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The author states:

>'The processor nominally maintained four separate stacks (one for each privilege level), plus a possible “shadow stack” for the operating system or hypervisor.'

Can someone elaborate on what the "shadow stack" is and what it's for exactly? This is the first time I've heard this nomenclature.

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That is probably referring to System Management Mode (privilege level/ring -2) and Intel Management Engine (privilege level/ring -3). Two very terrible things.

More likely to be https://en.wikipedia.org/wiki/Shadow_stack.

Probably not. Shadow stacks don't really have anything to do with privilege levels or interrupts. And Intel only started shipping CPUs with "Control-flow Enforcement Technology" (which has their implementation of shadow stacks, and other things) last year.

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