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From:   Jesse Barnes <jsbarnes@google.com>
Date:   Mon, 8 Jun 2020 11:29:58 -0700
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Subject: Re: [RFC] Restrict the untrusted devices, to bind to only a set of
 "whitelisted" drivers
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Rajat Jain <rajatja@google.com>, Rajat Jain <rajatxjain@gmail.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Krishnakumar, Lalithambika" <lalithambika.krishnakumar@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci <linux-pci@vger.kernel.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Prashant Malani <pmalani@google.com>,
        Benson Leung <bleung@google.com>,
        Todd Broch <tbroch@google.com>,
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> > 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.
>
> 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.

Thanks,
Jesse