CVE-2015-8963 in Android
Summary
by MITRE
Race condition in kernel/events/core.c in the Linux kernel before 4.4 allows local users to gain privileges or cause a denial of service (use-after-free) by leveraging incorrect handling of an swevent data structure during a CPU unplug operation.
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Analysis
by VulDB Data Team • 10/15/2024
The vulnerability described in CVE-2015-8963 represents a critical race condition within the Linux kernel's performance monitoring subsystem, specifically in the kernel/events/core.c file. This flaw exists in kernel versions prior to 4.4 and demonstrates a fundamental issue in how the system handles software event (swevent) data structures during CPU unplug operations. The race condition occurs when multiple threads or processes attempt to access and modify the same data structure simultaneously, creating a window where memory corruption can occur. This particular vulnerability falls under the category of use-after-free conditions as defined by CWE-416, where memory that has been freed is accessed by subsequent operations, leading to unpredictable behavior and potential privilege escalation.
The technical implementation of this vulnerability involves the improper synchronization mechanisms during CPU hot-unplug operations, which are designed to remove CPU cores from active use while the system is running. During these operations, the kernel must properly handle references to software events that are associated with the CPU being removed. When the race condition occurs, the swevent data structure may be freed while another process or thread is still attempting to access it, resulting in a use-after-free scenario. This condition can be exploited by local attackers who have access to the system to either execute arbitrary code with elevated privileges or to cause a system crash through denial of service. The vulnerability is particularly dangerous because it allows privilege escalation from a regular user account to root access, making it a high-severity issue for all Linux systems running affected kernel versions.
The operational impact of CVE-2015-8963 extends beyond simple system instability, as it provides a pathway for attackers to gain unauthorized administrative access to compromised systems. This vulnerability affects the core kernel functionality and operates at the most privileged level of the operating system, making it a prime target for exploitation in various attack scenarios. The race condition specifically manifests during CPU unplug operations, which are commonly used in power management, system maintenance, and virtualization environments. Attackers can leverage this vulnerability to maintain persistent access to systems, escalate privileges to gain root access, or cause sustained denial of service conditions that can disrupt critical services. The exploitation requires local access to the target system but does not require network connectivity, making it particularly concerning for environments where physical or logical access control is insufficient. According to ATT&CK framework, this vulnerability maps to privilege escalation techniques and can be categorized under T1068 (Local Privilege Escalation) and T1499 (Endpoint Denial of Service) with potential for T1543 (Create or Modify System Process) operations.
Mitigation strategies for CVE-2015-8963 primarily focus on kernel version updates, which provide the most effective solution through the implementation of proper synchronization mechanisms and memory management controls. System administrators should immediately upgrade to Linux kernel version 4.4 or later, where the race condition has been resolved through improved locking mechanisms and proper handling of swevent data structures during CPU hot-unplug operations. Additional protective measures include implementing strict access controls to limit local user privileges, monitoring for unusual CPU unplug events that might indicate exploitation attempts, and maintaining comprehensive system logging to detect potential abuse of this vulnerability. Organizations should also consider implementing kernel hardening techniques such as stack canaries, address space layout randomization, and control flow integrity checks to further reduce the exploitability of similar vulnerabilities. The fix implemented in kernel 4.4 addresses the root cause by ensuring proper reference counting and synchronization during the CPU unplug process, preventing the premature freeing of memory structures while they remain in use. This vulnerability serves as a reminder of the critical importance of proper race condition handling in kernel-level code and demonstrates how seemingly minor synchronization issues can lead to major security implications.