CVE-2019-3887 in Linux
Summary
by MITRE
A flaw was found in the way KVM hypervisor handled x2APIC Machine Specific Rregister (MSR) access with nested(=1) virtualization enabled. In that, L1 guest could access L0's APIC register values via L2 guest, when 'virtualize x2APIC mode' is enabled. A guest could use this flaw to potentially crash the host kernel resulting in DoS issue. Kernel versions from 4.16 and newer are vulnerable to this issue.
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Analysis
by VulDB Data Team • 08/28/2023
The vulnerability identified as CVE-2019-3887 represents a critical security flaw within the Kernel-based Virtual Machine (KVM) hypervisor implementation on Linux systems. This issue specifically affects systems running kernel versions 4.16 and newer where nested virtualization is enabled with x2APIC mode virtualization activated. The flaw manifests in the improper handling of Machine Specific Registers (MSRs) during x2APIC operations, creating a significant bypass of virtualization isolation mechanisms that should prevent guest operating systems from accessing hypervisor-level resources.
The technical root cause of this vulnerability lies in the insufficient validation of x2APIC MSR access when nested virtualization is active. When the 'virtualize x2APIC mode' feature is enabled, the hypervisor fails to properly enforce access controls between different virtualization levels. This allows a Level 1 (L1) guest operating system to indirectly access Level 0 (L0) APIC register values through a Level 2 (L2) guest, effectively breaking the virtualization isolation boundary that is fundamental to secure hypervisor operation. The vulnerability specifically exploits the interaction between nested virtualization and x2APIC functionality, where the hypervisor's MSR handling logic does not adequately distinguish between different privilege levels during virtualized x2APIC register access.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can be leveraged to cause a complete system denial of service through kernel crashes. An attacker controlling an L1 guest operating system could potentially craft malicious x2APIC MSR access patterns that trigger kernel memory corruption, leading to kernel panics and system-wide service disruption. This represents a serious threat to virtualized environments where multiple tenants share the same physical hardware, as a compromised guest could potentially affect the stability and availability of the entire host system. The vulnerability is particularly concerning in cloud computing environments where KVM is commonly used for virtualization, as it could enable attackers to cause widespread service disruption.
Mitigation strategies for CVE-2019-3887 primarily involve applying the relevant kernel patches that address the x2APIC MSR access validation issue. System administrators should ensure that all affected systems are updated to kernel versions that contain the fix, typically those released after the vulnerability disclosure. Alternative approaches include disabling nested virtualization when x2APIC mode is in use, or configuring the hypervisor to disable the problematic 'virtualize x2APIC mode' feature. Organizations should also implement monitoring for unusual kernel crash patterns that might indicate exploitation attempts. This vulnerability aligns with CWE-284 Access Control Issues, specifically focusing on insufficient access control in virtualized environments, and represents a technique that could be categorized under ATT&CK tactic TA0040 Privilege Escalation through hypervisor bypass methods. The remediation process should include comprehensive testing of updated kernels to ensure compatibility with existing virtualization workloads while maintaining the security posture of the virtualized infrastructure.