linux-kernel - Re: [PATCH v2 1/2] Revert "usb: gadget: u_serial: Add null pointer check in gs_start

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Message-ID: <2fadb854-f838-44c9-968e-96f0c00f4d3e@oss.qualcomm.com>
Date: Wed, 18 Jun 2025 17:58:16 +0530
From: Prashanth K <prashanth.k@....qualcomm.com>
To: Kuen-Han Tsai <khtsai@...gle.com>, gregkh@...uxfoundation.org,
        hulianqin@...o.com, krzysztof.kozlowski@...aro.org, mwalle@...nel.org,
        jirislaby@...nel.org
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] Revert "usb: gadget: u_serial: Add null pointer
 check in gs_start_io"



On 6/17/2025 10:37 AM, Kuen-Han Tsai wrote:
> This reverts commit ffd603f214237e250271162a5b325c6199a65382.
> 
> Commit ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in
> gs_start_io") adds null pointer checks at the beginning of the
> gs_start_io() function to prevent a null pointer dereference. However,
> these checks are redundant because the function's comment already
> requires callers to hold the port_lock and ensure port.tty and port_usb
> are not null. All existing callers already follow these rules.
> 
> The true cause of the null pointer dereference is a race condition. When
> gs_start_io() calls either gs_start_rx() or gs_start_tx(), the port_lock
> is temporarily released for usb_ep_queue(). This allows port.tty and
> port_usb to be cleared.
> 
> Fixes: ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io")
> Signed-off-by: Kuen-Han Tsai <khtsai@...gle.com>
> ---

Reviewed-by: Prashanth K <prashanth.k@....qualcomm.com>

> v2:
> - Remove Cc: stable
> 
> ---
>  drivers/usb/gadget/function/u_serial.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
> index ab544f6824be..c043bdc30d8a 100644
> --- a/drivers/usb/gadget/function/u_serial.c
> +++ b/drivers/usb/gadget/function/u_serial.c
> @@ -544,20 +544,16 @@ static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head,
>  static int gs_start_io(struct gs_port *port)
>  {
>  	struct list_head	*head = &port->read_pool;
> -	struct usb_ep		*ep;
> +	struct usb_ep		*ep = port->port_usb->out;
>  	int			status;
>  	unsigned		started;
> 
> -	if (!port->port_usb || !port->port.tty)
> -		return -EIO;
> -
>  	/* Allocate RX and TX I/O buffers.  We can't easily do this much
>  	 * earlier (with GFP_KERNEL) because the requests are coupled to
>  	 * endpoints, as are the packet sizes we'll be using.  Different
>  	 * configurations may use different endpoints with a given port;
>  	 * and high speed vs full speed changes packet sizes too.
>  	 */
> -	ep = port->port_usb->out;
>  	status = gs_alloc_requests(ep, head, gs_read_complete,
>  		&port->read_allocated);
>  	if (status)
> --
> 2.50.0.rc2.692.g299adb8693-goog
>