02-16-2018, 10:23 AM
https://arstechnica.com/gadgets/2018/02/...-to-solve/
Quote:Kocher suggests that Microsoft should offer a more pessimistic mode that protects every conditional access. But this will come with a heavy cost: in sample code he wrote to compute SHA-256 hashes, the version with lfence instructions after every branch had only 40 percent of the performance of the unmodified version. This poses a security-performance trade-off that's decidedly uncomfortable; even if the compiler offered such an option, few people are likely to be willing to accept that kind of performance penalty in general. But for smaller pieces of code that are known to be at risk, such an option may be useful.
Microsoft's much more restricted protection does have the virtue of having much lower impact; the company says that it has built Windows with the Spectre protection and found no real performance regression.
The work done on the compiler and the limitations faced underscore what a complex problem Spectre poses for the computing industry. The processors are working as they're supposed to. We can't do without speculative execution of this kind—we need the performance it offers—but equally, we have no good way of systematically addressing the security concerns it creates. Compiler changes of the kind Microsoft has made are well-meaning, but as Kocher's investigation has shown, they're a long way short of offering a complete solution.