2 ms·
These instructions only work if the CPU is in a particular, unusual state ("Red Unlocked"); even then, they require a value to be written to a MSR, which is a p
by duskwuff 5y ago
These instructions only work if the CPU is in a particular, unusual state ("Red Unlocked"); even then, they require a value to be written to a MSR, which is a privileged operation. It's highly unlikely that there is any way to activate this functionality from userspace. Indeed, it's not clear that it's possible to activate without physical intervention (e.g. JTAG access).
- ipython 5y agoBut in the paper they explicitly test the possibility that the undocumented instructions could be speculatively executed and have visible side effects. They found that the instructions are actually executed, but they couldn’t jump to arbitrary microcode (a side effect of one of the undocumented instructions) because of a single lfence ucode op. They proved that at least on the atom processor they could unlock, the speculative execution of the undocumented instruction calculated an offset and placed it in internal registers even though the instruction was executed from the “green” (normal) operating mode.