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

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

On Tue, Jun 09, 2020 at 04:04:00PM -0500, Bjorn Helgaas 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.
> 
>     This comes from either the DT "external-facing" property or the
>     ACPI "ExternalFacingPort" property.  

Good idea, but as people have pointed out, even these don't always work
so userspace will need to be able to override that somehow :(

>   - All devices present at boot are enumerated.  Any statically built
>     drivers will bind to them before any userspace code runs.
> 
>     If you want to keep statically built drivers from binding, you'd
>     need to invent some mechanism so pci_driver_init() could clear
>     drivers_autoprobe after registering pci_bus_type.
> 
>   - Early userspace code prevents modular drivers from automatically
>     binding to PCI devices:
> 
>       echo 0 > /sys/bus/pci/drivers_autoprobe
> 
>     This prevents modular drivers from binding to all devices, whether
>     present at boot or hot-added.
> 
>   - Userspace code uses the sysfs "bind" file to control which drivers
>     are loaded and can bind to each device, e.g.,
> 
>       echo 0000:02:00.0 > /sys/bus/pci/drivers/nvme/bind

Seems good to me, and also matches what the current USB tools do for
this type of thing.

thanks,

greg k-h