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AMD reveals Specter-like vulnerabilities in Zen3 CPUs




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AMD has published details of Specter-like vulnerabilities affecting Zen 3 CPUs. This is related to a new feature introduced by AMD in a modern architecture called Predictive Store Forwarding (PSF). AMD is unaware of the code that is actually exploiting this issue, but has released this information preemptively.

PSF is used to guess what the result of a load will be and execute instructions based on that assumption. PSF is based on a previous performance improvement called Store to Load Forwarding (STLF). STLF refers to a method of transferring data directly from a store to a load without first writing it to main memory. Before the STLF completes, the CPU verifies that the load and store addresses match.

PSF is built on STLF by guessing the load-store relationship without waiting for the address check to complete. PSF monitors execution patterns over time to learn possible outcomes. When this is done, it may speculatively perform STLF before confirming that STLF has occurred. Whenever the CPU talks about performing an operation without asking if the result of the operation is needed, it refers to a performance-enhancing technique called speculative execution.

All modern CPUs from all vendors execute instructions somewhat speculatively. In 2018, Intel was caught in a major PR trouble due to a series of security weaknesses called Specter and Meltdown. Specter and Meltdown have spawned side-channel attacks of all genres, but most of these attacks only applied to Intel. This is the first side-channel attack of this type that hit AMD.

According to AMD, false PSF predictions can occur “at least” for two reasons:

1). The store / load initially had a dependency, but either the store address or the load address has changed, so the dependency is gone.

2). There is an alias in the PSF prediction structure. The PSF predictor is supposed to track load / store pairs based on some of the relative instruction pointers. AMD writes: “A store / load pair that has a dependency may create an alias with another store / load pair that doesn’t.”

Comparison of Meltdown and Specter. The chart that started it all.

AMD’s security briefing states that the company has proposed a security patch for the Linux kernel, allowing customers to enable or disable speculative features and allow PSFs to leak data through side-channel attacks. AMD recommends leaving this feature enabled for performance benefits, and states that the risk of attack is considered “probably low.”

Side-channel attacks have not emerged as a serious threat

When Specter and Meltdown came out three years ago, it wasn’t clear how much they would be a problem in the long run. As far as we know, there are no public attacks that have attempted to steal data using these methods. Side-channel attacks are difficult and do not automatically leak the data that the attacker actually needs. It’s a problem of its own.

About a year ago, I realized that security disclosures about CPU flaws (mostly Intel-related, if not perfect) are becoming more and more historic. In many cases, the security PR / website tone and the actual report copy tone were not related to each other. For the same reasons that Intel is important to disclose, it is important for AMD to disclose these findings, but either Specter, Meltdown, Zombieload, Fallout, MDS, RIDL, or any other is used. There is no evidence that it is. In the real world.

This may change in the future, but the current risk of side-channel execution attacks on x86 or ARM chips is very low. It is much more likely to be the target of spear phishing emails than it is to encounter security flaws from side-channel attacks.

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