Re: [PATCH v4] usb: gadget: u_serial: Add null pointer check in gserial_resume

[Alan Stern]

On Mon, Feb 13, 2023 at 05:06:10PM +0530, Prashanth K wrote:
> Consider a case where gserial_disconnect has already cleared
> gser->ioport. And if a wakeup interrupt triggers afterwards,
> gserial_resume gets called, which will lead to accessing of
> gser->ioport and thus causing null pointer dereference.Add
> a null pointer check to prevent this.
> 
> Added a static spinlock to prevent gser->ioport from becoming
> null after the newly added check.
> 
> Fixes: aba3a8d01d62 ("usb: gadget: u_serial: add suspend resume callbacks")
> Signed-off-by: Prashanth K <quic_prashk@xxxxxxxxxxx>
> ---
> v4: Added short comment about serial_port_lock.
> 
>  drivers/usb/gadget/function/u_serial.c | 27 +++++++++++++++++++++++----
>  1 file changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
> index 840626e..3a6adf2 100644
> --- a/drivers/usb/gadget/function/u_serial.c
> +++ b/drivers/usb/gadget/function/u_serial.c
> @@ -82,6 +82,13 @@
>  #define WRITE_BUF_SIZE		8192		/* TX only */
>  #define GS_CONSOLE_BUF_SIZE	8192
>  
> +/*
> + * Since all the spinlocks used in this driver is part of struct gs_port (port_lock).
> + * Added a new static spinlock to prevent race conditions where gs_port is dereferenced
> + * after marking as null.
> + */

This comment could be a little shorter and more direct.  Something like:

/* Prevents race conditions when accessing gser->ioport */

or

/* Prevents races between disconnect and resume for gser->ioport */

or even just

/* Protects gser->ioport */

Everything else looks okay.

Alan Stern

> +static DEFINE_SPINLOCK(serial_port_lock);
> +
>  /* console info */
>  struct gs_console {
>  	struct console		console;
> @@ -1375,8 +1382,10 @@ void gserial_disconnect(struct gserial *gser)
>  	if (!port)
>  		return;
>  
> +	spin_lock_irqsave(&serial_port_lock, flags);
> +
>  	/* tell the TTY glue not to do I/O here any more */
> -	spin_lock_irqsave(&port->port_lock, flags);
> +	spin_lock(&port->port_lock);
>  
>  	gs_console_disconnect(port);
>  
> @@ -1391,7 +1400,8 @@ void gserial_disconnect(struct gserial *gser)
>  			tty_hangup(port->port.tty);
>  	}
>  	port->suspended = false;
> -	spin_unlock_irqrestore(&port->port_lock, flags);
> +	spin_unlock(&port->port_lock);
> +	spin_unlock_irqrestore(&serial_port_lock, flags);
>  
>  	/* disable endpoints, aborting down any active I/O */
>  	usb_ep_disable(gser->out);
> @@ -1425,10 +1435,19 @@ EXPORT_SYMBOL_GPL(gserial_suspend);
>  
>  void gserial_resume(struct gserial *gser)
>  {
> -	struct gs_port *port = gser->ioport;
> +	struct gs_port *port;
>  	unsigned long	flags;
>  
> -	spin_lock_irqsave(&port->port_lock, flags);
> +	spin_lock_irqsave(&serial_port_lock, flags);
> +	port = gser->ioport;
> +
> +	if (!port) {
> +		spin_unlock_irqrestore(&serial_port_lock, flags);
> +		return;
> +	}
> +
> +	spin_lock(&port->port_lock);
> +	spin_unlock(&serial_port_lock);
>  	port->suspended = false;
>  	if (!port->start_delayed) {
>  		spin_unlock_irqrestore(&port->port_lock, flags);
> -- 
> 2.7.4
>