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

[Rajat Jain <rajatja@google.com>]

On Tue, Jun 9, 2020 at 5:04 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Tue, Jun 09, 2020 at 04:23:54PM -0700, Rajat Jain wrote:
> > Hi Bjorn,
> >
> > Thanks for sending out the summary, I was about to send it out but got lazy.
> >
> > On Tue, Jun 9, 2020 at 2:04 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > >
> > > On Sun, Jun 07, 2020 at 01:36:32PM +0200, Greg Kroah-Hartman wrote:
> > >
> > > > Your "problem" I think can be summed up a bit more concise:
> > > >       - you don't trust kernel drivers to be "secure" for untrusted
> > > >         devices
> > > >       - you only want to bind kernel drivers to "internal" devices
> > > >         automatically as you "trust" drivers in that situation.
> > > >       - you want to only bind specific kernel drivers that you somehow
> > > >         feel are "secure" to untrusted devices "outside" of a system
> > > >         when those devices are added to the system.
> > > >
> > > > Is that correct?
> > > >
> > > > If so, fine, you can do that today with the bind/unbind ability of
> > > > drivers, right?  After boot with your "trusted" drivers bound to
> > > > "internal" devices, turn off autobind of drivers to devices and then
> > > > manually bind them when you see new devices show up, as those "must" be
> > > > from external devices (see the bind/unbind files that all drivers export
> > > > for how to do this, and old lwn.net articles, this feature has been
> > > > around for a very long time.)
> > > >
> > > > I know for USB you can do this, odds are PCI you can turn off
> > > > autobinding as well, as I think this is a per-bus flag somewhere.  If
> > > > that's not exported to userspace, should be trivial to do so, should be
> > > > somewere in the driver model already...
> > > >
> > > > Ah, yes, look at the "drivers_autoprobe" and "drivers_probe" files in
> > > > sysfs for all busses.  Do those not work for you?
> > > >
> > > > My other points are the fact that you don't want to put policy in the
> > > > kernel, and I think that you can do everything you want in userspace
> > > > today, except maybe the fact that trying to determine what is "inside"
> > > > and "outside" is not always easy given that most hardware does not
> > > > export this information properly, if at all.  Go work with the firmware
> > > > people on that issue please, that would be most helpful for everyone
> > > > involved to get that finally straightened out.
> > >
> > > To sketch this out, my understanding of how this would work is:
> > >
> > >   - Expose the PCI pdev->untrusted bit in sysfs.  We don't expose this
> > >     today, but doing so would be trivial.  I think I would prefer a
> > >     sysfs name like "external" so it's more descriptive and less of a
> > >     judgment.
> >
> > Yes. I think we should probably semantically differentiate between
> > "external" and "external facing" devices. Root ports and downstream
> > ports can be "external facing" but are actually internal devices.
> > Anything below an "external facing" device is "external". So the sysfs
> > attribute "external" should be set only for devices that are truly
> > external.
>
> Good point; we (maybe you? :)) should fix that edge case.

Sure, happy to. I will start a fresh conversation about that (in a
separate thread).

>
> > Just a suggestion: Do you think an enum attribute may be better
> > instead, whose values could be "internal" / "external" /
> > "external-facing" in case need arises later to distinguish between
> > them?
>
> I don't see the need for an enum yet.  Maybe we should add that
> if/when we do need it?

Sure, no problems. (I just wanted to slip the thought into the
conversation as UAPI is hard to change later).

>
> > >   - Early userspace code prevents modular drivers from automatically
> > >     binding to PCI devices:
> > >
> > >       echo 0 > /sys/bus/pci/drivers_autoprobe
> >
> > Yes.
> > I believe this setting will apply it equally to both modular and
> > statically linked drivers?
>
> Yes.  The test is in bus_probe_device(), and it does the same for both
> modular and statically linked drivers.
>
> But for statically linked drivers, it only prevents them from binding
> to *hot-added* devices.  They will claim devices present at boot even
> before userspace code can run.

Yes, understood.

>
> > The one thing that still needs more thought is how about the
> > "pcieport" driver that enumerates the PCI bridges. I'm unsure if it
> > needs to be whitelisted for further enumeration downstream. What do
> > you think?
>
> The pcieport driver is required for AER, PCIe native hotplug, PME,
> etc., and it cannot be a module, so the whitelist wouldn't apply to
> it.

Not that I see the need, but slight clarification needed just to make
sure I understand it clearly:

Since pcieport driver is statically compiled in, AER, pciehp, PME, DPC
etc will always be enabled for devices plugged in during boot. But I
can still choose to choose to allow or deny for devices added *after
boot* using the whitelist, right?

Also, denying pcieport driver for hot-added PCIe bridges only disables
these pcieport services on those bridges, but device enumeration
further downstream those bridges is not an issue?

> I assume you need hotplug support, so you would have pcieport
> enabled and built in statically.
>
> If you're using ACPI hotplug, that doesn't require pcieport.

Thank you, this was indeed a long and useful thread :-)

Best Regards,

Rajat