Re: USB fuzzing with syzbot

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Apr 24, 2019 at 6:05 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Fri, Apr 19, 2019 at 10:35 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > > 2. Is there an easy way to figure out which config options enable
> > > drivers reachable over USB?
> >
> > Looking for all options that depend on USB is a good start.
> >
> > > Right now our kernel config is based on one of the Debian kernel
> > > configs, that supposedly enables enough relevant USB drivers. At the
> > > same time it enables a lot of other unnecessary stuff, which makes the
> > > kernel big and long to compile. Ideally, we would to have a way to
> > > auto-generate a kernel config that enables all the relevant (enabled
> > > by at least one of the distros) USB drivers. I've looked at whether
> > > it's possible to figure out which particular options in some kernel
> > > config are related to USB, but it seems that neither the option names,
> > > nor the way they are grouped in the config file, are representative
> > > enough.
> >
> > Yeah, it's hard to just carve out this type of configuration, but here's
> > what I have done in the past to try to be sure I enabled all USB drivers
> > in my kernel configuration.
> >
> > First, start with a "minimally working configuration" by running:
> >         make localmodconfig
> > on a working system, with the needed modules for booting and operating
> > properly already loaded.
> >
> > That gives you a .config file that should take only minutes to build,
> > compared to much longer for the normal distro configuration (also be
> > sure to disable some debugging options so you don't spend extra time
> > building and stripping symbols).
> >
> > Boot and make sure that configuration works.
> >
> > Then, take that .config and do:
> >         - disable USB from the configuration by deleting the:
> >                 CONFIG_USB_SUPPORT=y
> >           option from your .config
> >         - run 'make oldconfig' to disable all USB drivers
> >         - turn USB back on by setting CONFIG_USB_SUPPORT=y back on in
> >           your .config
> >         - run 'make oldconfig' and answer 'y' or 'm' to all of the
> >           driver options you are presented with.
> >
> > That usually catches almost all of them.  Sometimes you need to make
> > sure you have some other subsystem enabled (like SCSI), but odds are, if
> > you start with a "normally stripped down" configuration that works, you
> > should be fine.
>
> I suspect that make localmodconfig (+ switching CONFIG_USB_SUPPORT off
> and on) would likely include a lot of stuff that we don't need (there
> are many options that are =y, but not related to USB at all), but it
> definitely sounds better than what I have right now (converting almost
> all =m into =y). I'll give it a shot, thanks!

I've tried this and unfortunately it doesn't work as desired. The
reason is that localmodconfig will only enable options for the modules
that are currently loaded, and if a module that some USB driver
depends on is not loaded, then this driver won't be enabled after yes
| make oldconfig. For example my machine didn't have the cfg80211
module loaded, and thus e.g. CONFIG_AT76C50X_USB didn't get enabled
after oldconfig. However when I plug in a wireless USB adapter,
cfg80211 gets loaded together with the USB driver for that adapter. I
guess the same applies to other kinds of dependency modules (e.g.
bluetooth). So this would only work if all the dependency modules are
already loaded.