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 <helg...@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. > 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? > > - 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. > 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. 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.
