Re: Use of usb_find_interface in open is racy

[Greg KH]

On Wed, Nov 18, 2009 at 10:31:47AM -0500, Alan Stern wrote:
> 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().

Yeah, I think so, sorry about that I think this is my fault :(

I'm travelling all today, could someone make up a patch for this before
Thursday if it's an issue?

Russ, have you ever hit this problem, or did you just find it by looking
at the code?

thanks,

greg k-h
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