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I think that's 'unfortunately', and I think that such updates should require the movement of a switch or jumper to be activated. Obviously the 'hands off' and 'lights out' trends in the datacenter make such a requirement a non-starter but from a security perspective it would make good sense.

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I think it's good. There's not much space in the microcode patch RAM, so hiding a malicious update like you would a root kit would be extremely difficult.

A stealth privilege escalation would be far more likely. Find a redundant, or unused bit, and...

It's too late to edit, but that's exactly what I would go for.

"There's probably not enough space for a meaningful exploit" is never a strategy that pans out.

Hardcoded signature validation can go a long way, but of course it really needs to be bulletproof, and also take e.g. downgrade protection into consideration.

An added physical switch, while more secure, is (as the previous commenter mentioned) so impractical, that it will more likely lead to less updating of vulnerable microcode.

One reason they do this is so bad fw can't brick an expensive CPU

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