No, not in theory, not in practice. But yeah, restricting an attacker's
ability to hack hardware (by controlling physical access) does take out a
whole class of attack vectors.
But, seriously, such discussion has the tendency to quickly get toooo
theoretical (thus losing practical significance). For example, would we
not also need to prevent the (userspace) superuser from being able to run
arbitrary executables that can modify firmware? Okay, let's say we have
a kernelspace infrastructure of verifying cryptographic signatures on
binaries before executing them ... but how practical/usable is this?
How practically/universally applicable is a system whose security derives
from keeping machines behind locked doors and protected by incorruptible,
armed guard?
Overall, I tend to be unenthusiastic about most schemes that claim to
have solved the user-kernel security problem (with no loss of usability/
practicality).
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