patch "usbcore/driver: Fix incorrect downcast" added to usb-linus

This is a note to let you know that I've just added the patch titled

usbcore/driver: Fix incorrect downcast

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 4df30e7603432704380b12fe40a604ee7f66746d Mon Sep 17 00:00:00 2001
From: "M. Vefa Bicakci" <[email protected]>
Date: Tue, 22 Sep 2020 14:07:02 +0300
Subject: usbcore/driver: Fix incorrect downcast

This commit resolves a minor bug in the selection/discovery of more specific USB device drivers for devices that are currently bound to generic USB device drivers.

The bug is related to the way a candidate USB device driver is compared against the generic USB device driver. The code in is_dev_usb_generic_driver() assumes that the device driver in question is a USB device driver by calling to_usb_device_driver(dev->driver) to downcast; however I have observed that this assumption is not always true, through code instrumentation.

This commit avoids the incorrect downcast altogether by comparing the USB device's driver (i.e., dev->driver) to the generic USB device driver directly. This method was suggested by Alan Stern.

This bug was found while investigating Andrey Konovalov's report indicating usbip device driver misbehaviour with the recently merged generic USB device driver selection feature. The report is linked below.

---
 drivers/usb/core/driver.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

index 715782995428..b00b7fb1aad1 100644
@@ -905,21 +905,14 @@ static int usb_uevent(struct device *dev, struct kobj_uevent_env *env)
 	return 0;
 }
 
-static bool is_dev_usb_generic_driver(struct device *dev)
-{
-	struct usb_device_driver *udd = dev->driver ?
-		to_usb_device_driver(dev->driver) : NULL;
-
-	return udd == &usb_generic_driver;
-}
-
 static int __usb_bus_reprobe_drivers(struct device *dev, void *data)
 {
 	struct usb_device_driver *new_udriver = data;
 	struct usb_device *udev;
 	int ret;
 
-	if (!is_dev_usb_generic_driver(dev))
+	/* Don't reprobe if current driver isn't usb_generic_driver */
+	if (dev->driver != &usb_generic_driver.drvwrap.driver)
 		return 0;
 
 	udev = to_usb_device(dev);

2.28.0