Re: [PATCH] driver core: take parent lock on deferred probe

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Oct 13, 2026 at 9:11 PM CEST, Alan Stern wrote:
> On Tue, Oct 13, 2026 at 07:43:08PM +0200, Danilo Krummrich wrote:
>> On Tue Oct 13, 2026 at 4:06 PM CEST, Johan Hovold wrote:
>> > +void bus_probe_device_deferred(struct device *dev)
>> > +{
>> > +	if (dev->parent && dev->bus->need_parent_lock)
>> > +		device_lock(dev->parent);
>> 
>> This looks like it has the same potential UAF as the other patch.  It's quite
>> unfortunate we need the whole need_parent_lock thing just for USB.  IIUC the
>> main reason USB needs this is because interface devices share resources of USB
>> devices and we need to keep the locking order to avoid inversions, right?  Is
>> there more to this from the USB side of things?
>
> That's pretty much it.
>
>> I think we can probably do something like this
>> 
>> 	struct device *parent = NULL;
>> 
>> 	scoped_guard(device, dev) {
>> 		if (dev->p->dead)
>> 			return;
>> 	
>> 		if (dev->bus->need_parent_lock)
>> 			parent = get_device(dev->parent);
>> 	}
>> 
>> to avoid the problem, but it's still a bit ugly.
>> 
>> > +	__bus_probe_device(dev);
>> > +	if (dev->parent && dev->bus->need_parent_lock)
>> > +		device_unlock(dev->parent);
>> > +}
>
> Wouldn't it be simpler just to make each USB interface take a reference 
> to its parent device?  And likewise for USB devices, if needed?

The driver core already holds a reference count of the parent between
device_add() and device_del(), which is where a "parent" is technically defined.
But the deferred retry can extend beyond device_del().

USB interfaces already take an additional reference to their USB device in
usb_set_configuration() and drop it in usb_release_interface(), so they should
already be protected against this particular UAF.

The issue concerns a USB device and its parent hub's USB device.  (This makes me
wonder whether probing a USB device actually requires the parent USB HUB
device's lock to be held, or whether this is just a consequence of sharing the
same usb_bus_type?)

In theory a USB device could also keep a reference count to its parent, but then
the driver core code would rely on an implementation detail of its users; I'd
prefer not to make driver core's access to dev->parent depend on references
maintained by individual subsystems.

I'd even go a step further any say that if subsystems try to keep this pointer
valid beyond what's guaranteed by the driver core, they should do this with a
separate bus specific pointer.  The reason is that otherwise the lifetime and
ownership definition of this pointer does not depend on API contracts, but on
individual implementation details, which is a very common source for all kinds
of bugs.