linux-kernel - Re: [greybus-dev] Re: [PATCH] greybus: Fix deref of NULL in __gb_lights_flash_brightness

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Date: Sat, 2 Mar 2024 08:57:17 -0600
From: Alex Elder <elder@...e.org>
To: Dan Carpenter <dan.carpenter@...aro.org>,
 Mikhail Lobanov <m.lobanov@...alinux.ru>, rmfrfs@...il.com
Cc: greybus-dev@...ts.linaro.org, linux-staging@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [greybus-dev] Re: [PATCH] greybus: Fix deref of NULL in
 __gb_lights_flash_brightness_set

On 3/2/24 3:59 AM, Dan Carpenter wrote:
> On Fri, Mar 01, 2024 at 02:04:24PM -0500, Mikhail Lobanov wrote:
>> Dereference of null pointer in the __gb_lights_flash_brightness_set function.
>> Assigning the channel the result of executing the get_channel_from_mode function
>> without checking for NULL may result in an error.
> 
> get_channel_from_mode() can only return NULL when light->channels_count
> is zero.
> 
> Although get_channel_from_mode() seems buggy to me.  If it can't
> find the correct mode, it just returns the last channel.  So potentially
> it should be made to return NULL.

I agree with you.  This looks quite wrong to me, and I
like your fix, *except* there is also no need to check
whether the channel pointer is null inside the loop.
It's the address of an object, and will always be non-null.

     static struct gb_channel *
     get_channel_from_mode(struct gb_light *light, u32 mode)
     {
         struct gb_channel *channel;
         u32 i;

         for (i = 0; i < light->channels_count; i++) {
             channel = &light->channels[i];
             if (channel->mode == mode)
                 return channel;
         }
         return NULL;
     }


Rui, could you please confirm what Dan says (and his
proposed change) was your intention?

If so (and assuming you also fix the check for a null
channel pointer inside the loop):

Reviewed-by: Alex Elder <elder@...aro.org>

					-Alex

> 
> diff --git a/drivers/staging/greybus/light.c b/drivers/staging/greybus/light.c
> index d62f97249aca..acd435f5d25d 100644
> --- a/drivers/staging/greybus/light.c
> +++ b/drivers/staging/greybus/light.c
> @@ -95,15 +95,15 @@ static struct led_classdev *get_channel_cdev(struct gb_channel *channel)
>   static struct gb_channel *get_channel_from_mode(struct gb_light *light,
>   						u32 mode)
>   {
> -	struct gb_channel *channel = NULL;
> +	struct gb_channel *channel;
>   	int i;
>   
>   	for (i = 0; i < light->channels_count; i++) {
>   		channel = &light->channels[i];
>   		if (channel && channel->mode == mode)
> -			break;
> +			return channel;
>   	}
> -	return channel;
> +	return NULL;
>   }
>   
>   static int __gb_lights_flash_intensity_set(struct gb_channel *channel,
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