Comment by ProblemFactory | Hacker News Reader

Comment by ProblemFactory | Hacker News Reader

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> how that ever got approved if the security model was clearly leaking

When you consider a theoretical model of the CPU, then it's not leaking - the speculative execution, cache, other parts of the CPU are designed carefully so that no data can "escape" and be read by processes that don't have the permission to do it. Speculated execution can happen, but before any results from that are released, the permissions are checked, and if they fail, the results are discarded.

What people did not consider is the timing attacks that do leak information. It's only "clear" to us now after the attack has been demonstrated, even if it has been present in CPU design for the last 20 years.

There are probably many more of these side channel data extraction paths possible. For example, in the recent years attacks on cryptographic algorithms have looked at similar timing measurements, and in some cases power consumption measurements.

Isn't the knowledge of potential for timing attacks using cache/memory fairly old? I am pretty sure I heard of the concept long ago.

Very old in the arena of cryptography.

No one in the CPU Architecture Design arena put 2 & 2 together [1] to realize that the same side channel that was devastating for cryptography work would also be quite devastating for bypassing memory permission protections in the CPU's they were designing.

[1] probably because the intersection of "cryptographers who can mount timing side channel attacks" and "CPU Architecture designers" is very close to zero.

I may be wrong, but the timing attack only reveals the cache line, but being able to read the cache line is something itself I would expect to fail even without the timing attack. How does that not cause an exception?

My understanding is that you never directly read memory to which you shouldn't have access. You arrange for the processor to read a specific address you control based on the value of the illegal read. By timing subsequent accesses to that user address, you can infer whether or not the processor brought it into the cache, and thus infer the value of the illegal read.

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