CVE-2016-9644 in Linux
Summary
by MITRE
The __get_user_asm_ex macro in arch/x86/include/asm/uaccess.h in the Linux kernel 4.4.22 through 4.4.28 contains extended asm statements that are incompatible with the exception table, which allows local users to obtain root access on non-SMEP platforms via a crafted application. NOTE: this vulnerability exists because of incorrect backporting of the CVE-2016-9178 patch to older kernels.
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Analysis
by VulDB Data Team • 10/04/2022
The vulnerability described in CVE-2016-9644 represents a critical privilege escalation flaw within the Linux kernel's memory access validation mechanisms. This issue specifically affects kernel versions 4.4.22 through 4.4.28 and stems from improper implementation of the __get_user_asm_ex macro in the x86 architecture's user access header file. The flaw enables local attackers to escalate their privileges to root level access on systems lacking SMEP (Supervisor Mode Execution Prevention) protection. The vulnerability's root cause lies in the incorrect backporting of a patch designed to address CVE-2016-9178, which created an incompatibility between the extended assembly statements and the kernel's exception table handling mechanism.
The technical implementation of this vulnerability exploits the interaction between kernel assembly code and the exception handling framework. The __get_user_asm_ex macro contains extended asm statements that fail to properly account for exception table entries, creating a scenario where memory access violations can be manipulated to bypass kernel security checks. This occurs because the macro's assembly implementation does not correctly handle the exception table entries that would normally be generated during memory access operations, allowing malicious code to exploit the gap in the kernel's privilege validation system. When a crafted application executes this vulnerable code path, it can manipulate the kernel's memory access controls to gain elevated privileges.
The operational impact of CVE-2016-9644 is severe and directly affects systems running affected kernel versions. Local users who can execute code on the target system can leverage this vulnerability to achieve full root access without requiring additional attack vectors or exploitation techniques. The vulnerability is particularly dangerous because it does not require network access or specialized hardware, making it accessible to any local user with basic execution privileges. Systems without SMEP protection are especially vulnerable, as this security feature would normally prevent such privilege escalation attacks from succeeding. The vulnerability affects a wide range of Linux distributions that utilize kernel versions within the affected range, including enterprise and desktop systems.
Mitigation strategies for CVE-2016-9644 primarily involve updating to patched kernel versions where the backporting error has been corrected. System administrators should prioritize upgrading to kernel versions that properly implement the CVE-2016-9178 patch without the problematic backporting artifacts. Additionally, organizations should consider enabling SMEP protection where available, as this hardware feature provides an additional layer of defense against such attacks. The vulnerability aligns with CWE-119 (Improper Access Control) and CWE-200 (Information Exposure) categories, while also mapping to ATT&CK techniques involving privilege escalation and defense evasion. Organizations should implement comprehensive patch management processes to ensure all affected systems receive timely updates and should monitor for potential exploitation attempts through system logs and security monitoring tools.