Re: usb/sound/bcd2000: warning in bcd2000_init_device

[Takashi Iwai]

On Wed, 04 Oct 2017 14:03:25 +0200,
Johan Hovold wrote:
> 
> On Wed, Oct 04, 2017 at 12:41:55PM +0200, Takashi Iwai wrote:
> > On Wed, 04 Oct 2017 12:23:11 +0200,
> > Johan Hovold wrote:
> > > 
> > > On Wed, Oct 04, 2017 at 12:04:06PM +0200, Takashi Iwai wrote:
> > > > On Wed, 04 Oct 2017 11:24:42 +0200, Johan Hovold wrote:
> > > > > On Wed, Oct 04, 2017 at 08:10:59AM +0200, Takashi Iwai wrote:
> > > 
> > > > > > Well, what I had in my mind is just a snippet from usb_submit_urb(),
> > > > > > something like:
> > > > > > 
> > > > > > bool usb_sanity_check_urb_pipe(struct urb *urb)
> > > > > > {
> > > > > > 	struct usb_host_endpoint *ep;
> > > > > > 	int xfertype;
> > > > > > 	static const int pipetypes[4] = {
> > > > > > 		PIPE_CONTROL, PIPE_ISOCHRONOUS, PIPE_BULK, PIPE_INTERRUPT
> > > > > > 	};
> > > > > > 
> > > > > > 	ep = usb_pipe_endpoint(urb->dev, urb->pipe);
> > > > > > 	xfertype = usb_endpoint_type(&ep->desc);
> > > > > > 	return usb_pipetype(urb->pipe) != pipetypes[xfertype];
> > > > > > }
> > > > > > 
> > > > > > And calling this before usb_submit_urb() in each place that assigns
> > > > > > the fixed EP as device-specific quirks.
> > > > > > Does it make sense?
> > > > > 
> > > > > Not really. Your driver should not even bind to an interface which lacks
> > > > > the expected endpoints (rather than check this at a potentially later
> > > > > point in time when URBs are submitted).
> > > > 
> > > > The endpoint may exist but it may be invalid, as the problem is
> > > > triggered by a VM.  It doesn't parse but tries a fixed EP as it's no
> > > > compliant device.
> > > 
> > > Yes, that's why a driver should verify that the endpoints it expects are
> > > indeed present (and of the right type) already at probe.
> > > 
> > > In Andrey's fuzzing it's triggered by in a VM using the dummy_hcd
> > > driver, but this could just as well be a (malicious) physical device
> > > with unexpected descriptors.
> > > 
> > > > > The new helper which Greg mentioned would allow this to implemented with
> > > > > just a few lines of code. Just add it to bcd2000_init_midi() or similar.  
> > > > 
> > > > Could you give an example?  Then I can ask Andrey whether such a call
> > > > really addresses the issue.
> > > 
> > > If you grep for usb_find_common_endpoints you'll find a few examples
> > > of how that function may be used (e.g. in drivers/usb/misc/usblcd.c).
> > > 
> > > The helper iterates of the endpoint descriptors of an interface
> > > alt-setting and returns a descriptor for each requested type if found.
> > > After a vetting of our current drivers I concluded that this would
> > > cover the needs of the vast majority of drivers.
> > > 
> > > So for the driver in question you'd only need to add something like:
> > > 
> > > 	struct usb_endpoint_descriptor *int_in, *int_out;
> > > 	int ret;
> > > 
> > > 	ret = usb_find_common_endpoints(interface->cur_altsetting,
> > > 					NULL, NULL, &int_in, &int_out);
> > > 	if (ret) {
> > > 		dev_err(&interface->dev, "required endpoints not found\n");
> > > 		return -ENODEV;
> > > 	}
> > > 
> > > Then you can use int_in->bEndpointAddress etc. when initialising your
> > > URBs.
> > 
> > OK, but in our cases, it's not about using the returned one but
> > checking whether it's the expected address, right?  The device is
> > non-compliant and that's the reason the driver takes the fixed EP.
> 
> There's nothing preventing you from verifying that the returned
> descriptors have the expected addresses if tightening the constraints
> this ways makes sense for your application.

OK.

> Or you can implement your own sanity checks, just do it at probe.
> 
> But note that you'd introduce NULL-deref that can be triggered by a
> malicious device with your outlined helper above, as
> 
> 	ep = usb_pipe_endpoint(urb->dev, urb->pipe);
> 
> will be NULL when the corresponding descriptor is missing.

Yes, if we do that.  But I'm working on a version using
usb_find_*_endpoint*() instead.

> > In anyway, the check will be shortly before the URB submission because
> > the EP is often determined a late stage of probe, as most of errors
> > happened for the MIDI interface that are device-specific.
> 
> As long as you do the check during probe and refuse to bind to a
> non-compliant device you should be fine. Some drivers do not submit URBs
> until the user tries to use whatever interface the driver exposes (e.g.
> when opening a character device), which IMO is too late for such sanity
> checks.

Right, and this won't be a problem as the issue is triggered before
the actual device registration (ALSA has a staged registration
scheme).


thanks,

Takashi