CVE-2021-4093 in Linux
Summary
by MITRE • 02/18/2022
A flaw was found in the KVM's AMD code for supporting the Secure Encrypted Virtualization-Encrypted State (SEV-ES). A KVM guest using SEV-ES can trigger out-of-bounds reads and writes in the host kernel via a malicious VMGEXIT for a string I/O instruction (for example, outs or ins) using the exit reason SVM_EXIT_IOIO. This issue results in a crash of the entire system or a potential guest-to-host escape scenario.
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Analysis
by VulDB Data Team • 02/18/2022
This vulnerability represents a critical security flaw in the Linux Kernel-based Virtual Machine implementation specifically affecting AMD processors with Secure Encrypted Virtualization-Encrypted State support. The issue manifests when a malicious KVM guest utilizes SEV-ES functionality to execute string I/O instructions that trigger VMGEXIT events with exit reason SVM_EXIT_IOIO. The vulnerability stems from inadequate bounds checking within the host kernel's handling of these specific VM exits, creating an opportunity for unauthorized memory access patterns that can compromise system integrity.
The technical implementation flaw resides in the KVM hypervisor's AMD code path responsible for managing SEV-ES features, where string I/O operations such as outs and ins instructions fail to properly validate memory boundaries during VMGEXIT processing. This allows a guest virtual machine to manipulate host kernel memory through crafted I/O operations that bypass normal access controls and memory protection mechanisms. The vulnerability operates at the hypervisor level, making it particularly dangerous as it can potentially enable full system compromise or facilitate guest-to-host escape attacks that violate fundamental virtualization security boundaries.
The operational impact of this vulnerability extends beyond simple system crashes, presenting a serious threat to virtualized environments where multiple tenants share the same physical infrastructure. When exploited successfully, the out-of-bounds memory access can result in arbitrary code execution within the host kernel context, potentially enabling attackers to escalate privileges from guest level to host level access. This creates a pathway for attackers to compromise other virtual machines running on the same physical host, effectively breaking the isolation guarantees that virtualization platforms are designed to provide. The vulnerability particularly affects systems utilizing AMD EPYC processors with SEV-ES capabilities and can be exploited by malicious guests with limited privileges.
From a cybersecurity framework perspective, this vulnerability maps directly to CWE-129 and CWE-787 which address improper bounds checking and out-of-bounds read/write conditions respectively. The attack pattern aligns with ATT&CK technique T1059.001 for command and scripting interpreter usage and T1068 for exploit for privilege escalation. Organizations should prioritize immediate mitigation through kernel updates and patch management, implementing additional monitoring for suspicious VMGEXIT patterns, and considering temporary disablement of SEV-ES functionality until comprehensive security measures are in place to prevent exploitation.
The remediation approach requires deployment of updated KVM hypervisor versions that include proper bounds checking for string I/O operations within the SVM_EXIT_IOIO exit handler. System administrators should also implement enhanced monitoring solutions to detect anomalous VMGEXIT patterns and consider network segmentation to limit potential lateral movement if exploitation occurs. Additionally, organizations using SEV-ES should evaluate their virtualization security posture and potentially implement additional host-based security controls such as kernel lockdown modes or hypervisor hardening measures to reduce the attack surface and mitigate potential exploitation scenarios.