CVE-2016-4440 in Linux
Summary
by MITRE • 01/25/2023
arch/x86/kvm/vmx.c in the Linux kernel through 4.6.3 mishandles the APICv on/off state, which allows guest OS users to obtain direct APIC MSR access on the host OS, and consequently cause a denial of service (host OS crash) or possibly execute arbitrary code on the host OS, via x2APIC mode.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 01/25/2023
The vulnerability described in CVE-2016-4440 resides within the virtual machine monitor implementation of the Linux kernel version 4.6.3 and earlier, specifically within the vmx.c file that handles Intel VT-x virtualization extensions. This flaw represents a critical security issue in the KVM (Kernel-based Virtual Machine) subsystem that manages virtualization capabilities on x86 architectures. The vulnerability stems from improper handling of Advanced Programmable Interrupt Controller virtualization (APICv) state management during the transition between standard APIC and x2APIC modes, creating a potential pathway for privilege escalation and system compromise.
The technical flaw manifests when the kernel fails to properly validate or enforce the APICv state during virtual machine execution, allowing malicious guest operating systems to manipulate the virtualized APIC state in ways that bypass normal security boundaries. This mismanagement occurs specifically when transitioning between standard APIC mode and x2APIC mode, where the virtualization layer should maintain strict isolation between guest and host APIC access. The vulnerability exploits a race condition or state inconsistency that enables guest code to directly access host APIC Model Specific Registers (MSRs), which are typically protected from guest access. This direct access capability represents a fundamental breakdown in the virtualization isolation model that the kernel should maintain between virtual machines and the underlying host system.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, presenting a potential path for arbitrary code execution on the host system. When a guest OS user successfully exploits this flaw, they can potentially execute code with the privileges of the host kernel, effectively compromising the entire virtualization environment. The vulnerability is particularly dangerous because it allows for both denial of service through host kernel crashes and privilege escalation attacks that could lead to complete system compromise. Attackers could leverage this vulnerability to establish persistent access to the host system, potentially using it as a foothold for further attacks on the broader network infrastructure. The x2APIC mode specifically presents additional attack surface due to its extended register space and different access patterns compared to traditional APIC implementations.
Mitigation strategies for CVE-2016-4440 require immediate kernel updates to versions 4.6.4 and later, where the APICv state management has been corrected to properly enforce isolation between guest and host APIC access. System administrators should also consider disabling x2APIC mode in virtualized environments until proper patches are applied, though this may impact performance and compatibility with newer guest operating systems. Additional defensive measures include implementing strict virtual machine monitoring, employing hypervisor-level security controls, and conducting regular security assessments of virtualization environments. Organizations should also consider using hardware-assisted virtualization features that provide better isolation boundaries and regularly review their virtualization security configurations to prevent similar vulnerabilities from being exploited in other components of their virtualization infrastructure. This vulnerability aligns with CWE-284 Access Control Issues and represents a significant concern for organizations relying on KVM-based virtualization platforms, as it directly undermines the fundamental security assumptions of virtualization technologies.