The new Downfall vulnerability is a big one. This impacts Intel’s desktop CPUs up to Alder Lake and Intel’s server CPUs up to Ice Lake, the generation that was still top-of-the-line on the first day of 2023. Found by Daniel Moghimi, now at Google, the vulnerability targets AVX2 and AVX-512 pipelines in what Intel is calling a Gather Data Sampling (GDS) attack.
Intel Downfall is a Big Deal
The new transient vector speculative execution vulnerability targets vector processing pipelines in AVX2 and AVX-512 paths during a gather data process. There are a few caveats. First, like many side-channel attacks, this is something that is easiest to reproduce in a lab. Also, an attacker cannot specifically target any data sitting in memory. Instead, they are limited to what happens to be available in their context.
Intel and Daniel have been working for some time on this before its disclosure today. Intel and its firmware and OS partners have been coordinating a microcode update that will mitigate the vulnerability. This is an important enough vulnerability that the default will be “ON” for the mitigation. Administrators will be able to turn the mitigations off if they wish.
Here is Intel’s statement to STH on this:
“The security researcher, working within the controlled conditions of a research environment, demonstrated the GDS issue which relies on software using Gather instructions. While this attack would be very complex to pull off outside of such controlled conditions, affected platforms have an available mitigation via a microcode update. Recent Intel processors, including Alder Lake, Raptor Lake and Sapphire Rapids, are not affected. Many customers, after reviewing Intel’s risk assessment guidance, may determine to disable the mitigation via switches made available through Windows and Linux operating systems as well as VMMs. In public cloud environments, customers should check with their provider on the feasibility of these switches.” (Source: Intel)
From what we have heard, the mitigations for the vulnerability have some significant impact on AVX2/ AVX-512 workloads. Intel sent us this statement on the potential performance impacts:
For most workloads, Intel has not observed reduced performance due to this mitigation. However, certain vectorization-heavy workloads may see some impact. Intel encourages customers to review our technical documentation to understand options available to reduce or eliminate any performance impact. (Source: Intel)
It is interesting that Intel uses features like AVX-512 as a key competitive differentiator, but says here that “most workloads” do not heavily use the feature and thus do not suffer performance loss. AVX2 and AVX-512 are most common in high-performance computing (HPC) computing environments. Intel told us the vulnerability is only present when another user is on the same core at the target process. In theory, many HPC workloads dedicate cores to users so this would not be an issue and the mitigation can be turned off to increase performance to pre-mitigation levels.
For STH readers, if you have AVX-512 or AVX2 heavy pipelines, then we would advise looking at the pre/post mitigation performance and doing a risk assessment based on the likelihood that an attack can occur in your environment.
One area where we would be particularly concerned is at the edge. Intel’s last two edge P-core architectures the Skylake-D and Ice Lake-D are both impacted by this. Intel has been pushing AVX-512 as a workload accelerator at the edge without needing a PCIe accelerator. Hopefully, our readers who work in that space take some time to evaluate the impacts of the vulnerability and mitigation. Unlike HPC clusters that typically have local management teams and have an upgrade path to Sapphire Rapids, Xeon Max, or AMD’s Genoa/ Genoa-X, there is no replacement architecture from either Intel or AMD for the edge. Even if there was a replacement architecture, edge deployments can be very costly to upgrade as we saw with the Intel Atom C2000 AVR54 bug.
Which CPUs are Impacted by Downfall?
Intel told us that the following CPU architectures are impacted:
- Skylake family (Skylake, Cascade Lake, Cooper Lake, Amber Lake, Kaby Lake, Coffee Lake, Whiskey Lake, Comet Lake)
- Tiger Lake family
- Ice Lake family (Ice Lake, Rocket Lake)
Those are both desktop and server parts. Intel also said that the latest generation Sapphire Rapids chips are not impacted by the vulnerability.
To the best of our knowledge, AMD CPUs are not impacted by this. Still, a large portion of STH readers will need to evaluate the vulnerability and the impacts of the mitigation for this one. We do not cover every security vulnerability, but this one seems like it is going to be a big deal for a large portion of our readers.
You can learn more about this under INTEL-SA-00828.