Re: [PATCH 5/5] USB: serial: quatech2: handle unbound ports

[Greg KH <greg@kroah.com>]

On Fri, Jan 17, 2020 at 10:50:26AM +0100, Johan Hovold wrote:
> Check for NULL port data in the event handlers to avoid dereferencing a
> NULL pointer in the unlikely case where a port device isn't bound to a
> driver (e.g. after an allocation failure on port probe).
> 
> Fixes: f7a33e608d9a ("USB: serial: add quatech2 usb to serial driver")
> Cc: stable <stable@vger.kernel.org>     # 3.5
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  drivers/usb/serial/quatech2.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/usb/serial/quatech2.c b/drivers/usb/serial/quatech2.c
> index a62981ca7a73..c76a2c0c32ff 100644
> --- a/drivers/usb/serial/quatech2.c
> +++ b/drivers/usb/serial/quatech2.c
> @@ -470,6 +470,13 @@ static int get_serial_info(struct tty_struct *tty,
>  
>  static void qt2_process_status(struct usb_serial_port *port, unsigned char *ch)
>  {
> +	struct qt2_port_private *port_priv;
> +
> +	/* May be called from qt2_process_read_urb() for an unbound port. */
> +	port_priv = usb_get_serial_port_data(port);
> +	if (!port_priv)
> +		return;
> +

Where is the null dereference here?  Will port be NULL somehow?

>  	switch (*ch) {
>  	case QT2_LINE_STATUS:
>  		qt2_update_lsr(port, ch + 1);
> @@ -484,14 +491,27 @@ static void qt2_process_status(struct usb_serial_port *port, unsigned char *ch)
>  static void qt2_process_xmit_empty(struct usb_serial_port *port,
>  				   unsigned char *ch)
>  {
> +	struct qt2_port_private *port_priv;
>  	int bytes_written;
>  
> +	/* May be called from qt2_process_read_urb() for an unbound port. */
> +	port_priv = usb_get_serial_port_data(port);
> +	if (!port_priv)
> +		return;
> +
>  	bytes_written = (int)(*ch) + (int)(*(ch + 1) << 4);

What's the harm in doing a pointless calculation here?  Nothing seems to
happen in this function at all.

>  }
>  
>  /* not needed, kept to document functionality */
>  static void qt2_process_flush(struct usb_serial_port *port, unsigned char *ch)
>  {
> +	struct qt2_port_private *port_priv;
> +
> +	/* May be called from qt2_process_read_urb() for an unbound port. */
> +	port_priv = usb_get_serial_port_data(port);
> +	if (!port_priv)
> +		return;
> +
>  	return;
>  }

This whole function can just be removed, right?

thanks,

greg k-h