Good and interesting paper! Especially last paragraph before conclusion.
“Lastly, a so-called ‘halt and catch fire’ instruction was discovered on an as-yet unnamed x86 processor. This instruction, executed in ring 3 from an unprivileged process, appears to lock the processor entirely. […] At the time of this paper’s publishing, the vendor has not been provided sufficient time to respond to the issue. The details of the instruction and the processors affected will be enumerated when responsible disclosure is complete, and an updated version of this whitepaper will be released. Such instructions pose a critical security risk, as they allow unprivileged users to mount denial of service attacks against shared systems.”
I wonder if any freezes in otherwise reliable systems came from a bitflip of a CPU instruction in memory that turned it into catch fire instruction.