linux-kernel - Re: [PATCH 2/5] media: uvc: don't do DMA on stack

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Date:   Mon, 21 Jun 2021 15:09:10 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc:     linuxarm@...wei.com, mauro.chehab@...wei.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 2/5] media: uvc: don't do DMA on stack

Hi Mauro,

Thank you for the patch.

On Mon, Jun 21, 2021 at 01:56:46PM +0200, Mauro Carvalho Chehab wrote:
> As warned by smatch:
> 	drivers/media/usb/uvc/uvc_v4l2.c:911 uvc_ioctl_g_input() error: doing dma on the stack (&i)
> 	drivers/media/usb/uvc/uvc_v4l2.c:943 uvc_ioctl_s_input() error: doing dma on the stack (&i)
> 
> those two functions call uvc_query_ctrl passing a pointer to
> a data at the DMA stack. those are used to send URBs via
> usb_control_msg(). Using DMA stack is not supported and should
> not work anymore on modern Linux versions.
> 
> So, use a temporary buffer, allocated together with
> struct uvc_video_chain.

DMA in a memory location that may share a cache line with something else
isn't a great idea either. The buffer should be kmalloc()ed in
uvc_ioctl_g_input() and uvc_ioctl_s_input().

> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
> ---
>  drivers/media/usb/uvc/uvc_v4l2.c | 10 ++++------
>  drivers/media/usb/uvc/uvcvideo.h |  3 +++
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> index 252136cc885c..e60d4675881a 100644
> --- a/drivers/media/usb/uvc/uvc_v4l2.c
> +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> @@ -900,7 +900,6 @@ static int uvc_ioctl_g_input(struct file *file, void *fh, unsigned int *input)
>  	struct uvc_fh *handle = fh;
>  	struct uvc_video_chain *chain = handle->chain;
>  	int ret;
> -	u8 i;
>  
>  	if (chain->selector == NULL ||
>  	    (chain->dev->quirks & UVC_QUIRK_IGNORE_SELECTOR_UNIT)) {
> @@ -910,11 +909,11 @@ static int uvc_ioctl_g_input(struct file *file, void *fh, unsigned int *input)
>  
>  	ret = uvc_query_ctrl(chain->dev, UVC_GET_CUR, chain->selector->id,
>  			     chain->dev->intfnum,  UVC_SU_INPUT_SELECT_CONTROL,
> -			     &i, 1);
> +			     &chain->input, 1);
>  	if (ret < 0)
>  		return ret;
>  
> -	*input = i - 1;
> +	*input = chain->input - 1;
>  	return 0;
>  }
>  
> @@ -923,7 +922,6 @@ static int uvc_ioctl_s_input(struct file *file, void *fh, unsigned int input)
>  	struct uvc_fh *handle = fh;
>  	struct uvc_video_chain *chain = handle->chain;
>  	int ret;
> -	u32 i;
>  
>  	ret = uvc_acquire_privileges(handle);
>  	if (ret < 0)
> @@ -939,10 +937,10 @@ static int uvc_ioctl_s_input(struct file *file, void *fh, unsigned int input)
>  	if (input >= chain->selector->bNrInPins)
>  		return -EINVAL;
>  
> -	i = input + 1;
> +	chain->input = input + 1;
>  	return uvc_query_ctrl(chain->dev, UVC_SET_CUR, chain->selector->id,
>  			      chain->dev->intfnum, UVC_SU_INPUT_SELECT_CONTROL,
> -			      &i, 1);
> +			      &chain->input, 1);
>  }
>  
>  static int uvc_ioctl_queryctrl(struct file *file, void *fh,
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index cce5e38133cd..3c0ed90d6912 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -475,6 +475,9 @@ struct uvc_video_chain {
>  	struct mutex ctrl_mutex;		/* Protects ctrl.info */
>  
>  	struct v4l2_prio_state prio;		/* V4L2 priority state */
> +
> +	u8 input;				/* buffer for set/get input */
> +
>  	u32 caps;				/* V4L2 chain-wide caps */
>  };
>  

-- 
Regards,

Laurent Pinchart