Re: [PATCH] tty: serial: owl: Fix data race in owl_uart_remove

[Greg KH <gregkh@linuxfoundation.org>]

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.
> 
> Found by Linux Driver Verification project (linuxtesting.org).
> 
> Signed-off-by: Saubhik Mukherjee <saubhik.mukherjee@gmail.com>
> ---
>  drivers/tty/serial/owl-uart.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/owl-uart.c b/drivers/tty/serial/owl-uart.c
> index 91f1eb0058d7..ac4e3aae2719 100644
> --- a/drivers/tty/serial/owl-uart.c
> +++ b/drivers/tty/serial/owl-uart.c
> @@ -751,8 +751,15 @@ static int owl_uart_probe(struct platform_device *pdev)
>  static int owl_uart_remove(struct platform_device *pdev)
>  {
>  	struct owl_uart_port *owl_port = platform_get_drvdata(pdev);
> +	struct uart_port *port = &owl_port->port;
>  
> -	uart_remove_one_port(&owl_uart_driver, &owl_port->port);
> +	/* It is possible to release device without closing a port.
> +	 * Thus, need to check it and call shutdown.
> +	 */
> +	if (owl_uart_read(port, OWL_UART_CTL) & OWL_UART_CTL_EN)
> +		owl_uart_shutdown(port);

How is this read determining if the device is here or not?  And what
happens if the state change happens right _after_ the check?

Also, your comment style is for networking, not the rest of the kernel
:)

thanks,

greg k-h