linux-kernel - Re: [PATCH 1/2] media: uvcvideo: Avoid invalid memory access

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Date:   Wed, 1 Dec 2021 04:36:49 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] media: uvcvideo: Avoid invalid memory access

Hi Ricardo,

Thank you for the patch.

On Tue, Nov 30, 2021 at 03:50:25PM +0000, Ricardo Ribalda wrote:
> If mappings points to an invalid memory, we will be invalid accessing
> it.
> Solve it by initializing the value of the variable mapping and by
> changing the order in the conditional statement (to avoid accessing
> mapping->id if not needed).
> 
> Fix:
> kasan: GPF could be caused by NULL-ptr deref or user memory access
> general protection fault: 0000 [#1] PREEMPT SMP KASAN NOPTI
> 
> Fixes: 6350d6a4ed487 ("media: uvcvideo: Set error_idx during ctrl_commit errors")
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_ctrl.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> index 30bfe9069a1fb..f7b7add3cfa59 100644
> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> @@ -852,8 +852,8 @@ static void __uvc_find_control(struct uvc_entity *entity, u32 v4l2_id,
>  				return;
>  			}
>  
> -			if ((*mapping == NULL || (*mapping)->id > map->id) &&
> -			    (map->id > v4l2_id) && next) {
> +			if (next && (map->id > v4l2_id) &&
> +			    (*mapping == NULL || (*mapping)->id > map->id)) {
>  				*control = ctrl;
>  				*mapping = map;
>  			}
> @@ -1638,7 +1638,7 @@ static int uvc_ctrl_find_ctrl_idx(struct uvc_entity *entity,
>  				  struct v4l2_ext_controls *ctrls,
>  				  struct uvc_control *uvc_control)
>  {
> -	struct uvc_control_mapping *mapping;
> +	struct uvc_control_mapping *mapping = NULL;

It seems to me that either change will fix the bug, we don't need both,
is that right ? If so I'd drop the change to __uvc_find_control(), as it
seems quite fragile to allow mapping to be uninitialized.

>  	struct uvc_control *ctrl_found;
>  	unsigned int i;
>  

-- 
Regards,

Laurent Pinchart