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

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Date:	Wed, 18 Nov 2009 10:35:16 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Jiri Kosina <jkosina@...e.cz>, 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, Oliver Neukum wrote:

> Am Mittwoch, 18. November 2009 11:41:25 schrieb Jiri Kosina:
> > > 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.
> > 
> 
> Device core code is hard to follow, but I tried.
> How about simply covering all of usb_register_dev() with minor_rwsem?

That won't help.  The window is between the end of usb_register_dev()  
and the end of skel_probe(), during which time the mutex isn't held.

Alan Stern

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