  Since the initial discovery of Meltdown and Spectre in 2017, different
variants of these attacks have been discovered. One often overlooked variant is
Meltdown 3a, also known as Meltdown-CPL-REG. Even though Meltdown-CPL-REG was
initially discovered in 2018, the available information regarding the
vulnerability is still sparse. In this paper, we analyze Meltdown-CPL-REG on 19
different CPUs from different vendors using an automated tool. We observe that
the impact is more diverse than documented and differs from CPU to CPU.
Surprisingly, while the newest Intel CPUs do not seem affected by
Meltdown-CPL-REG, the newest available AMD CPUs (Zen3+) are still affected by
the vulnerability. Furthermore, given our attack primitive CounterLeak, we show
that besides up-to-date patches, Meltdown-CPL-REG can still be exploited as we
reenable performance-counter-based attacks on cryptographic algorithms, break
KASLR, and mount Spectre attacks. Although Meltdown-CPL-REG is not as powerful
as other transient-execution attacks, its attack surface should not be
underestimated.
