linux-kernel - Re: [PATCH] tty: serial: owl: Fix data race in owl_uart

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Message-ID: <a9d43126-acd7-efb0-bf1a-86b06965f0e2@gmail.com>
Date:   Thu, 24 Jun 2021 17:37:38 +0530
From:   Saubhik Mukherjee <saubhik.mukherjee@...il.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org, andrianov@...ras.ru
Subject: Re: [PATCH] tty: serial: owl: Fix data race in owl_uart_remove

On 6/23/21 12:46 PM, Johan Hovold wrote:
> On Wed, Jun 23, 2021 at 11:06:53AM +0530, Saubhik Mukherjee wrote:
>> On 6/17/21 4:52 PM, Greg KH wrote:
>>> On Thu, Jun 17, 2021 at 04:34:43PM +0530, Saubhik Mukherjee wrote:
>>>> Suppose the driver is registered and a UART port is added. Once an
>>>> application opens the port, owl_uart_startup is called which registers
>>>> the interrupt handler owl_uart_irq.
>>>>
>>>> We could have the following race condition:
>>>>
>>>> When device is removed, owl_uart_remove is called, which calls
>>>> uart_remove_one_port, which calls owl_uart_release_port, which writes
>>>> NULL to port->membase. At this point parallely, an interrupt could be
>>>> handled by owl_uart_irq which reads port->membase.
>>>>
>>>> This is because it is possible to remove device without closing a port.
>>>> Thus, we need to check it and call owl_uart_shutdown in owl_uart_remove.
> 
> No, this makes no sense at all. The port is deregistered and hung up by
> uart_remove_one_port() (and the interrupt line is consequently disabled
> by the driver) before it is released so this can never happen.

Thanks for the reply. I am not sure I understand. I could not find any 
interrupt disabling in owl_uart_remove. Could you point out where/how is 
the interrupt line is disabled before releasing the port?

A related question question was asked in 
https://lore.kernel.org/linux-serial/YMcpBXd1vtipueQi@kroah.com/.

>>>> Found by Linux Driver Verification project (linuxtesting.org).
> 
> And you clearly did not test this, which you should mention.

This race warning was found by a static analysis tool. The code changes 
are untested.