Critical Use-After-Free Vulnerability in Linux Kernel’s POSIX CPU Timers Exposed
A significant security flaw, identified as CVE-2025-38352, has been discovered in the Linux kernel’s implementation of POSIX CPU timers. This vulnerability arises from a race condition within the `handle_posix_cpu_timers()` function, potentially allowing attackers to exploit use-after-free (UAF) conditions in kernel memory. Such exploitation could lead to privilege escalation and full system compromise.
Understanding the Vulnerability
The core of this issue lies in the `handle_posix_cpu_timers()` function, responsible for processing timer signals during CPU scheduler ticks. A race condition occurs between the collection and processing of firing timers, creating a window where an attacker can free timer structures while they are still in use. This premature deallocation results in UAF scenarios, where the kernel continues to reference memory that has already been freed, leading to unpredictable behavior and potential security breaches.
Affected Systems
The vulnerability predominantly affects systems with the `CONFIG_POSIX_CPU_TIMERS_TASK_WORK` configuration disabled. This configuration is commonly found in 32-bit Android devices, making them particularly susceptible. To exploit this flaw, an attacker must achieve a zombie process state and precisely time their actions to trigger the race condition, highlighting the complexity and specificity required for successful exploitation.
Proof-of-Concept Exploit
Security researcher Faraz Sth has publicly released a proof-of-concept (PoC) exploit demonstrating the practical implications of this vulnerability. The exploit involves:
1. Creating a POSIX CPU timer set to fire after a designated CPU time interval.
2. Inducing a thread into a zombie state during critical kernel operations.
3. Reaping the zombie task while timer processing is ongoing.
4. Deleting the timer via the `timer_delete()` syscall, leading to premature memory deallocation.
5. Triggering a UAF condition as the kernel continues to access the freed timer.
Successful execution of this exploit results in Kernel Address Sanitizer (KASAN) warnings, indicating UAF write operations within the `posix_timer_queue_signal()` function. On systems without KASAN, kernel warnings appear in the `send_sigqueue()` function.
Implications and Risks
While exploiting this vulnerability requires local system access and precise timing, the potential consequences are severe. Attackers could gain elevated kernel privileges, leading to full system control. Reports suggest that this flaw has been actively exploited in limited, targeted attacks, underscoring the urgency for remediation.
Mitigation and Patching
The Linux kernel development team has addressed this vulnerability by releasing patches through stable branches. The fix involves preventing zombie processes from executing timer handling code, effectively eliminating the race condition window.
Recommendations for System Administrators
1. Immediate Kernel Update: Administrators should promptly update to the patched kernel versions to mitigate this vulnerability.
2. Configuration Review: Verify the status of the `CONFIG_POSIX_CPU_TIMERS_TASK_WORK` configuration, especially on 32-bit Android devices and embedded Linux systems, to assess exposure risk.
3. Monitor for Exploitation Attempts: Implement monitoring tools to detect unusual system behavior indicative of exploitation attempts, such as unexpected kernel warnings or system crashes.
4. User Privilege Management: Restrict local access to trusted users and enforce the principle of least privilege to minimize potential attack vectors.
Conclusion
The disclosure of CVE-2025-38352 highlights the critical importance of timely vulnerability management within the Linux ecosystem. System administrators and users must remain vigilant, ensuring that systems are updated and configurations are reviewed to protect against potential exploits. The availability of a public PoC accelerates the risk timeline, making immediate action imperative to maintain system integrity and security.
