linux-kernel - Re: Use of usb_find_interface in open is racy

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Date:	Wed, 18 Nov 2009 10:31:47 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jiri Kosina <jkosina@...e.cz>
cc:	Russ Dill <russ.dill@...il.com>, <linux-kernel@...r.kernel.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: Use of usb_find_interface in open is racy

On Wed, 18 Nov 2009, Jiri Kosina wrote:

> On Tue, 17 Nov 2009, Russ Dill wrote:
> 
> > Many usb drivers that create character devices use "struct
> > usb_class_driver", a set of fops, and a usb_find_interface in their
> > open call. A prime example is drivers/usb/usb-skeleton.c. A race
> > occurs when userspace receives a hotplug event for the addition for
> > the interface and then opens the associated device file before the
> > device is added to the driver's klist_devices.
> > 
> > The usb core senses a new usb device (usb_new_device) and calls
> > device_add. This eventually gets down to really_probe and the
> > usb-skeleton probe function, skel_probe. skel_probe calls
> > usb_register_dev() which registers the associated character device for
> > skel_class. The hotplug events for the class device get emitted.
> > 
> > User space receives the hotplug event for the class device, makes the
> > device node and notifies another program that opens the device node.
> > The program opens the device node which calls into usb_open and then
> > skel_open. skel_open calls usb_find_interface. usb_find_interfaces
> > searches the klist_devices of skel_driver, finds no device associated
> > with the minor number and returns NULL. skel_open returns -ENODEV.
> > 
> > Control returns to really_probe and really_probe calls driver_bound
> > which adds the device to the list of devices associated with
> > skel_driver (klist_devices).
> > 
> > I'm not sure what the right way to solve this is. A call to
> > wait_for_device_probe() in the skel_open call before calling
> > usb_find_interface fixes the problem, but it is a rather large hammer.

I think the proper answer is for usb_find_interface() to use 
bus_for_each_dev() instead of driver_for_each_device().

Alan Stern

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