thermal: max77620: fix device-node reference imbalance

The thermal child device reuses the parent MFD-device device-tree node
when registering a thermal zone, but did not take a reference to the
node.

This leads to a reference imbalance, and potential use-after-free, when
the node reference is dropped by the platform-bus device destructor
(once for the child and later again for the parent).

Fix this by dropping any reference already held to a device-tree node
and getting a reference to the parent's node which will be balanced on
reprobe or on platform-device release, whichever comes first.

Note that simply clearing the of_node pointer on probe errors and on
driver unbind would not allow the use of device-managed resources as
specifically thermal_zone_of_sensor_unregister() claims that a valid
device-tree node pointer is needed during deregistration (even if it
currently does not seem to use it).

Fixes: ec4664b3 ("thermal: max77620: Add thermal driver for reporting junction temp")
Cc: stable <stable@vger.kernel.org>     # 4.9
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thermal/max77620_thermal.c

@@ -104,8 +104,6 @@ static int max77620_thermal_probe(struct platform_device *pdev)
 		return -EINVAL;
 	}
 
-	pdev->dev.of_node = pdev->dev.parent->of_node;
-
 	mtherm->dev = &pdev->dev;
 	mtherm->rmap = dev_get_regmap(pdev->dev.parent, NULL);
 	if (!mtherm->rmap) {
@@ -113,6 +111,14 @@ static int max77620_thermal_probe(struct platform_device *pdev)
 		return -ENODEV;
 	}
 
+	/*
+	 * Drop any current reference to a device-tree node and get a
+	 * reference to the parent's node which will be balanced on reprobe or
+	 * on platform-device release.
+	 */
+	of_node_put(pdev->dev.of_node);
+	pdev->dev.of_node = of_node_get(pdev->dev.parent->of_node);
+
 	mtherm->tz_device = devm_thermal_zone_of_sensor_register(&pdev->dev, 0,
 				mtherm, &max77620_thermal_ops);
 	if (IS_ERR(mtherm->tz_device)) {