Quick Overview

We show that the measurement used in AMD SEV’s attestation is block permutation-agnostic, meaning that changing the order of measured memory blocks does not affect the attestation outcome, and thus allows the attacker to modify the execution flow without detection by the VM’s owner. Using this, we construct an universal attack primitive, which reorders the measured blocks of an initially loaded UEFI and sets up a ROP chain to load and execute arbitrary code, giving the attacker full control over the VM’s operating system.

Questions & Answers

What is AMD SEV and how does it work?

AMD SEV and its incremental extensions SEV-ES and SEV-SNP aim to protect virtual machines from a malicious hypervisor by encrypting each VM’s RAM with a unique key. The keys are managed by the so-called Secure Processor and are inaccessible to the hypervisor. To bootstrap trust, the initial code image (usually an UEFI binary) of the VM is cryptographically attested by the Secure processor.

Which platforms are affected?

Our attack affects all currently available AMD EPYC server processors, from the Naples (Zen 1) to the Milan microarchitecture (Zen 3) as well as the EPYC embedded series.

Are there countermeasures?

Right now, there are no firmware patches for Zen 1 and Zen 2 available, leaving these systems vulnerable. For Zen 3, the SEV-SNP extension reliably prevents our attack.

Responsible Disclosure

We’ve responsibly disclosed our findings to AMD via email on January 19th, 2021. AMD has issued CVE-2021-26311 and stated that the problem is resolved with SEV-SNP. As this is only available on 3rd gen EPYC CPUs, previous generations, limited to SEV-ES, will remain vulnerable. AMD Security Bulletin

Videos

Cite

@article{undeservedTrust,
  author    = {Luca Wilke and
               Jan Wichelmann and
               Florian Sieck and
               Thomas Eisenbarth},
  title     = {undeSErVed trust: Exploiting Permutation-Agnostic Remote Attestation},
  year      = {2021},
  url       = {https://uzl-its.github.io/undeserved-trust/},
}

Will be updated once the WOOT’21 proceedings are available.