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

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Date:	Wed, 18 Nov 2009 09:58:29 -0700
From:	Russ Dill <russ.dill@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oliver@...kum.org>, Jiri Kosina <jkosina@...e.cz>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: Use of usb_find_interface in open is racy

On Wed, Nov 18, 2009 at 8:35 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> 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.
>

Like I said above, the window goes all the way till the device gets
added to the klist_devices on the driver.
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