Re: [RFC] Restrict the untrusted devices, to bind to only a set of "whitelisted" drivers

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jun 08, 2020 at 11:41:19AM -0700, Rajat Jain wrote:
> Hi Jesse and Greg,
> 
> On Mon, Jun 8, 2020 at 11:30 AM Jesse Barnes <jsbarnes@google.com> wrote:
> >
> > > > I think your suggestion to disable driver binding once the initial
> > > > bus/slot devices have been bound will probably work for this
> > > > situation.  I just wanted to be clear that without some auditing,
> > > > fuzzing, and additional testing, we simply have to assume that drivers
> > > > are *not* secure and avoid using them on untrusted devices until we're
> > > > fairly confident they can handle them (whether just misbehaving or
> > > > malicious), in combination with other approaches like IOMMUs of
> > > > course.  And this isn't because we don't trust driver authors or
> > > > kernel developers to dtrt, it's just that for many devices (maybe USB
> > > > is an exception) I think driver authors haven't had to consider this
> > > > case much, and so I think it's prudent to expect bugs in this area
> > > > that we need to find & fix.
> > >
> > > For USB, yes, we have now had to deal with "untrusted devices" lieing
> > > about their ids and sending us horrible data.  That's all due to the
> > > fuzzing tools that have been written over the past few years, and now we
> > > have some of those in the kernel tree itself to help with that testing.
> 
> This is great to hear! I tried to look up but didn't find anything
> else in-kernel, except the kcov support to export coverage info for
> userspace fuzzers. Can you please give us some pointers for in-kernel
> fuzzing tools?

For USB, it's a combination of using syzbot with the "raw gadget" driver
and the loopback gadget/host controller.  See many posts from Andrey
Konovalov <andreyknvl@google.com> on the linux-usb@vger.kernel.org list
for details as to how he does this.

> > > For PCI, heh, good luck, those assumptions about "devices sending valid
> > > data" are everywhere, if our experience with USB is any indication.
> > >
> > > But, to take USB as an example, this is exactly what the USB
> > > "authorized" flag is there for, it's a "trust" setting that userspace
> > > has control over.  This came from the wireless USB spec, where it was
> > > determined that you could not trust devices.  So just use that same
> > > model here, move it to the driver core for all busses to use and you
> > > should be fine.
> > >
> > > If that doesn't meet your needs, please let me know the specifics of
> > > why, with patches :)
> >
> > Yeah will do for sure.  I don't want to carry a big infra for this on our own!
> >
> > > Now, as to you all getting some sort of "Hardware flag" to determine
> > > "inside" vs. "outside" devices, hah, good luck!  It took us a long time
> > > to get that for USB, and even then, BIOSes lie and get it wrong all the
> > > time.  So you will have to also deal with that in some way, for your
> > > userspace policy.
> >
> > I think that's inherently platform specific to some extent.  We can do
> > it with our coreboot based firmware, but there's no guarantee other
> > vendors will adopt the same approach.  But I think at least for the
> > ChromeOS ecosystem we can come up with something that'll work, and
> > allow us to dtrt in userspace wrt driver binding.
> 
> Agree, we can work with our firmware teams to get that right, and then
> expose it from kernel to userspace to help it implement the policy we
> want.

This is already in the spec for USB, I suggest working to get this added
to the other bus type specs that you care about as well (UEFI, PCI,
etc.)

thanks,

greg k-h