linux-kernel - Re: [PATCH] intel_menlow: set cdev after null device check to avoid null pointer dereference

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Date:	Mon, 28 Mar 2016 11:18:05 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Colin King <colin.king@...onical.com>,
	Rafael Wysocki <rjw@...ysocki.net>
Cc:	Sujith Thomas <sujith.thomas@...el.com>,
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] intel_menlow: set cdev after null device check to avoid
 null pointer dereference

On Mon, Mar 28, 2016 at 05:18:39PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> intel_menlow_memory_remove sanity checks to see if device is null, however,
> this check is performed after we have already passed device into a call
> to acpi_driver_data.  If device is null, then acpi_driver_data will produce
> a null pointer dereference on device. The correct action is to sanity check
> device, then assign cdev, then check if cdev is null.
> 

Hrm, looking at this locally, that all makes sense.

Taking a step back however, I notice that intel_menlow_memory_remove is an ops
function pointer inside the acpi_driver structure itself, which is called from
acpi_device_remove() (and probe) (drivers/acpi/bus.c). This already verifies
acpi_driver is not NULL and can't get acpi_driver if acpi_device is NULL. So
unless there is some other use case for this callback I'm unaware of (certainly
possible) it appears to be totally redundant to do this checking here.

+Rafael - is there a best practices for these acpi callbacks with respect to
input validation?

> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/platform/x86/intel_menlow.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/platform/x86/intel_menlow.c b/drivers/platform/x86/intel_menlow.c
> index 0a919d8..185a1bd 100644
> --- a/drivers/platform/x86/intel_menlow.c
> +++ b/drivers/platform/x86/intel_menlow.c
> @@ -196,9 +196,13 @@ static int intel_menlow_memory_add(struct acpi_device *device)
>  
>  static int intel_menlow_memory_remove(struct acpi_device *device)
>  {
> -	struct thermal_cooling_device *cdev = acpi_driver_data(device);
> +	struct thermal_cooling_device *cdev;
> +
> +	if (!device)
> +		return -EINVAL;
>  
> -	if (!device || !cdev)
> +	cdev = acpi_driver_data(device);
> +	if (!cdev)
>  		return -EINVAL;
>  
>  	sysfs_remove_link(&device->dev.kobj, "thermal_cooling");
> -- 
> 2.7.4
> 
> 

-- 
Darren Hart
Intel Open Source Technology Center