislaby@kernel.org, linux-serial@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <ab5d9322-6bea-9845-c61b-fb68e3bb3a87@gmail.com>
 <YfUPlYwoWpMjhvpR@kroah.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <027e2ecd-ccb3-d772-dc31-951039fdeedf@gmail.com>
Date:   Tue, 1 Feb 2022 14:51:09 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.1
MIME-Version: 1.0
In-Reply-To: <YfUPlYwoWpMjhvpR@kroah.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Precedence: bulk


On 2022/1/29 17:57, Greg KH wrote:
> On Sat, Jan 29, 2022 at 05:34:05PM +0800, Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports a possible deadlock in the tty driver in
>> Linux 5.10:
> 5.10 was released over a year ago and over 100 thousand changes ago.
> Please redo your check on 5.16 at the oldest.

My static analysis tool checks the tty driver in Linux 5.16, and also 
finds this possible deadlock:

uart_remove_one_port()
   mutex_lock(&port->mutex); --> Line 3032 (Lock A)
   wait_event(state->remove_wait, ...); --> Line 3034 (Wait X)
   mutex_unlock(&port->mutex); --> Line 3036 (Unlock A)

uart_hangup()
   mutex_lock(&port->mutex); --> Line 1669 (Lock A)
   uart_flush_buffer()
     uart_port_unlock()
       uart_port_deref()
         wake_up(&uport->state->remove_wait); --> Line 68 (Wake X)
   mutex_unlock(&port->mutex); --> Line 1686 (Unlock A)

When uart_remove_one_port() is executed, "Wait X" is performed by 
holding "Lock A". If uart_hangup() is executed at this time, "Wake X" 
cannot be performed to wake up "Wait X" in uart_remove_one_port(), 
because "Lock A" has been already hold by uart_remove_one_port(), 
causing a possible deadlock.

I am not quite sure whether this possible problem is real and how to fix 
it if it is real.
Maybe we can call wait_event() before mutex_lock() in 
uart_remove_one_port().
Any feedback would be appreciated, thanks :)


Best wishes,
Jia-Ju Bai