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Share on Reddit The Meltdown and Spectre attacks that use processor speculative execution to leak sensitive information have resulted in a wide range of software changes to try to limit the scope for harm. Many of these are operating system-level fixessome of which depend on processor microcode updates.

But Spectre isn't a simple attack to solve; operating system changes help a great deal, but application-level changes are also needed. Apple has talked about some of the updates it has made to the WebKit rendering engine, used in its Safari browserbut this is only a single application.

Microsoft is offering a compiler-level change for Spectre. The "Spectre" label actually covers two different attacks. The one that Microsoft's compiler is addressing, known as "variant 1," concerns checking the size of an array: Other languages, like JavaScript and Java, perform them automatically.

Either way, the test has to be done; attempts to access array members that don't exist are a whole class of bugs all on their own. Speculative information leaks The Spectre problem is that the processor doesn't always wait to see if the Nth element exists before it tries to access it.

It can speculatively try to access the Nth element while waiting for the check to finish. This access is still "safe," insofar as it doesn't introduce any programming bugs.

But as security researchers found, it can leak information. The processor will try to load the Nth element regardless of what it is, or if it even existsand this can change the data stored in the processor's cache.

The change can be detected and can be used to leak secret information. This kind of speculative execution is important in modern processors.


Current Intel chips can run just shy of instructions speculatively. They'll essentially guess how comparisons will be evaluated and what the path through the code will be, rolling back everything that they guessed if those guesses turn out to be wrong.

This is all meant to be transparent to running programs; the security issue is because it isn't, thanks to those cache changes. The generally agreed-on fix for this problem is to make the processor wait: The difficulty with this—and the reason that Microsoft is investigating a compiler-level change—is that identifying exactly which accesses are risky and ensuring that they're fixed requires careful line-by-line inspection of program source code.

The goal of Microsoft's change is to avoid that and insert instructions to make the processor stop speculating in the right places automatically. As a confounding issue, there aren't really any truly good ways of making the processor stop speculating and wait.

Because the speculative execution is designed to be transparent, a hidden implementation detail of the way the processor works, processors don't give a lot of control over how it works. There's no explicit instruction—for now—that tells the processor "don't speculate beyond this instruction" without having any other effects.

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What we do have, however, are instructions that just happen to act as a speculation block. The oldest and most widely used of these is an instruction called cpuid.

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The processor contains various tables of information describing, for example, which extensions it supports things like SSE, AVX, bit, and so on or what its cache topology looks like, and cpuid is used to read those tables.HP SmartFriend Solve My Problem.

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