Re: usb/sound/bcd2000: warning in bcd2000_init_device

[Takashi Iwai]

On Tue, 03 Oct 2017 16:21:57 +0200,
Alan Stern wrote:
> 
> On Tue, 3 Oct 2017, Takashi Iwai wrote:
> 
> > On Mon, 25 Sep 2017 14:39:51 +0200,
> > Andrey Konovalov wrote:
> > > 
> > > Hi!
> > > 
> > > I've got the following report while fuzzing the kernel with syzkaller.
> > > 
> > > On commit e19b205be43d11bff638cad4487008c48d21c103 (4.14-rc2).
> > > 
> > > It seems that there's no check of the endpoint type.
> > > 
> > > usb 1-1: BOGUS urb xfer, pipe 1 != type 3
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 0 PID: 1846 at drivers/usb/core/urb.c:449
> > 
> > How is this bug triggered?  As it's syzkaller with QEMU, it looks
> > hitting an inconsistent state the driver didn't expect (it sets the
> > fixed endpoint), then USB-core detects the inconsistency and spews the
> > kernel warning with stack trace.  If so, it's no serious problem as it
> > appears.
> > 
> > Suppose my guess is right, I'm not sure what's the best way to fix
> > this.  Certainly we can add more sanity check in the caller side.
> > OTOH, I find the reaction of USB core too aggressive, it's not
> > necessary to be dev_WARN() but a normal dev_err().
> > Or I might be looking at a wrong place?
> 
> 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.


thanks,

Takashi