[unclepresent]

I doubt it is possible to be done on JavaScript. Timing cache access is a challenging task for such high level language. The key to the attack is to figure out latency of memory access.

A JavaScript app that is dealing with 100 layers of intermediate code before it actually gets to the physical memory could not see a difference between reading from actual memory or from cache. It is too slow to notice any change. Should be a pure assembler code to reliably estimate the effect of caching.

[jnordwick]

If you are already running untrusted binaries, there are bigger issues. Without a JS exploit, I'm not sure this is a big problem.

And we haven't seen a real world binary version either. The versions I've seen all take running starts so to speak.