CVE-2016-9777 in Linux
Summary
by MITRE
KVM in the Linux kernel before 4.8.12, when I/O APIC is enabled, does not properly restrict the VCPU index, which allows guest OS users to gain host OS privileges or cause a denial of service (out-of-bounds array access and host OS crash) via a crafted interrupt request, related to arch/x86/kvm/ioapic.c and arch/x86/kvm/ioapic.h.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-9777 represents a critical privilege escalation flaw within the Linux kernel's KVM virtualization subsystem, specifically affecting versions prior to 4.8.12. This issue manifests when the I/O Advanced Programmable Interrupt Controller is enabled, creating a scenario where guest operating systems can exploit improper bounds checking on Virtual CPU indexing. The flaw exists in the ioapic.c and ioapic.h kernel source files, which handle interrupt routing within virtualized environments, making it particularly dangerous for cloud computing and virtualization platforms where multiple tenants share the same physical host.
The technical implementation of this vulnerability stems from inadequate input validation within the KVM subsystem's interrupt handling mechanism. When a guest OS attempts to generate an interrupt request through the I/O APIC, the system fails to properly validate the VCPU index parameter, allowing malicious guest users to specify an out-of-bounds index value. This improper restriction enables attackers to manipulate memory structures within the host kernel, potentially leading to arbitrary code execution with host privileges. The vulnerability operates at the intersection of virtualization security and kernel memory management, where the boundary between guest and host execution contexts becomes compromised.
The operational impact of CVE-2016-9777 extends beyond simple privilege escalation to encompass potential system stability threats. Attackers can leverage this flaw to either gain unauthorized administrative access to the host system, effectively compromising the entire virtualization environment, or to induce a denial of service condition through controlled out-of-bounds memory access that results in host kernel crashes. This dual nature makes the vulnerability particularly attractive to adversaries seeking either persistent access to infrastructure or the ability to disrupt services within virtualized environments. The vulnerability directly relates to CWE-129, which addresses insufficient input validation, and maps to ATT&CK technique T1055 for privilege escalation through kernel exploits.
Mitigation strategies for CVE-2016-9777 primarily involve upgrading to Linux kernel versions 4.8.12 or later where the vulnerability has been patched. System administrators should prioritize patching their virtualization infrastructure, particularly in cloud environments where multiple tenants operate on shared hardware. Additional protective measures include implementing strict virtual machine resource limits, monitoring for anomalous interrupt patterns, and maintaining comprehensive logging of virtualization activities. Organizations should also consider disabling I/O APIC when it is not required for specific workloads, though this may impact system functionality. The fix implemented in the kernel addresses the core bounds checking issue by properly validating VCPU index values before processing interrupt requests, ensuring that guest operating systems cannot manipulate host kernel memory structures through crafted interrupt sequences.