AMD CPUs Improper Microcode Signature Verification Vulnerability Allowing Malicious Microcode Execution

A vulnerability exists in the microcode patch loader of AMD CPUs, specifically in the Zen 1-4 architectures. This issue arises from improper signature verification, allowing an attacker with local administrator privileges to load malicious microcode patches. The vulnerability could compromise the confidentiality and integrity of sensitive workloads running under AMD's Secure Encrypted Virtualization with Secure Nested Paging (SEV-SNP) feature.

Impact

Exploitation of this vulnerability could lead to unauthorized loading of microcode patches, allowing manipulation of CPU behavior. This could disrupt confidential computing tasks, particularly those protected by SEV-SNP, and potentially compromise the integrity of the data processed in these environments.

Reproduction

The vulnerability can be reproduced by crafting a malicious microcode patch that exploits the flawed signature verification process. This requires access to the AES-CMAC key used in the validation, which can be obtained from an affected CPU model. Once the key is extracted, it can be used to create a microcode patch that bypasses the signature checks, effectively allowing the injection of harmful microcode into the CPU.

Remediation

Users should update to the latest microcode version provided by AMD, which addresses the signature verification flaw by implementing a more secure validation process. Additionally, an SEV firmware update may be necessary for some platforms to support SEV-SNP attestation, which could also require a BIOS update.