On 8/4/2021 2:28 PM, Dan Williams wrote
The "hardware" in this case is virtual devices presented by the VMM to the VM. So if a driver misbehaves in a useful way for an attacker to exploit, they can stimulate that behavior with a custom crafted virtual device, and that driver will autoload unaware of the threat without this filter for vetted drivers.
Another way to see it is: the confidential guest is protected against the host, except for the places where it chooses to communicate with the host through MMIOs, port IOs, some (not all) MSRs. It's somewhat analogous to a network server in a hostile network which can be attacked through network packets. We typically use a firewall to limit the network exposure only to especially hardened network services. Each low level MMIO etc. is like a network access communicating with a hostile network. The device filter is the firewall for these vulnerable low level interactions. It reduces the hardening problem from being completely infeasible to tractable.
-Andi
