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In retrospect, yes this is of course a big problem. But regarding how it could ever be approved... Until this news broke, a CPU designer would tell you that s

In retrospect, yes this is of course a big problem. But regarding how it could ever be approved... Until this news broke, a CPU designer would tell you that speculative execution is a well-understood, proven approach to gaining a lot of performance. Branch predictors are really good at figuring out where the next instruction is going to come from, and are a really important tool for avoiding stalling out the whole machine while you wait for the next instruction to come back from memory. And intuitively, it seems really "safe." All you're doing is having the CPU get ready to perform "future" calculations more quickly. It gets to guess where the program is going to go, and start fetching resources that it thinks will be needed. As long as nothing architecturally visible is changed by these preparations, everything is functionally the same, so what could go wrong? And intuitively, you wouldn't think that something like a cache, which the program has no way to access directly, should be architecturally visible. Even putting on a security-minded hat, you would think that it doesn't matter what's in the cache, because if a program tries to access kernel memory, the access still has to undergo a permissions check. The attack is pretty damn clever. And disheartening.

stordoff 9y ago
More or less the conclusions I came to during university. Speculative execution felt dicey (because you're doing things before you know if they are the right things), so I spent some time thinking about it and realised that you are (basically) throwing away or rolling back the erroneous execution. At worst I thought it would be wasted executions if, e.g, your branch predictor is poor - I never even thought about timing or indirect attacks. It's a pretty subtle thing IMO. IIRC, one of my professors at one point explained modern CPU behaviour (branch prediction/OOOE etc.) as, roughly, the CPU can do whatever it likes because the program never sees what happened under the hood - it just has to order the result/make sure the result is correct.

stordoff 9y ago

More or less the conclusions I came to during university. Speculative execution felt dicey (because you're doing things before you know if they are the right things), so I spent some time thinking about it and realised that you are (basically) throwing away or rolling back the erroneous execution. At worst I thought it would be wasted executions if, e.g, your branch predictor is poor - I never even thought about timing or indirect attacks. It's a pretty subtle thing IMO. IIRC, one of my professors at one point explained modern CPU behaviour (branch prediction/OOOE etc.) as, roughly, the CPU can do whatever it likes because the program never sees what happened under the hood - it just has to order the result/make sure the result is correct.