“We verified this worked even inside an unprivileged guest VM, so this already has serious security implications for cloud providers.”
A meaningless rex prefix in front of rep movsb puts some Intel processors into a glitch state. Run it on several cores and the machine throws machine check exceptions and halts. One unprivileged guest can take down a shared cloud host. Intel shipped microcode, and the only other fix is disabling fast strings at a big performance cost.