Meltdown-Spectre flaws: We've found new attack variants, say researchers

Intel and AMD may need to revisit their microcode fixes for Meltdown and Spectre.

Video: Fake Meltdown-Spectre patch emails hiding Smoke Loader malware

Researchers have developed a tool to uncover new ways of attacking the Meltdown and Spectre CPU side-channel flaws, which may force chipmakers like Intel to re-examine already difficult hardware mitigations.

The tool allowed the researchers to synthesize a software-attack based on a description of a CPU's microarchitecture and an execution pattern that could be attacked.

Though the software attack is specific to a microarchitecture and represent exploits "in their most abstracted form", they can be used to develop fully fledged attacks.

The approach is described in a new paper from Caroline Trippel and Daniel Lustig of Princeton University and Margaret Martonosi from Nvidia called: MeltdownPrime and SpectrePrime: Automatically synthesized attacks exploiting invalidation-based coherence protocols.

The researchers were able to create new variants of Meltdown and Spectre with a separate class of cache timing side-channel attack known as Prime+Probe, described in 2015 by several of the researchers who found Meltdown and Spectre independently of Google's Project Zero.

The result is MeltdownPrime and SpectrePrime, which can leak the same type of information with the same level of precision as Meltdown and Spectre. The Prime variants rely on "invalid-based coherence protocols".

Meltdown attacks allow malware to access a system's memory and its secrets, while Spectre can leak secrets by breaking memory isolation between applications.

As The Register reports, a key difference between Meltdown and Spectre and their Prime variants are that the Primes attack the host by using two-cores against each other and a CPU's memory caches to discover privileged information about an application as it executes.

The other type of cache timing side-channel attack the paper explores is Flush+Reload, a technique the researchers who found Spectre used as a side channel in combination with speculative execution.

"In the context of Spectre and Meltdown, leveraging coherence invalidations enables a Prime+Probe attack to achieve the same level of precision as a Flush+Reload attack and leak the same type of information.

"By exploiting cache invalidations, MeltdownPrime and SpectrePrime -- two variants of Meltdown and Spectre, respectively -- can leak victim memory at the same granularity as Meltdown and Spectre while using a Prime+Probe timing side-channel."

The researchers developed proof-of-concept malware for SpectrePrime and ran it on a MacBook with an Intel Core i7 Processor running a version of macOS Sierra that hadn't received Apple's Meltdown and Spectre patches.

"Averaged over 100 runs, we observed SpectrePrime to achieve the same average accuracy as Spectre on the same hardware -- 97.9 percent for Spectre and 99.95 percent for SpectrePrime," they write.

The mitigations for Meltdown and Spectre have involved a combination of software fixes, such as Microsoft and Linux versions of 'kernel page table isolation', and hardware fixes such as Intel's microcode updates. Both can cause performance overheads.

But while existing software mitigations will probably suffice for these new variants of Meltdown and Spectre, chipmakers like Intel and AMD are likely to need to develop different hardware mitigations, according to the researchers.

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