CVE-2015-0239 in Linux
Summary
by MITRE
The em_sysenter function in arch/x86/kvm/emulate.c in the Linux kernel before 3.18.5, when the guest OS lacks SYSENTER MSR initialization, allows guest OS users to gain guest OS privileges or cause a denial of service (guest OS crash) by triggering use of a 16-bit code segment for emulation of a SYSENTER instruction.
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Analysis
by VulDB Data Team • 03/08/2022
The vulnerability described in CVE-2015-0239 represents a critical privilege escalation flaw within the Linux kernel's KVM virtualization subsystem. This issue affects versions prior to 3.18.5 and specifically targets the em_sysenter function located in arch/x86/kvm/emulate.c. The vulnerability manifests when a guest operating system fails to properly initialize the SYSENTER Model Specific Registers, creating a condition that can be exploited by malicious guest users to escalate their privileges or cause system crashes. The flaw operates at the intersection of virtualization security and kernel privilege management, making it particularly dangerous in virtualized environments where multiple guests share the same host system.
The technical implementation of this vulnerability stems from improper handling of 16-bit code segment emulation during SYSENTER instruction processing. When a guest OS attempts to execute a SYSENTER instruction without proper SYSENTER MSR initialization, the em_sysenter function fails to validate the code segment descriptor properly. This validation gap allows attackers to manipulate the execution flow and potentially elevate their privileges from guest user level to guest OS level. The vulnerability specifically exploits the lack of proper bounds checking and segment descriptor validation when dealing with 16-bit code segments, which should normally be restricted in modern x86 architectures but become exploitable in the KVM emulation context. The flaw aligns with CWE-122, which describes insufficient validation of buffer access, and represents a classic case of improper input validation in kernel space code.
The operational impact of CVE-2015-0239 extends beyond simple privilege escalation to encompass potential denial of service conditions that can crash entire guest operating systems. An attacker with access to a guest VM can leverage this vulnerability to either gain elevated privileges within that guest environment or cause the guest OS to crash entirely, thereby disrupting services and potentially affecting other VMs running on the same host system. This makes the vulnerability particularly dangerous in multi-tenant cloud environments where guest isolation is paramount for security. The attack vector requires the guest user to execute specific code that triggers the SYSENTER instruction with improperly initialized registers, making it somewhat more complex than some other privilege escalation vulnerabilities but still feasible in environments where guest users have execution capabilities.
Mitigation strategies for CVE-2015-0239 primarily involve upgrading to Linux kernel versions 3.18.5 or later, where the vulnerability has been patched through proper validation of SYSENTER MSR initialization. System administrators should implement comprehensive patch management processes to ensure all virtualized environments are updated promptly. Additional mitigations include implementing strict guest OS configuration policies that enforce proper SYSENTER MSR initialization, monitoring for suspicious code execution patterns, and maintaining robust virtualization security controls. The fix addresses the root cause by ensuring that the em_sysenter function properly validates code segment descriptors before proceeding with emulation, preventing the privilege escalation pathway. Organizations should also consider implementing hypervisor-level security controls and monitoring for anomalous behavior in virtualized environments that might indicate exploitation attempts. This vulnerability demonstrates the importance of thorough input validation in kernel space code and highlights the risks associated with incomplete virtualization security implementations that can create privilege escalation pathways between guest and host environments.