Re: [PATCH] media: v4l: async: Fix NULL pointer when v4l2 flash subdev binding

[Sakari Ailus]

Hi Chi Yuan,

On Wed, May 08, 2024 at 10:51:49AM +0800, cy_huang@xxxxxxxxxxx wrote:
> From: ChiYuan Huang <cy_huang@xxxxxxxxxxx>
> 
> In v4l2_async_create_ancillary_links(), if v4l2 async notifier is
> created from v4l2 device, the v4l2 flash subdev async binding will enter
> the logic to create media link. Due to the subdev of notifier is NULL,
> this will cause NULL pointer to access the subdev entity. Therefore, add
> the check to bypass it.
> 
> Fixes: aa4faf6eb271 ("media: v4l2-async: Create links during v4l2_async_match_notify()")
> Signed-off-by: ChiYuan Huang <cy_huang@xxxxxxxxxxx>
> ---
> Hi,
> 
>   I'm trying to bind the v4l2 subdev for flashlight testing. It seems
> some logic in v4l2 asynd binding is incorrect.
> 
> From the change, I modified vim2m as the test driver to bind mt6370 flashlight.
> 
> Here's the backtrace log.
> 
>  vim2m soc:vim2m: bound [white:flash-2]
>  Unable to handle kernel NULL pointer dereference at virtual address 0000000000000058
>  ......skipping
>  Call trace:
>   media_create_ancillary_link+0x48/0xd8 [mc]
>   v4l2_async_match_notify+0x17c/0x208 [v4l2_async]
>   v4l2_async_register_subdev+0xb8/0x1d0 [v4l2_async]

There's something wrong obviously somewhere but where?

A sub-notifier does have a sub-device after the notifier initialisation.
Maybe the initialisation does not happen in the right order?

>   __v4l2_flash_init.part.0+0x3b4/0x4b0 [v4l2_flash_led_class]
>   v4l2_flash_init+0x28/0x48 [v4l2_flash_led_class]
>   mt6370_led_probe+0x348/0x690 [leds_mt6370_flash]
> 
> After tracing the code, it will let the subdev labeled as F_LENS or
> F_FLASH function to create media link. To prevent the NULL pointer
> issue, the simplest way is add a check when 'n->sd' is NULL and bypass
> the later media link creataion.
> ---
>  drivers/media/v4l2-core/v4l2-async.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
> index 3ec323bd528b..9d3161c51954 100644
> --- a/drivers/media/v4l2-core/v4l2-async.c
> +++ b/drivers/media/v4l2-core/v4l2-async.c
> @@ -324,6 +324,9 @@ static int v4l2_async_create_ancillary_links(struct v4l2_async_notifier *n,
>  	    sd->entity.function != MEDIA_ENT_F_FLASH)
>  		return 0;
>  
> +	if (!n->sd)
> +		return 0;

This isn't the right fix: the ancillary link won't be created as a result.

> +
>  	link = media_create_ancillary_link(&n->sd->entity, &sd->entity);
>  
>  #endif

-- 
Regards,

Sakari Ailus