linux-kernel - Re: [PATCH] usb: gadget: u_serial: Add null pointer check in gserial

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Message-ID: <d05be5e3-a994-4924-a86f-d8f19d800193@rowland.harvard.edu>
Date:   Sat, 6 May 2023 11:45:31 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Prashanth K <quic_prashk@...cinc.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: gadget: u_serial: Add null pointer check in
 gserial_suspend

On Fri, May 05, 2023 at 02:48:37PM +0530, Prashanth K wrote:
> Consider a case where gserial_disconnect has already cleared
> gser->ioport. And if gserial_suspend gets called afterwards,
> it will lead to accessing of gser->ioport and thus causing
> null pointer dereference.
> 
> Avoid this by adding a null pointer check. Added a static
> spinlock to prevent gser->ioport from becoming null after
> the newly added null pointer check.
> 
> Fixes: aba3a8d01d62 ("usb: gadget: u_serial: add suspend resume callbacks")
> Signed-off-by: Prashanth K <quic_prashk@...cinc.com>
> ---
>  drivers/usb/gadget/function/u_serial.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
> index a0ca47f..e5d522d 100644
> --- a/drivers/usb/gadget/function/u_serial.c
> +++ b/drivers/usb/gadget/function/u_serial.c
> @@ -1420,10 +1420,19 @@ EXPORT_SYMBOL_GPL(gserial_disconnect);
>  
>  void gserial_suspend(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 = true;
>  	spin_unlock_irqrestore(&port->port_lock, flags);
>  }

This looks fine to me, but I'm not a serial-gadget maintainer.  
In fact, it looks like we don't have a serial-gadget maintainer.

Alan Stern