[PATCH 2/3] usbcore/driver: Fix incorrect downcast

["M. Vefa Bicakci" <m.v.b@runbox.com>]

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.

Given that USB device drivers are bound to struct device instances with
of the type &usb_device_type, this commit checks the return value of
is_usb_device() before the call to is_dev_usb_generic_driver(), and
therefore ensures that incorrect type downcasts do not occur. The use
of is_usb_device() was suggested by Bastien Nocera.

This bug was found while investigating Andrey Konovalov's report
indicating USB/IP subsystem's misbehaviour with the generic USB device
driver matching code.

Fixes: d5643d2249 ("USB: Fix device driver race")
Link: https://lore.kernel.org/linux-usb/CAAeHK+zOrHnxjRFs=OE8T=O9208B9HP_oo8RZpyVOZ9AJ54pAA@mail.gmail.com/
Cc: <stable@vger.kernel.org> # 5.8
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Bastien Nocera <hadess@hadess.net>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: <syzkaller@googlegroups.com>
Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
---
 drivers/usb/core/driver.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
index 950044a6e77f..ba7acd6e7cc4 100644
--- a/drivers/usb/core/driver.c
+++ b/drivers/usb/core/driver.c
@@ -919,7 +919,7 @@ static int __usb_bus_reprobe_drivers(struct device *dev, void *data)
 	struct usb_device *udev;
 	int ret;
 
-	if (!is_dev_usb_generic_driver(dev))
+	if (!is_usb_device(dev) || !is_dev_usb_generic_driver(dev))
 		return 0;
 
 	udev = to_usb_device(dev);
-- 
2.26.2