So that's the why - breaking the trust barrier results in revocation. The how is a little more difficult. There's two ways you can revoke binaries. The first is to add an update of the specific SHA256. That makes sense in many cases, but probably isn't the best choice here - any older kernels are presumably also compromisable. So instead you just revoke the individual signing key and sign your kernels with a new one. How will that scale? Great question. We'll find out.
And yes obviously there's the risk of implementation flaws. The cryptographic model in use is believed to be sound, and we assume that Microsoft have learned from their mistakes with the Terminal Server key. Is that a guarantee? No. But then SSL isn't guaranteed to be safe either, and people still rely on that. Proving security is hard.
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