CVE-2016-9178 in Linux
Summary
by MITRE
The __get_user_asm_ex macro in arch/x86/include/asm/uaccess.h in the Linux kernel before 4.7.5 does not initialize a certain integer variable, which allows local users to obtain sensitive information from kernel stack memory by triggering failure of a get_user_ex call.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability identified as CVE-2016-9178 resides within the Linux kernel's architecture-specific user access handling mechanisms, specifically in the __get_user_asm_ex macro located in arch/x86/include/asm/uaccess.h. This flaw represents a classic information disclosure vulnerability that exploits improper variable initialization within kernel space code. The vulnerability affects Linux kernel versions prior to 4.7.5 and demonstrates how seemingly minor coding oversights in kernel-level implementations can lead to significant security implications. The issue stems from the macro failing to properly initialize a critical integer variable during its execution flow, creating a scenario where sensitive data from kernel stack memory may be inadvertently exposed to unprivileged users.
The technical root cause of this vulnerability aligns with CWE-755 weakness category, which deals with improper handling of exceptional conditions and uninitialized variables. When the get_user_ex call fails, the uninitialized integer variable retains its previous stack contents, which may include sensitive kernel data such as cryptographic keys, memory addresses, or other confidential information. This occurs because the macro operates in kernel space where memory management and access controls are more stringent, but the improper initialization creates a window where stack data leaks occur. The vulnerability is particularly concerning as it operates at the kernel level, where access to sensitive information could enable further exploitation or compromise of system integrity.
From an operational perspective, this vulnerability presents a local privilege escalation vector that allows users to extract kernel memory contents without requiring elevated privileges. The impact extends beyond simple information disclosure, as the leaked kernel stack data could contain pointers, kernel configuration details, or other sensitive information that could aid attackers in subsequent exploitation attempts. The vulnerability operates silently without requiring specific user interaction, making it particularly dangerous as it can be exploited continuously without detection. Attackers can repeatedly trigger the get_user_ex failure condition to gather increasing amounts of sensitive kernel memory, potentially leading to full system compromise through information gathering that enables more sophisticated attacks.
The exploitation of CVE-2016-9178 aligns with several ATT&CK tactics including privilege escalation and credential access, as the ability to extract kernel memory content provides attackers with valuable information for further compromise. This vulnerability demonstrates the importance of proper variable initialization in kernel space code and highlights the need for comprehensive code review processes that examine memory management and access control mechanisms. Organizations should prioritize patching this vulnerability through kernel updates to version 4.7.5 or later, as this specific issue has been addressed in the official kernel releases. Additionally, system administrators should implement monitoring for unusual kernel memory access patterns and ensure that all systems are running patched kernel versions to prevent potential exploitation. The vulnerability underscores the critical nature of kernel-level security and the necessity of rigorous code quality assurance processes in security-critical software components.