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Downfall and Zenbleed: Googlers helping secure the ecosystem

Finding and mitigating security vulnerabilities is critical to keeping Internet users safe.  However, the more complex a system becomes, the harder it is to secure—and that is also the case with computing hardware and processors, which have developed highly advanced capabilities over the years. This post will detail this trend by exploring Downfall and Zenbleed, two new security vulnerabilities (one of which was disclosed today) that prior to mitigation had the potential to affect billions of personal and cloud computers, signifying the importance of vulnerability research and cross-industry collaboration. Had these vulnerabilities not been discovered by Google researchers, and instead by adversaries, they would have enabled attackers to compromise Internet users. For both vulnerabilities, Google worked closely with our partners in the industry to develop fixes, deploy mitigations and gather details to share widely and better secure the ecosystem.

What are Downfall and Zenbleed?

Downfall (CVE-2022-40982) and Zenbleed (CVE-2023-20593) are two different vulnerabilities affecting CPUs – Intel Core (6th – 11th generation) and AMD Zen2, respectively. They allow an attacker to violate the software-hardware boundary established in modern processors. This could allow an attacker to access data in internal hardware registers that hold information belonging to other users of the system (both across different virtual machines and different processes). 

These vulnerabilities arise from complex optimizations in modern CPUs that speed up applications: 

  1. Preemptive multitasking and simultaneous multithreading enable users and applications to share CPU cores, while the CPU enforces security boundaries at the architecture level to stop a malicious user accessing data from other users. 

  2. Speculative execution allows the CPU core to execute instructions from a single execution thread without waiting for prior instructions to be completed.

  3. SIMD enables data-level parallelism where an instruction computes the same function multiple times with different data.

Downfall, affecting Intel CPUs, exploits the speculative forwarding of data from the SIMD Gather instruction. The Gather instruction helps the software access scattered data in memory quickly, which is crucial for high-performance computing workloads performing data encoding and processing. Downfall shows that this instruction forwards stale data from the internal physical hardware registers to succeeding instructions. Although this data is not directly exposed to software registers, it can trivially be extracted via similar exploitation techniques as Meltdown. Since these physical hardware register files are shared across multiple users sharing the same CPU core, an attacker can ultimately extract data from other users. 

Zenbleed, affecting AMD CPUs, shows that incorrectly implemented speculative execution of the SIMD Zeroupper instruction leaks stale data from physical hardware registers to software registers. Zeroupper instructions should clear the data in the upper-half of SIMD registers (e.g., 256-bit register YMM) which on Zen2 processors is done by just setting a flag that marks the upper half of the register as zero. However, if on the same cycle as a register to register move the Zeroupper instruction is mis-speculated, the zero flag doesn’t get rolled back properly, leading to the upper-half of the YMM register to hold stale data rather than the value of zero. Similar to Downfall, leaking stale data from physical hardware registers expose the data from other users who share the same CPU core and its internal physical registers. 

Comparison

Downfall

Zenbleed

Affects

Intel Core (6th-11th Gen)

AMD Zen 2

Leaks

Entire XMM/YMM/ZMM Register

Upper-half of 256-bit YMM Registers

Exploit

Gather Data Sampling

Architectural Data Leak

Discovered by

Microarchitectural Analysis

Fuzzing

Fix

Microcode blocking speculative forwarding from Gather

Microcode properly wiping out YMM register when Zeroupper 

Mitigation overhead

0-50% depending on the workload 

Statistically insignificant

Reported on

August 24, 2022

May, 15 2023

Fixed on

August 8, 2023

July 19, 2023

How did we protect our users?

Vulnerability research continues to be at the heart of our security work at Google. We invest in not only vulnerability research, but in the community as a whole in order to encourage further research that keeps all users safe. These vulnerabilities were no exception, and we worked closely with our industry partners to make them aware of the vulnerabilities, coordinate on mitigations, align on disclosure timelines and a plan to get details out to the ecosystem. 

Upon disclosures, we immediately published Security Bulletins for both Downfall and Zenbleed that detailed how Google responded to each vulnerability, and provided guidance for the industry. In addition to our bulletins, we posted technical details for insights on both Downfall and Zenbleed. It’s imperative that vulnerability research continues to be supported by the industry, and we’re dedicated to doing our part to helping protect those that do this important work.

Lessons learned 

These long existing vulnerabilities, their discovery and the mitigations that followed have provided several lessons learned that will help the industry move forward in vulnerability research, including: 

  • There are fundamental challenges in designing secure hardware that requires further research and understanding.

  • There are gaps in automated testing and verification of hardware for vulnerabilities. 

  • Optimization features that are supposed to make computation faster are closely related to security and can introduce new vulnerabilities, if not implemented properly.

As Downfall and Zenbleed, suggest, computer hardware is only becoming more complex everyday, and so we will see more vulnerabilities, which is why Google is investing in CPU/hardware security research. We look forward to continuing to share our insights and encourage the wider industry to join us in helping to expand on this work. 

Want to learn more?

Downfall will be presented at Blackhat USA 2023 on August 9 at 1:30pm. You can also read more about Zenbleed on this advisory.


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Author: Kimberly Samra