CVE-2017-12154 in Linux
Summary
by MITRE
The prepare_vmcs02 function in arch/x86/kvm/vmx.c in the Linux kernel through 4.13.3 does not ensure that the "CR8-load exiting" and "CR8-store exiting" L0 vmcs02 controls exist in cases where L1 omits the "use TPR shadow" vmcs12 control, which allows KVM L2 guest OS users to obtain read and write access to the hardware CR8 register.
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Analysis
by VulDB Data Team • 12/30/2022
The vulnerability described in CVE-2017-12154 represents a critical hypervisor-level flaw within the Linux kernel's KVM virtualization implementation, specifically affecting the Intel VT-x virtualization subsystem. This issue resides in the prepare_vmcs02 function located in arch/x86/kvm/vmx.c, which handles the preparation of virtual machine control structures for nested virtualization scenarios. The vulnerability manifests when KVM processes nested virtualization requests where the guest operating system fails to properly configure certain virtualization controls, creating a pathway for privilege escalation and unauthorized hardware register access.
The technical flaw stems from inadequate validation of virtual machine control structures within the nested virtualization framework. Specifically, when L1 guest operating system omits the "use TPR shadow" vmcs12 control, the prepare_vmcs02 function fails to properly enforce the existence of "CR8-load exiting" and "CR8-store exiting" controls in the L0 vmcs02 structure. This oversight creates a condition where the hypervisor does not adequately validate the virtualization controls that govern how the CR8 register is handled during nested virtualization operations. The CR8 register, which controls the Task Priority Register on x86 systems, is particularly sensitive because it directly influences interrupt handling and system scheduling behavior.
The operational impact of this vulnerability is significant, as it allows malicious L2 guest operating system users to gain unauthorized read and write access to the hardware CR8 register. This capability represents a fundamental breach in virtualization security isolation, as it enables guest operating systems to directly manipulate critical system scheduling parameters that should remain protected from guest-level access. The vulnerability essentially undermines the hypervisor's ability to maintain proper privilege separation between different virtualization layers, potentially allowing for privilege escalation attacks that could compromise the entire virtualization environment. Attackers could leverage this access to manipulate interrupt priority levels, potentially causing system instability or enabling further exploitation techniques.
This vulnerability aligns with CWE-284, which addresses improper access control in software systems, and demonstrates characteristics consistent with ATT&CK technique T1055 related to privilege escalation through kernel exploits. The flaw specifically affects the virtualization security model by creating an unauthorized access path that bypasses normal privilege boundaries. The vulnerability affects Linux kernel versions through 4.13.3, representing a substantial attack surface given the widespread use of these kernel versions in production environments. The issue is particularly concerning in multi-tenant virtualization environments where multiple guests share the same physical hardware, as it could enable one guest to potentially interfere with or compromise the operation of other guests.
Mitigation strategies for this vulnerability include immediate kernel updates to versions that contain the patched implementation of the prepare_vmcs02 function, which properly validates the existence of required CR8 controls regardless of the presence or absence of TPR shadow controls. Organizations should also implement monitoring for suspicious virtualization activities and consider reducing the privileges granted to L2 guest operating systems where possible. The patch addresses the root cause by ensuring proper validation of virtual machine control structures before allowing guest operating systems to access hardware registers, thereby restoring the intended security boundaries within the virtualization environment.