Re: [PATCHv2 2/2] hwmon: core: fix potential memory leak in *hwmon_device_register*

[Guenter Roeck <linux@roeck-us.net>]

On Wed, May 29, 2019 at 07:56:05PM -0700, Eduardo Valentin wrote:
> When registering a hwmon device with HWMON_C_REGISTER_TZ flag
> in place, the hwmon subsystem will attempt to register the device
> also with the thermal subsystem. When the of-thermal registration
> fails, __hwmon_device_register jumps to ida_remove, leaving
> the locally allocated hwdev pointer.
> 
> This patch fixes the leak by jumping to a new label that
> will first unregister hdev and then fall into the kfree of hwdev
> to finally remove the idas and propagate the error code.
> 

Hah, actually this is wrong. hwdev is freed indirectly with the
device_unregister() call. See commit 74e3512731bd ("hwmon: (core)
Fix double-free in __hwmon_device_register()").

It may make sense to add a respective comment to the code, though.

Guenter

> Cc: Jean Delvare <jdelvare@suse.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Cc: linux-hwmon@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Eduardo Valentin <eduval@amazon.com>
> ---
> V1->V2: removed the device_unregister() before jumping
> into the new label, as suggested in the first review round.
> 
>  drivers/hwmon/hwmon.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwmon/hwmon.c b/drivers/hwmon/hwmon.c
> index 429784edd5ff..620f05fc412a 100644
> --- a/drivers/hwmon/hwmon.c
> +++ b/drivers/hwmon/hwmon.c
> @@ -652,10 +652,8 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
>  				if (info[i]->config[j] & HWMON_T_INPUT) {
>  					err = hwmon_thermal_add_sensor(dev,
>  								hwdev, j);
> -					if (err) {
> -						device_unregister(hdev);
> -						goto ida_remove;
> -					}
> +					if (err)
> +						goto device_unregister;
>  				}
>  			}
>  		}
> @@ -663,6 +661,8 @@ __hwmon_device_register(struct device *dev, const char *name, void *drvdata,
>  
>  	return hdev;
>  
> +device_unregister:
> +	device_unregister(hdev);
>  free_hwmon:
>  	kfree(hwdev);
>  ida_remove: