linux-kernel - Re: [PATCH v2] bus: fsl-mc: fix invalid free in fsl_mc_device

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Date:   Thu, 9 Jul 2020 15:37:52 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     trix@...hat.com
Cc:     stuyoder@...il.com, laurentiu.tudor@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] bus: fsl-mc: fix invalid free in fsl_mc_device_add

On Thu, Jul 09, 2020 at 05:41:15AM -0700, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> 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@...hat.com>
> ---
>  v1: add a comment to explain freeing uniqueness
>  
>  drivers/bus/fsl-mc/fsl-mc-bus.c | 10 ++++++++--
>  1 file changed, 8 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..df10ed430baa 100644
> --- a/drivers/bus/fsl-mc/fsl-mc-bus.c
> +++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
> @@ -691,8 +691,14 @@ 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);

Close, but that still doesn't help with anyone coming along later, how
about adding:

> +	/*
> +	 * 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.

thanks,

greg k-h