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Comment by ams6110 | Hacker News Reader

Conventional thinking is that if a malicious user can run code on your system, you've lost. In that sense, this is just another local exploit, of which there are many.

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This includes fully sandboxed code, however, which is an important distinction. The fact that a driveby javascript exploit could potentially result in leaking every important secret from your system (logins, encryption keys, what-have-you) is why people are taking this so seriously.

Algorithmically is it possible to pass only 'safe' interpreted code to the CPU? The majority of Javacript engines (Chakra, V8, Spidemonkey and JavasciptCore) are under a security-auditable FOSS license. Browsers claiming Spectre/Meltdown immunity would be in the next release notes, surely.

I'm not discounting that CPUs should operate correctly into the future but the current sandboxes would seem to rely on whims of a CPU's microcode. And the question for browser makers is whether to trust them.

>Algorithmically is it possible to pass only 'safe' interpreted code to the CPU?

in other words, the halting problem?

It's not the halting problem because you can have false positives. If the code can't be proven safe it is rejected.

Its not about algorithms. The issue is mentioned in the paper: even if the cache-based side channels are stopped (lets say, by manually clearing any cached memory as soon as its accessed, reducing performance to 0.1% of what it is now) there are other potential side channels like through limited CPU resources. A lot of the "fixes" might just be mitigations, that don't solve the problem entirely, but make it take much longer. Maybe you can cover up enough holes, and make the timers available poor enough that it takes a few hours or days to read a single byte. In other words, mitigations to make things less simple.

I assume exploitability through javascript depends very strongly on how the browser treats it. If it's jit-ed to optimal machine code it should be easy. If it's interpreted it should be harder.

So question is, can the Javascript engine generate suboptimal but safe machine code?

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