staging:iio:dummy: Fix potential NULL pointer dereference

If the config contains CONFIG_IIO_BUFFER=y and CONFIG_IIO_SIMPLE_DUMMY_BUFFER=n
iio_simple_dummy_configure_buffer() is stubbed out and iio_buffer_register() is
not. As a result we try to register a buffer which has not been configured.
This will causes a NULL pointer deref in iio_buffer_register. To solve this
issue move the iio_buffer_register() call to iio_simple_dummy_configure_buffer(),
so it will only be called if iio_simple_dummy_configure_buffer() has been called.

Reported-by: Fengguang Wu <[e-mail]>
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/iio_simple_dummy.c

···
 	if (ret < 0)
 		goto error_free_device;
 
-	/* Configure buffered capture support. */
-	ret = iio_simple_dummy_configure_buffer(indio_dev);
+	/*
+	 * Configure buffered capture support and register the channels with the
+	 * buffer, but avoid the output channel being registered by reducing the
+	 * number of channels by 1.
+	 */
+	ret = iio_simple_dummy_configure_buffer(indio_dev, iio_dummy_channels, 5);
 	if (ret < 0)
 		goto error_unregister_events;
 
-	/*
-	 * Register the channels with the buffer, but avoid the output
-	 * channel being registered by reducing the number of channels by 1.
-	 */
-	ret = iio_buffer_register(indio_dev, iio_dummy_channels, 5);
+	ret = iio_device_register(indio_dev);
 	if (ret < 0)
 		goto error_unconfigure_buffer;
 
-	ret = iio_device_register(indio_dev);
-	if (ret < 0)
-		goto error_unregister_buffer;
-
 	return 0;
-error_unregister_buffer:
-	iio_buffer_unregister(indio_dev);
 error_unconfigure_buffer:
 	iio_simple_dummy_unconfigure_buffer(indio_dev);
 error_unregister_events:
···
 	/* Device specific code to power down etc */
 
 	/* Buffered capture related cleanup */
-	iio_buffer_unregister(indio_dev);
 	iio_simple_dummy_unconfigure_buffer(indio_dev);
 
 	ret = iio_simple_dummy_events_unregister(indio_dev);
···
 		instances = 1;
 		return -EINVAL;
 	}
+
 	/* Fake a bus */
 	iio_dummy_devs = kcalloc(instances, sizeof(*iio_dummy_devs),
 				 GFP_KERNEL);

drivers/staging/iio/iio_simple_dummy.h

···
 };
 
 #ifdef CONFIG_IIO_SIMPLE_DUMMY_BUFFER
-int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev);
+int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev,
+	const struct iio_chan_spec *channels, unsigned int num_channels);
 void iio_simple_dummy_unconfigure_buffer(struct iio_dev *indio_dev);
 #else
-static inline int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev)
+static inline int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev,
+	const struct iio_chan_spec *channels, unsigned int num_channels)
 {
 	return 0;
 };

drivers/staging/iio/iio_simple_dummy_buffer.c

···
 	.predisable = &iio_triggered_buffer_predisable,
 };
 
-int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev)
+int iio_simple_dummy_configure_buffer(struct iio_dev *indio_dev,
+	const struct iio_chan_spec *channels, unsigned int num_channels)
 {
 	int ret;
 	struct iio_buffer *buffer;
···
 	 * driven by a trigger.
 	 */
 	indio_dev->modes |= INDIO_BUFFER_TRIGGERED;
+
+	ret = iio_buffer_register(indio_dev, channels, num_channels);
+	if (ret)
+		goto error_dealloc_pollfunc;
+
 	return 0;
 
+error_dealloc_pollfunc:
+	iio_dealloc_pollfunc(indio_dev->pollfunc);
 error_free_buffer:
 	iio_kfifo_free(indio_dev->buffer);
 error_ret:
···
  */
 void iio_simple_dummy_unconfigure_buffer(struct iio_dev *indio_dev)
 {
+	iio_buffer_unregister(indio_dev);
 	iio_dealloc_pollfunc(indio_dev->pollfunc);
 	iio_kfifo_free(indio_dev->buffer);
 }