CVE-2016-9756 in Linux
Summary
by MITRE
arch/x86/kvm/emulate.c in the Linux kernel before 4.8.12 does not properly initialize Code Segment (CS) in certain error cases, which allows local users to obtain sensitive information from kernel stack memory via a crafted application.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-9756 resides within the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically in the file arch/x86/kvm/emulate.c. This flaw represents a critical information disclosure issue that affects Linux kernel versions prior to 4.8.12, demonstrating a fundamental failure in proper memory initialization during error handling scenarios. The vulnerability occurs when the kernel fails to correctly initialize the Code Segment (CS) register during certain error conditions within the KVM emulation code path, creating a pathway for unauthorized information leakage.
The technical nature of this vulnerability stems from improper initialization of the CS register, which is a fundamental component of x86 architecture that defines the code segment for instruction execution. When error conditions occur during KVM instruction emulation, the kernel does not properly reset or initialize the CS register, leaving residual data from previous operations in memory accessible to user-space applications. This memory leakage occurs through the kernel stack where sensitive information from previous kernel operations, including potentially privileged data, remains accessible due to the uninitialized register state. The flaw operates at the intersection of virtualization and privilege escalation, exploiting the gap between user and kernel space boundaries within the KVM framework.
The operational impact of CVE-2016-9756 is significant for local attackers who can leverage this vulnerability to extract sensitive information from kernel stack memory. This information disclosure can potentially include kernel pointers, stack canaries, credential information, or other privileged data that could aid in further exploitation attempts. The vulnerability is particularly dangerous because it requires no special privileges beyond normal user access, making it an attractive target for attackers seeking to gain deeper insights into the kernel's operational state. This type of information leakage can serve as a stepping stone for more sophisticated attacks, including privilege escalation or bypassing security mechanisms. The vulnerability aligns with CWE-248, which addresses "Uncaught Exception" conditions where error handling fails to properly manage program state, and relates to ATT&CK technique T1063 for "Security Software Discovery" as it could reveal kernel-level security information.
Mitigation strategies for this vulnerability center on applying the official kernel patch released as part of Linux kernel version 4.8.12, which properly initializes the CS register during error handling scenarios within the KVM subsystem. System administrators should prioritize updating their kernel versions to ensure protection against this information disclosure vulnerability. Additionally, organizations should implement monitoring for unusual memory access patterns or potential exploitation attempts that might indicate an active attack leveraging this vulnerability. The fix addresses the root cause by ensuring proper register initialization regardless of error conditions, preventing the leakage of sensitive kernel stack information. Regular kernel updates and security audits remain essential practices for maintaining system integrity against similar vulnerabilities in virtualization components, particularly given the widespread use of KVM-based virtualization environments across enterprise and cloud deployments.