linux-kernel - Re: usb/sound/bcd2000: warning in bcd2000_init

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Date:   Wed, 04 Oct 2017 12:04:06 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        alsa-devel@...a-project.org,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Sakamoto <o-takashi@...amocchi.jp>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>, linux-usb@...r.kernel.org
Subject: Re: usb/sound/bcd2000: warning in bcd2000_init_device

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:
> > On Tue, 03 Oct 2017 19:42:21 +0200,
> > Greg Kroah-Hartman wrote:
> > > 
> > > On Tue, Oct 03, 2017 at 12:50:08PM -0400, Alan Stern wrote:
> > > > On Tue, 3 Oct 2017, Takashi Iwai wrote:
> > > > 
> > > > > > It's a dev_WARN because it indicates a potentially serious error in the 
> > > > > > driver: The driver has submitted an interrupt URB to a bulk endpoint.  
> > > > > > That may not sound bad, but the same check gets triggered if a driver 
> > > > > > submits a bulk URB to an isochronous endpoint, or any other invalid 
> > > > > > combination.
> > > > > > 
> > > > > > Most likely the explanation here is that the driver doesn't bother to
> > > > > > check the endpoint type because it expects the endpoint will always be
> > > > > > interrupt.  But that is not a safe strategy.  USB devices and their
> > > > > > firmware should not be trusted unnecessarily.
> > > > > > 
> > > > > > The best fix is, like you said, to add a sanity check in the caller.
> > > > > 
> > > > > OK, but then do we have some handy helper for the check?
> > > > > As other bug reports by syzkaller suggest, there are a few other
> > > > > drivers that do the same, submitting a urb with naive assumption of
> > > > > the fixed EP for specific devices.  In the end we'll need to put the
> > > > > very same checks there in multiple places.
> > > > 
> > > > Perhaps we could add a helper routine that would take a list of 
> > > > expected endpoint types and check that the actual endpoints match the 
> > > > types.  But of course, all the drivers you're talking about would have 
> > > > to add a call to this helper routine.
> > > 
> > > We have almost this type of function, usb_find_common_endpoints(),
> > > what's wrong with using that?  Johan has already swept the tree and
> > > added a lot of these checks, odds are no one looked at the sound/
> > > subdir...
> 
> Yeah, I only swept the tree for instances were a missing endpoint could
> lead to a NULL-deref. This is not the case here were the endpoint
> addresses are hardcoded in the driver.
> 
> I also never got around to applying the new helper outside of
> drivers/usb.
> 
> > 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.

> 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.


thanks,

Takashi