CVE-2014-8480 in Linux
Summary
by MITRE
The instruction decoder in arch/x86/kvm/emulate.c in the KVM subsystem in the Linux kernel before 3.18-rc2 lacks intended decoder-table flags for certain RIP-relative instructions, which allows guest OS users to cause a denial of service (NULL pointer dereference and host OS crash) via a crafted application.
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Analysis
by VulDB Data Team • 02/23/2022
The vulnerability identified as CVE-2014-8480 resides within the kernel-based virtual machine subsystem of Linux, specifically in the x86 architecture emulation component. This flaw exists in the instruction decoder implementation located at arch/x86/kvm/emulate.c, which is responsible for handling virtualized instruction execution when guest operating systems run on KVM hypervisors. The issue manifests as an incomplete implementation of decoder table flags for RIP-relative instructions, which are critical for proper instruction decoding and execution in x86 architectures. The vulnerability represents a significant security concern as it allows unprivileged guest users to manipulate the hypervisor's instruction processing mechanisms, potentially leading to system instability and service disruption.
The technical nature of this vulnerability stems from the improper handling of RIP-relative addressing modes within the KVM hypervisor's emulation layer. When guest operating systems execute certain RIP-relative instructions, the decoder fails to properly validate or process the instruction flags that should normally be present in the instruction table entries. This omission creates a scenario where a malicious guest application can craft specific instruction sequences that, when executed within the virtualized environment, cause the hypervisor to attempt to dereference a NULL pointer. The missing decoder table flags essentially create a gap in the instruction processing logic that allows malformed instruction data to bypass normal validation checks and propagate through the emulation layer directly to the host kernel's instruction decoder.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can potentially lead to complete system crashes and compromise the stability of the entire virtualization environment. When a guest user successfully exploits this vulnerability, the resulting NULL pointer dereference causes the host operating system to crash and restart, effectively creating a denial of service condition that affects not only the targeted virtual machine but potentially other VMs running on the same host. This vulnerability is particularly dangerous in multi-tenant environments where multiple users share the same physical hardware, as a single compromised guest can affect the entire system. The flaw affects Linux kernel versions prior to 3.18-rc2, representing a substantial window of vulnerable systems that could be exploited by attackers with access to a guest operating system.
Mitigation strategies for CVE-2014-8480 primarily focus on updating the Linux kernel to versions 3.18-rc2 or later, where the decoder table flags have been properly implemented and validated. System administrators should prioritize patching affected systems, particularly in virtualized environments where the risk of exploitation is highest. Additional protective measures include implementing strict guest operating system monitoring and limiting guest user privileges to reduce the attack surface. From a cybersecurity perspective, this vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions, and represents a type of hypervisor escape or privilege escalation vector that could be mapped to ATT&CK techniques involving privilege escalation and defense evasion. Organizations should also consider implementing virtualization-specific monitoring solutions that can detect anomalous instruction execution patterns that might indicate exploitation attempts. The vulnerability underscores the critical importance of proper input validation and robust instruction decoding mechanisms in hypervisor implementations, as even seemingly minor flaws in instruction processing can lead to catastrophic system failures.