Spectre, Meltdown: Critical CPU Security Flaws Explained

Over the past few days we’ve covered major new security risks that struck at a number of modern microprocessors from Intel and to a much lesser extent, ARM and AMD. Information on the attacks and their workarounds initially leaked out slowly, but Google has pushed up its timeline for disclosing the problems and some vendors, like AMD, have issued their own statements. The two flaws in question are known as Spectre and Meltdown, and they both relate to one of the core capabilities of modern CPUs, known as speculative execution.

Speculative execution is a performance-enhancing technique virtually all modern CPUs include to one degree or another. One way to increase CPU performance is to allow the core to perform calculations it may need in the future. The different between speculative execution and “execution” is that the CPU performs these calculations before it knows whether it’ll actually be able to use the results.

Here’s how Google’s Project Zero summarizes the problem: “We have discovered that CPU data cache timing can be abused to efficiently leak information out of mis-speculated execution, leading to (at worst) arbitrary virtual memory read vulnerabilities across local security boundaries in various contexts.”

Meltdown is Variant 3 in ARM, AMD, and Google parlance. Spectre accounts for Variant 1 and Variant 2.


“On affected systems, Meltdown enables an adversary to read memory of other processes or virtual machines in the cloud without any permissions or privileges, affecting millions of customers and virtually every user of a personal computer.”

Intel is badly hit by Meltdown because its speculative execution methods are fairly aggressive. Specifically, Intel CPUs are allowed to access kernel memory when performing speculative execution, even when the application in question is running in user memory space. The CPU does check to see if an invalid memory access occurs, but it performs the check after speculative execution, not before. Architecturally, these invalid branches never execute — they’re blocked — but it’s possible to read data from affected cache blocks even so.

ARM defines its own 3a variant of meltdown, but doesn’t believe a software fix is required. Only the Cortex-A75 is affected by Meltdown (Variant 3).

The various OS-level fixes going into macOS, Windows, and Linux all concern Meltdown. The formal PDF on Meltdown notes that the software patches Google, Apple, and Microsoft are working on are a good start, but that the problem can’t be completely fixed in software. AMD and ARM appear largely immune to Meltdown, though ARM’s upcoming Cortex-A75 is apparently impacted.


Meltdown is bad, but Meltdown can at least be ameliorated in software (with updates), even if there’s an associated performance penalty. Spectre is the name given to a set of attacks that “involve inducing a victim to speculatively perform operations that would not occur during correct program execution, and which leak the victim’s confidential information via a side channel to the adversary.”

Unlike Meltdown, which impacts mostly Intel CPUs, Spectre’s proof of concept works against everyone, including ARM and AMD. Its attacks are pulled off differently — one variant targets branch prediction — and it’s not clear there are hardware solutions to this class of problems, for anyone.

What Happens Next

Intel, AMD, and ARM aren’t going to stop using speculative execution in their processors; it’s been key to some of the largest performance improvements we’ve seen in semiconductor history. But as Google’s extensive documentation makes clear, these proof-of-concept attacks are serious. Neither Spectre nor Meltdown relies on any kind of software bug to work. Meltdown can be solved through hardware design and software rearchitecting; Spectre may not.

When reached for comment on the matter, Linux creator Linux Torvalds responded with the tact that’s made him legendary. “I think somebody inside of Intel needs to really take a long hard look at their CPU’s, and actually admit that they have issues instead of writing PR blurbs that say that everything works as designed,” Torvalds writes. “And that really means that all these mitigation patches should be written with ‘not all CPU’s are crap’ in mind. Or is Intel basically saying ‘We are committed to selling you shit forever and ever, and never fixing anything? Because if that’s the case, maybe we should start looking towards the ARM64 people more.”

It does appear, as of this writing, that Intel is disproportionately exposed on these security flaws. While Spectre-style attacks can affect all CPUs, Meltdown is pretty Intel-specific. Thus far, user applications and games don’t seem much impacted, but web servers and potentially other workloads that access kernel memory frequently could run markedly slower once patched.

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