Re: [PATCH v2 4/9] ALSA: line6: Add a sanity check for invalid EPs

[Johan Hovold <johan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Wed, Oct 11, 2017 at 04:45:16PM +0200, Takashi Iwai wrote:
> On Wed, 11 Oct 2017 16:28:37 +0200,
> Johan Hovold wrote:
> > 
> > On Wed, Oct 11, 2017 at 12:36:41PM +0200, Takashi Iwai wrote:
> > > As syzkaller spotted, currently line6 drivers submit a URB with the
> > > fixed EP without checking whether it's actually available, which may
> > > result in a kernel warning like:
> > >   usb 1-1: BOGUS urb xfer, pipe 3 != type 1
> > >   ------------[ cut here ]------------
> > >   WARNING: CPU: 0 PID: 24 at drivers/usb/core/urb.c:449
> > >   usb_submit_urb+0xf8a/0x11d0
> > >   Modules linked in:
> > >   CPU: 0 PID: 24 Comm: kworker/0:1 Not tainted 4.14.0-rc2-42613-g1488251d1a98 #238
> > >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> > >   Workqueue: usb_hub_wq hub_event
> > >   Call Trace:
> > >    line6_start_listen+0x55f/0x9e0 sound/usb/line6/driver.c:82
> > >    line6_init_cap_control sound/usb/line6/driver.c:690
> > >    line6_probe+0x7c9/0x1310 sound/usb/line6/driver.c:764
> > >    podhd_probe+0x64/0x70 sound/usb/line6/podhd.c:474
> > >    usb_probe_interface+0x35d/0x8e0 drivers/usb/core/driver.c:361
> > >    ....
> > > 
> > > This patch adds a sanity check of validity of EPs at the device
> > > initialization phase for avoiding the call with an invalid EP.
> > > 
> > > Reported-by: Andrey Konovalov <andreyknvl-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> > > Tested-by: Andrey Konovalov <andreyknvl-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> > > Signed-off-by: Takashi Iwai <tiwai-l3A5Bk7waGM@public.gmane.org>
> > > ---
> > >  sound/usb/line6/driver.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/sound/usb/line6/driver.c b/sound/usb/line6/driver.c
> > > index 0ff5a7d2e19f..0da6f68761e3 100644
> > > --- a/sound/usb/line6/driver.c
> > > +++ b/sound/usb/line6/driver.c
> > > @@ -78,6 +78,13 @@ static int line6_start_listen(struct usb_line6 *line6)
> > >  			line6->buffer_listen, LINE6_BUFSIZE_LISTEN,
> > >  			line6_data_received, line6);
> > >  	}
> > > +
> > > +	/* sanity checks of EP before actually submitting */
> > > +	if (usb_urb_ep_type_check(line6->urb_listen)) {
> > > +		dev_err(line6->ifcdev, "invalid control EP\n");
> > > +		return -EINVAL;
> > > +	}
> > 
> > You're now doing this check twice (here and in usb_submit_urb) every
> > time this URB is submitted rather just checking it once in probe.
> > 
> > Seems like a quick band-aid to me.
> 
> Yes, it is :)
> 
> We may split line6_start_listen() to two functions, one for
> initializing URBs and another for submission, but it'll be more codes
> and the check is fairly cheap.  So again it's a question whether
> easier to read or to get minor optimization.

I'd say it's also a question about long-term maintainability. Those
quick band-aids tend to keep piling up over time.

Johan
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