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

[Rajat Jain <rajatja@google.com>]

Hello,

I spent some more thoughts into this...

On Wed, Jun 3, 2020 at 5:16 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Wed, Jun 03, 2020 at 04:51:18AM -0700, Rajat Jain wrote:
> > Hello,
> >
> > >
> > > > Thanks for the pointer! I'm still looking at the details yet, but a
> > > > quick look (usb_dev_authorized()) seems to suggest that this API is
> > > > "device based". The multiple levels of "authorized" seem to take shape
> > > > from either how it is wired or from userspace choice. Once authorized,
> > > > USB device or interface is authorized to be used by *anyone* (can be
> > > > attached to any drivers). Do I understand it right that it does not
> > > > differentiate between drivers?
> > >
> > > Yes, and that is what you should do, don't fixate on drivers.  Users
> > > know how to control and manage devices.  Us kernel developers are
> > > responsible for writing solid drivers and getting them merged into the
> > > kernel tree and maintaining them over time.  Drivers in the kernel
> > > should always be trusted, ...
> >
> > 1) Yes, I agree that this would be ideal, and this should be our
> > mission. I should clarify that I may have used the wrong term
> > "Trusted/Certified drivers". I didn't really mean that the drivers may
> > be malicious by intent. What I really meant is that a driver may have
> > an attack surface, which is a vulnerability that may be exploited.
>
> Any code has such a thing, proving otherwise is a tough problem :)
>
> > Realistically speaking, finding vulnerabilities in drivers, creating
> > attacks to exploit them, and fixing them is a never ending cat and
> > mouse game. At Least "identifying the vulnerabilities" part is better
> > performed by security folks rather than driver writers.
>
> Are you sure about that?  It's hard to prove a negative :)
>
> > Earlier in the
> > thread I had mentioned certain studies/projects that identified and
> > exploited such vulnerabilities in the drivers. I should have used the
> > term "Vetted Drivers" maybe to convey the intent better - drivers that
> > have been vetted by a security focussed team (admin). What I'm
> > advocating here is an administrator's right to control the drivers
> > that he wants to allow for external ports on his systems.
>
> That's an odd thing, but sure, if you want to write up such a policy for
> your systems, great.  But that policy does not belong in the kernel, it
> belongs in userspace.
>
> > 2) In addition to the problem of driver negligences / vulnerabilities
> > to be exploited, we ran into another problem with the "whitelist
> > devices only" approach. We did start with the "device based" approach
> > only initially - but quickly realized that anything we use to
> > whitelist an external device can only be based on the info provided by
> > *that device* itself. So until we have devices that exchange
> > certificates with kernel [1], it is easy for a malicious device to
> > spoof a whitelisted device (by presenting the same VID:DID or any
> > other data that is used by us to whitelist it).
> >
> > [1] https://www.intel.com/content/www/us/en/io/pci-express/pcie-device-security-enhancements-spec.html
> >
> > I hope that helps somewhat clarify how / why we reached here?
>
> Kind of, I still think all you need to do is worry about controling the
> devices and if a driver should bind to it or not.  Again, much like USB
> has been doing for a very long time now.  The idea of "spoofing" ids
> also is not new, and has been around for a very long time as well, and
> again, the controls that the USB core gives you allows you to make any
> type of policy decision you want to, in userspace.

Er, *currently* it doesn't allow the userspace to make the particular
policy I want to, right? Specifically, today an administrator can not
control which USB *drivers* he wants to allow on an *external* USB
port. He can only control which USB devices he wants to authorize, but
once authorized, they are free to bind to any of the USB drivers. So
if I want to allow the administrator to implement a policy that allows
him to control the drivers for external ports, we'll need to enhance
the current code (whether we want to do it specific to a bus, or more
generically in the driver core). Are we on the same page?

To implement the policy that I want to in the driver core, what is
missing today in driver core is a distinction between "internal" and
"external" devices. Some buses have this knowledge locally today (PCI
has "untrusted" flag which can be used, USB uses hcd->wireless and
hub->port->connect_type) but it is not shared with the core.

So just to make sure if I'm thinking in the right direction, this is
what I'm thinking:

1) The device core needs a notion of internal vs external devices (a
flag) - a knowledge that needs to be filled in by the bus as it
discovers the device.

2) The driver core needs to allow an admin to provide a whitelist of
drivers for external devices. (Via Command line or a driver flag.
Default = everything is whitelisted).

3) While matching a driver to a device, the driver core needs to
impose the whitelist if the device is external, and if the
administrator has provided a whitelist.

Any bus that wants to use this can use it if it wants to, for external devices.

Thoughts?

Thanks & Best Regards,

Rajat

>
> So please, in summary:
>         - don't think this is some new type of thing, it's an old issue
>           transferred to yet-another-hardware-bus.  Not to say this is
>           not important, just please look at the work that others have
>           done in the past to help mitigate and solve this (reading the
>           Wireless USB spec should help you out here too, as they
>           detailed all of this.)
>         - do copy what USB did, by moving that logic into the driver
>           core so that all busses who want to take advantage of this
>           type of functionality, easily can do so.
>
> thanks,
>
> greg k-h