driver core: platform: use the correct callback type for bus_find_device

platform_find_device_by_driver calls bus_find_device and passes
platform_match as the callback function. Casting the function to a
mismatching type trips indirect call Control-Flow Integrity (CFI) checking.

This change adds a callback function with the correct type and instead
of casting the function, explicitly casts the second parameter to struct
device_driver* as expected by platform_match.

Fixes: 36f3313d ("platform: Add platform_find_device_by_driver() helper")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191112214156.3430-1-samitolvanen@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/platform.c

@@ -1310,6 +1310,11 @@ struct bus_type platform_bus_type = {
 };
 EXPORT_SYMBOL_GPL(platform_bus_type);
 
+static inline int __platform_match(struct device *dev, const void *drv)
+{
+	return platform_match(dev, (struct device_driver *)drv);
+}
+
 /**
  * platform_find_device_by_driver - Find a platform device with a given
  * driver.
@@ -1320,7 +1325,7 @@ struct device *platform_find_device_by_driver(struct device *start,
 					      const struct device_driver *drv)
 {
 	return bus_find_device(&platform_bus_type, start, drv,
-			       (void *)platform_match);
+			       __platform_match);
 }
 EXPORT_SYMBOL_GPL(platform_find_device_by_driver);