Spectre/Meltdown nominated for most over hyped bug at Pwnies
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The chip vendors are adding speculation barriers, but the big problem is adding the required compiler analysis to figure out where to insert them. Requiring users to strategically sprinkle __builtin_speculation_safe_value (untrusted) around code is not an ideal approach.
That's not a fix. That's a pathetic tool for minimal workarounds on a case-by-case basis. A fix is an MSR to disable speculative execution across branches or entirely.
That’s what is being proposed. BTW Disabling all speculation is a big sledgehammer when it is possible to design CPUs that speculate without visible side effects.
I believe it's plausible to design a CPU that speculates without visible side effects, but it sounds difficult. I don't believe it's possible to add that via microcode. An MSR to disable all speculation should be possible as a microcode update.
and I believe even that approach needs microcode update; over CPU vendors are proposing other mechanisms to introduce these “speculation barriers”. If it were easy it would be easily fixed. There needs to be fundamental design changes for side-effect free speculation.
Fundamental design changes are needed to make it fast, specifically fast enough to be acceptable to most users. Just fixing it is trivial: nuke speculation.
Nuking speculation is only trivial if that is already “designed in”. Perhaps okay for new processors. The “speculation barrier” is what is on the table for current designs that may be able to be introduced by micro-code.
Nuking (all, not just BTB) branch prediction should also work (aside from meltdown I'm not aware of other ways speculation could be exploited, but not 100% sure) and seems likely to be possible with microcode.
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