CVE-2016-8630 in Linux
Summary
by MITRE
The x86_decode_insn function in arch/x86/kvm/emulate.c in the Linux kernel before 4.8.7, when KVM is enabled, allows local users to cause a denial of service (host OS crash) via a certain use of a ModR/M byte in an undefined instruction.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability described in CVE-2016-8630 represents a critical denial of service flaw within the Linux kernel's KVM (Kernel-based Virtual Machine) implementation. This issue affects systems running Linux kernel versions prior to 4.8.7 where KVM virtualization is enabled, creating a potential vector for malicious actors to disrupt host system operations. The vulnerability specifically resides in the x86_decode_insn function located within the arch/x86/kvm/emulate.c file, which handles instruction emulation for virtualized x86 environments.
The technical root cause stems from inadequate validation of ModR/M bytes during instruction decoding processes within the KVM subsystem. ModR/M bytes are essential components of x86 instruction encoding that specify operand locations and addressing modes. When the x86_decode_insn function encounters certain undefined instructions with malformed ModR/M byte combinations, it fails to properly handle these edge cases, leading to unpredictable behavior in the virtualization layer. This improper handling can result in memory corruption or invalid pointer dereferences that ultimately trigger a kernel panic and system crash.
From an operational perspective, this vulnerability presents a significant risk to virtualized environments where multiple guests may be running concurrently on a single host system. Local users within a guest virtual machine could exploit this flaw to crash the host operating system, effectively creating a denial of service condition that impacts all other virtual machines sharing the same physical hardware. The attack requires local access within a guest environment but can have widespread consequences across the entire virtualization infrastructure, making it particularly dangerous in multi-tenant cloud deployments or server environments where KVM virtualization is prevalent.
The impact of this vulnerability aligns with CWE-121, which addresses stack-based buffer overflow conditions, and can be categorized under ATT&CK technique T1499.200 for endpoint denial of service. Organizations running Linux systems with KVM enabled must prioritize patching to version 4.8.7 or later, as this update includes proper validation mechanisms for ModR/M byte processing. Mitigation strategies should also include monitoring for unusual system crashes and implementing proper access controls to limit local user privileges within virtualized environments. Additionally, system administrators should consider disabling KVM virtualization for systems where the risk of exploitation cannot be adequately mitigated through patching, particularly in environments where guest operating systems cannot be fully trusted or controlled.