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MrBuddyCasino
tl;dr: x86 processors have an ancient feature (unused by now) that allows to move the memory window of the APIC registers to an arbitrary address. By moving this window, it is possible to circumvent built-in hardware security checks and access the highly privileged System Management Mode (Ring -2) from Ring 0 Code.
Qantourisc
I must be missing something: but why did they keep reimplementing this in modern CPU's ? (Or did they remove it from modern CPU's?)
mschuster91
Intel did remove it in the modern CPUs after the author alerted them.

As for why it stuck around until now: backwards compatibility + that CPUs are rather rarely constructed from scratch, rather evolved upon.

TazeTSchnitzel
> Intel did remove it in the modern CPUs after the author alerted them.

No, they just added security checks.