iio: proximity: as3935: fix use-after-free on device remove

This driver's probe() uses a mix of devm_ and non-devm_ functions. This
means that the remove order will not be the exact opposite of the probe
order.

Remove order:
1. remove() executes:
	iio_device_unregister
	iio_triggered_buffer_cleanup
	iio_trigger_unregister
	(A)
2. core frees devm resources in reverse order:
	free_irq
	iio_trigger_free
	iio_device_free

In (A) the trigger has been unregistered, but the irq handler is still
registered and active, so the trigger may still be touched via
interrupt -> as3935_event_work. This is a potential use-after-unregister.

Given that the delayed work is never canceled explicitly, it may run even
after iio_device_free. This is a potential use-after-free.

Solution: convert all probe functions to their devm_ equivalents.
Add a devm callback, called by the core on remove right after irq_free,
which explicitly cancels the delayed work. This will guarantee that all
resources are freed in the correct order.

As an added bonus, some boilerplate code can be removed.
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/proximity/as3935.c

@@ -345,6 +345,14 @@ static SIMPLE_DEV_PM_OPS(as3935_pm_ops, as3935_suspend, as3935_resume);
 #define AS3935_PM_OPS NULL
 #endif
 
+static void as3935_stop_work(void *data)
+{
+	struct iio_dev *indio_dev = data;
+	struct as3935_state *st = iio_priv(indio_dev);
+
+	cancel_delayed_work_sync(&st->work);
+}
+
 static int as3935_probe(struct spi_device *spi)
 {
 	struct iio_dev *indio_dev;
@@ -368,7 +376,6 @@ static int as3935_probe(struct spi_device *spi)
 
 	spi_set_drvdata(spi, indio_dev);
 	mutex_init(&st->lock);
-	INIT_DELAYED_WORK(&st->work, as3935_event_work);
 
 	ret = of_property_read_u32(np,
 			"ams,tuning-capacitor-pf", &st->tune_cap);
@@ -414,22 +421,28 @@ static int as3935_probe(struct spi_device *spi)
 	iio_trigger_set_drvdata(trig, indio_dev);
 	trig->ops = &iio_interrupt_trigger_ops;
 
-	ret = iio_trigger_register(trig);
+	ret = devm_iio_trigger_register(&spi->dev, trig);
 	if (ret) {
 		dev_err(&spi->dev, "failed to register trigger\n");
 		return ret;
 	}
 
-	ret = iio_triggered_buffer_setup(indio_dev, iio_pollfunc_store_time,
-		&as3935_trigger_handler, NULL);
+	ret = devm_iio_triggered_buffer_setup(&spi->dev, indio_dev,
+					      iio_pollfunc_store_time,
+					      as3935_trigger_handler, NULL);
 
 	if (ret) {
 		dev_err(&spi->dev, "cannot setup iio trigger\n");
-		goto unregister_trigger;
+		return ret;
 	}
 
 	calibrate_as3935(st);
 
+	INIT_DELAYED_WORK(&st->work, as3935_event_work);
+	ret = devm_add_action(&spi->dev, as3935_stop_work, indio_dev);
+	if (ret)
+		return ret;
+
 	ret = devm_request_irq(&spi->dev, spi->irq,
 				&as3935_interrupt_handler,
 				IRQF_TRIGGER_RISING,
@@ -438,35 +451,15 @@ static int as3935_probe(struct spi_device *spi)
 
 	if (ret) {
 		dev_err(&spi->dev, "unable to request irq\n");
-		goto unregister_buffer;
+		return ret;
 	}
 
-	ret = iio_device_register(indio_dev);
+	ret = devm_iio_device_register(&spi->dev, indio_dev);
 	if (ret < 0) {
 		dev_err(&spi->dev, "unable to register device\n");
-		goto unregister_buffer;
+		return ret;
 	}
 	return 0;
-
-unregister_buffer:
-	iio_triggered_buffer_cleanup(indio_dev);
-
-unregister_trigger:
-	iio_trigger_unregister(st->trig);
-
-	return ret;
-}
-
-static int as3935_remove(struct spi_device *spi)
-{
-	struct iio_dev *indio_dev = spi_get_drvdata(spi);
-	struct as3935_state *st = iio_priv(indio_dev);
-
-	iio_device_unregister(indio_dev);
-	iio_triggered_buffer_cleanup(indio_dev);
-	iio_trigger_unregister(st->trig);
-
-	return 0;
 }
 
 static const struct of_device_id as3935_of_match[] = {
@@ -488,7 +481,6 @@ static struct spi_driver as3935_driver = {
 		.pm	= AS3935_PM_OPS,
 	},
 	.probe		= as3935_probe,
-	.remove		= as3935_remove,
 	.id_table	= as3935_id,
 };
 module_spi_driver(as3935_driver);