On Thu, Sep 30, 2021 at 6:41 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Sep 30, 2021 at 01:52:59PM -0700, Dan Williams wrote: > > On Thu, Sep 30, 2021 at 1:44 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Thu, Sep 30, 2021 at 12:23:36PM -0700, Andi Kleen wrote: > > > > > > > > > I don't think the current mitigations under discussion here are about > > > > > keeping the system working. In fact most encrypted VM configs tend to > > > > > stop booting as a preferred way to handle security issues. > > > > > > > > Maybe we should avoid the "trusted" term here. We're only really using it > > > > because USB is using it and we're now using a common framework like Greg > > > > requested. But I don't think it's the right way to think about it. > > > > > > > > We usually call the drivers "hardened". The requirement for a hardened > > > > driver is that all interactions through MMIO/port/config space IO/MSRs are > > > > sanitized and do not cause memory safety issues or other information leaks. > > > > Other than that there is no requirement on the functionality. In particular > > > > DOS is ok since a malicious hypervisor can decide to not run the guest at > > > > any time anyways. > > > > > > > > Someone loading an malicious driver inside the guest would be out of scope. > > > > If an attacker can do that inside the guest you already violated the > > > > security mechanisms and there are likely easier ways to take over the guest > > > > or leak data. > > > > > > > > The goal of the device filter mechanism is to prevent loading unhardened > > > > drivers that could be exploited without them being themselves malicious. > > > > > > If all you want to do is prevent someone from loading a bunch of > > > drivers that you have identified as unhardened, why not just use a > > > modprobe blacklist? Am I missing something? > > > > modules != drivers (i.e. multi-driver modules are a thing) and builtin > > modules do not adhere to modprobe policy. > > > > There is also a desire to be able to support a single kernel image > > across hosts and guests. So, if you were going to say, "just compile > > all unnecessary drivers as modules" that defeats the common kernel > > image goal. For confidential computing the expectation is that the > > necessary device set is small. As you can see in the patches in this > > case it's just a few lines of PCI ids and a hack to the virtio bus to > > achieve the goal of disabling all extraneous devices by default. > > > > If your goal is to prevent some unwanted _drivers_ from operating -- > or all but a few desired drivers, as the case may be -- why extend > the "authorized" API to all _devices_? Why not instead develop a > separate API (but of similar form) for drivers? > > Wouldn't that make more sense? It corresponds a lot more directly > with what you say you want to accomplish. This was v1. v1 was NAKd [1] [2]: [1]: https://lore.kernel.org/all/YQwpa+LAYt7YZ5dh@xxxxxxxxx/ [2]: https://lore.kernel.org/all/YQzDqm6FOezM6Rnu@xxxxxxxxx/ > What would you do in the theoretical case where two separate drivers > can manage the same device, but one of them is desired (or hardened) > and the other isn't? Allow for user override, just like we do today for new_id, remove_id, bind, and unbind when default driver policy is insufficient. echo 1 > /sys/bus/$bus/devices/$device/authorized echo $device > /sys/bus/$bus/drivers/$desired_driver/bind The device filter is really only necessary to bootstrap until you can run override policy scripts. The driver firewall approach was overkill in that regard.
