Hardware Security
What is the principle of the vulnerability called Meltdown, which affected some CPUs?
- AThe CPU melts from heat and data leaks out
- BEven an instruction reading kernel memory without permission speculatively reads the value before the permission exception takes effect, leaving value-dependent traces in the cache
- CA misconfiguration exposing kernel memory to all processes
Show answer and explanation
Answer: B. Even an instruction reading kernel memory without permission speculatively reads the value before the permission exception takes effect, leaving value-dependent traces in the cache
When a user program tries to read kernel memory, it should take a permission exception and get no value. On affected CPUs, however, in the brief window before the exception took effect, later instructions using the loaded value ran speculatively and brought a value-dependent location into the cache. The results were discarded by the exception, but kernel data could be inferred through a cache side channel.
- A. Nothing physically melts.
- B. Correct. The problem was the ordering of permission checks and speculative reads.
- C. It was a CPU implementation issue, not a misconfiguration.