meltdown's Introduction

meltdown

Reimplementing Meltdown based on just the Wikipedia summary.

The CPU encounters an instruction accessing the value, A, at an address forbidden to the process by the virtual memory system and the privilege check. Because of speculative execution, the instruction is scheduled and dispatched to an execution unit. This execution unit then schedules both the privilege check and the memory access.
The CPU encounters an instruction accessing address Base+A, with Base chosen by the attacker. This instruction is also scheduled and dispatched to an execution unit.
The privilege check informs the execution unit that the address of the value, A, involved in the access is forbidden to the process (per the information stored by the virtual memory system), and thus the instruction should fail and subsequent instructions should have no effect. Because these instructions were speculatively executed, however, the data at Base+A may have been cached before the privilege check – and may not have been undone by the execution unit (or any other part of the CPU). If this is indeed the case, the mere act of caching constitutes a leak of information in and of itself. At this point, Meltdown intervenes.
The process executes a timing attack by executing instructions referencing memory operands directly. To be effective, the operands of these instructions must be at addresses which cover the possible address, Base+A, of the rejected instruction's operand. Because the data at the address referred to by the rejected instruction, Base+A, was cached nevertheless, an instruction referencing the same address directly will execute faster. The process can detect this timing difference and determine the address, Base+A, that was calculated for the rejected instruction – and thus determine the value A at the forbidden memory address.

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