spi: fix use-after-free at controller deregistration

The controller is typically freed as part of device_unregister() so
store the bus id before deregistration to avoid use-after-free when the
id is later released.

Fixes: 9b61e302210e ("spi: Pick spi bus number from Linux idr or spi alias")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable <stable@vger.kernel.org>

drivers/spi/spi.c

@@ -2240,11 +2240,12 @@ static int __unregister(struct device *dev, void *null)
 void spi_unregister_controller(struct spi_controller *ctlr)
 {
 	struct spi_controller *found;
+	int id = ctlr->bus_num;
 	int dummy;
 
 	/* First make sure that this controller was ever added */
 	mutex_lock(&board_lock);
-	found = idr_find(&spi_master_idr, ctlr->bus_num);
+	found = idr_find(&spi_master_idr, id);
 	mutex_unlock(&board_lock);
 	if (found != ctlr) {
 		dev_dbg(&ctlr->dev,
@@ -2264,7 +2265,7 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 	device_unregister(&ctlr->dev);
 	/* free bus id */
 	mutex_lock(&board_lock);
-	idr_remove(&spi_master_idr, ctlr->bus_num);
+	idr_remove(&spi_master_idr, id);
 	mutex_unlock(&board_lock);
 }
 EXPORT_SYMBOL_GPL(spi_unregister_controller);