CVE-2021-26342 in EPYC
Summary
by MITRE • 05/11/2022
In SEV guest VMs, the CPU may fail to flush the Translation Lookaside Buffer (TLB) following a particular sequence of operations that includes creation of a new virtual machine control block (VMCB). The failure to flush the TLB may cause the microcode to use stale TLB translations which may allow for disclosure of SEV guest memory contents. Users of SEV-ES/SEV-SNP guest VMs are not impacted by this vulnerability.
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Analysis
by VulDB Data Team • 05/13/2022
The vulnerability described in CVE-2021-26342 represents a critical flaw in AMD's Secure Encrypted Virtualization (SEV) technology, specifically affecting SEV guest virtual machines. This issue manifests in the CPU's inability to properly flush the Translation Lookaside Buffer (TLB) when certain operational sequences occur, particularly following the creation of a new virtual machine control block (VMCB). The TLB serves as a crucial component in virtual memory management by caching virtual-to-physical address translations, and its proper operation is fundamental to maintaining memory isolation between virtual machines and the host system. When the TLB fails to flush correctly, it creates a persistent cache of stale translations that can be exploited to access memory contents that should remain isolated.
The technical flaw stems from a specific sequence of operations within the SEV implementation where the microcode does not adequately invalidate TLB entries when transitioning between different VMCB states. This failure creates a potential attack vector where malicious actors could leverage the stale TLB entries to gain unauthorized access to sensitive data within the SEV guest VM. The vulnerability is particularly concerning because it directly impacts the core security promise of SEV technology, which is designed to protect guest memory contents from both the hypervisor and potentially compromised host environments. The flaw operates at the microcode level, making it difficult to detect and address through traditional software patches, requiring careful consideration of the underlying CPU architecture and memory management mechanisms.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally undermines the memory isolation guarantees that SEV is designed to provide. Attackers who can exploit this vulnerability may be able to extract confidential data from other VMs running on the same physical host, potentially accessing encryption keys, sensitive application data, or system credentials. The attack surface is particularly significant in multi-tenant cloud environments where multiple customers share the same physical infrastructure, as the vulnerability could enable cross-tenant memory access. This weakness directly violates the fundamental security principles of virtualization, where one VM should not be able to access another VM's memory space, and represents a critical failure in the isolation mechanisms that protect virtualized environments.
The mitigation strategies for this vulnerability involve both firmware and software updates to ensure proper TLB flushing mechanisms are implemented during VMCB transitions. System administrators should prioritize updating AMD CPU microcode and hypervisor firmware to versions that address this specific TLB flush issue. Additionally, organizations should implement monitoring solutions to detect potential exploitation attempts and consider temporarily disabling SEV functionality for workloads that cannot be immediately updated. This vulnerability aligns with CWE-116, which addresses improper encoding or handling of security-relevant information, and maps to ATT&CK technique T1004, which involves the use of multiple techniques to achieve memory access in virtualized environments. The remediation process requires careful coordination between hardware vendors, hypervisor providers, and cloud service providers to ensure comprehensive coverage of affected systems.