Re: [PATCH v3] bus: fsl-mc: fix invalid free in fsl_mc_device_add

[Laurentiu Tudor]

On 7/9/2020 6:31 PM, trix@xxxxxxxxxx wrote:
> From: Tom Rix <trix@xxxxxxxxxx>
> 
> clang static analysis flags this error
> 
> fsl-mc-bus.c:695:2: warning: Attempt to free released memory [unix.Malloc]
>         kfree(mc_dev);
>         ^~~~~~~~~~~~~
> 
> The problem block of code is
> 
> 		mc_bus = kzalloc(sizeof(*mc_bus), GFP_KERNEL);
> 		if (!mc_bus)
> 			return -ENOMEM;
> 
> 		mc_dev = &mc_bus->mc_dev;
> 
> mc_bus's structure contains a mc_dev element,
> freeing it later is not appropriate.
> 
> So check if mc_bus was allocated before freeing mc_dev
> 
> This is a case where checkpatch
> 
> WARNING: kfree(NULL) is safe and this check is probably not required
> +	if (mc_bus)
> +		kfree(mc_bus);
> 
> is wrong.
> 
> Fixes: a042fbed0290 ("staging: fsl-mc: simplify couple of deallocations")
> 
> Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> ---
>  v1: add a comment to explain freeing uniqueness
>  v2: add gregkh's suggestion to comment.
>  
>  drivers/bus/fsl-mc/fsl-mc-bus.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
> index 40526da5c6a6..839d96d03f0d 100644
> --- a/drivers/bus/fsl-mc/fsl-mc-bus.c
> +++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
> @@ -691,8 +691,16 @@ int fsl_mc_device_add(struct fsl_mc_obj_desc *obj_desc,
>  
>  error_cleanup_dev:
>  	kfree(mc_dev->regions);
> -	kfree(mc_bus);
> -	kfree(mc_dev);
> +	/*
> +	 * mc_bus allocates a private version of mc_dev
> +	 * it is not appropriate to free the private version.
> +	 * Which means we have to check the pointer before freeing it.
> +	 * Do not remove this check.
> +	 */
> +	if (mc_bus)
> +		kfree(mc_bus);
> +	else
> +		kfree(mc_dev);
>  
>  	return error;
>  }
> 

Thanks, Tom!

Reviewed-by: Laurentiu Tudor <laurentiu.tudor@xxxxxxx>

---
Best Regards, Laurentiu