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

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

On 17/09/2020 18.01, Alan Stern wrote:
> On Thu, Sep 17, 2020 at 05:41:50PM +0300, M. Vefa Bicakci wrote:
>> 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
> 
> Why not simplify the whole thing by testing the underlying driver
> pointer?
> 
> 	/* Don't reprobe if current driver isn't usb_generic_driver */
> 	if (dev->driver != &usb_generic_driver.drvwrap.driver)
> 		return 0;
> 
> Then is_dev_usb_generic_driver can be removed entirely.

Alan, sorry for the delay, and thanks for the review! I had not thought
of this. I will apply the changes you have suggested with the next version
of the patch series.

Thanks again,

Vefa