CVE-2023-0468: Linux Kernel Use-After-Free Vulnerability

CVE-2023-0468 Overview

A use-after-free vulnerability has been identified in the Linux Kernel's io_uring subsystem, specifically within the io_poll_check_events function in io_uring/poll.c. This flaw arises from a race condition involving poll_refs, which can lead to a NULL pointer dereference. The io_uring interface is a high-performance asynchronous I/O API in Linux, making this vulnerability particularly relevant for systems utilizing modern I/O operations.

Critical Impact

Local attackers with low privileges can exploit this race condition to cause system denial of service through NULL pointer dereference, potentially crashing the kernel and disrupting system availability.

Affected Products

• Linux Kernel versions prior to 6.1
• Linux Kernel 6.1 release candidates (rc1 through rc6)
• Linux Kernel 6.1 initial release

Discovery Timeline

• January 26, 2023 - CVE-2023-0468 published to NVD
• April 1, 2025 - Last updated in NVD database

Technical Details for CVE-2023-0468

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption flaw that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of CVE-2023-0468, the vulnerability manifests in the io_uring polling mechanism, a critical component of the Linux Kernel's asynchronous I/O infrastructure.

The io_uring subsystem provides efficient asynchronous I/O operations by using ring buffers shared between user space and kernel space. The polling functionality allows applications to efficiently wait for I/O events without blocking. However, a race condition in the poll_refs reference counting mechanism can cause the kernel to access memory after it has been freed.

Root Cause

The root cause of this vulnerability lies in improper synchronization of the poll_refs reference counter within the io_poll_check_events function. When multiple threads or operations concurrently access and modify the poll reference count, a race condition can occur where:

1. One execution path decrements the reference count and triggers memory deallocation
2. Another concurrent execution path attempts to access the same memory region
3. The subsequent access results in a use-after-free condition, leading to a NULL pointer dereference

This synchronization failure violates the expected memory safety guarantees of the reference counting mechanism, allowing freed memory to be accessed.

Attack Vector

The attack vector for CVE-2023-0468 requires local access to the system. An attacker must be able to execute code on the target machine to trigger the race condition. The exploitation scenario involves:

The vulnerability requires local access with low privileges but no user interaction. An attacker would need to craft specific io_uring operations that trigger the race condition in io_poll_check_events. By rapidly creating and destroying poll operations, an attacker can increase the likelihood of hitting the vulnerable timing window where memory is freed but still referenced.

Successful exploitation results in a NULL pointer dereference, which typically causes a kernel panic or system crash, leading to denial of service. The attack complexity is high due to the need to win the race condition, which requires precise timing.

Detection Methods for CVE-2023-0468

Indicators of Compromise

• Unexpected kernel panics or system crashes with call traces referencing io_poll_check_events or io_uring/poll.c
• Kernel oops messages indicating NULL pointer dereference in io_uring subsystem
• System instability when applications heavily utilize io_uring for asynchronous I/O operations
• Core dumps showing memory corruption patterns consistent with use-after-free conditions

Detection Strategies

• Monitor kernel logs for oops or panic messages containing references to io_uring, poll.c, or io_poll_check_events
• Implement kernel crash dump analysis to identify patterns consistent with use-after-free exploitation
• Deploy runtime kernel integrity monitoring to detect abnormal memory access patterns in io_uring subsystem
• Use kernel memory debugging tools like KASAN (Kernel Address Sanitizer) to detect use-after-free conditions in development environments

Monitoring Recommendations

• Enable kernel auditing for io_uring system calls to track unusual patterns of usage
• Configure automated alerting for kernel panic events with io_uring-related stack traces
• Monitor system stability metrics and correlate with io_uring-intensive workloads
• Implement centralized log collection to aggregate kernel messages across multiple systems for pattern analysis

How to Mitigate CVE-2023-0468

Immediate Actions Required

• Update affected Linux Kernel installations to patched versions that address the race condition in io_poll_check_events
• Review systems running kernel versions 6.1 and its release candidates for vulnerability exposure
• Consider temporarily disabling io_uring functionality if immediate patching is not possible and the feature is not critical
• Prioritize patching systems where untrusted local users have access

Patch Information

The vulnerability has been tracked and addressed by the Linux Kernel maintainers. System administrators should consult the Red Hat Bug Report #2164024 for detailed patch information and vendor-specific guidance. Linux distribution vendors have released updated kernel packages that incorporate the fix for this race condition.

The fix involves proper synchronization of the poll_refs reference counting to prevent the race condition that leads to use-after-free. Organizations should update to kernel versions that include this fix through their standard package management processes.

Workarounds

• Restrict local access to systems to minimize the attack surface for local privilege users who could exploit this vulnerability
• If feasible, disable io_uring subsystem by setting io_uring_disabled=2 boot parameter on systems where this functionality is not required
• Implement system call filtering using seccomp to restrict io_uring-related syscalls for untrusted processes
• Monitor and limit resources available to local users to reduce the likelihood of successful race condition exploitation

# Configuration example
# Disable io_uring at boot time by adding kernel parameter
# Add to GRUB_CMDLINE_LINUX in /etc/default/grub:
GRUB_CMDLINE_LINUX="io_uring_disabled=2"

# Update GRUB configuration
sudo update-grub

# Alternatively, disable at runtime (requires kernel support)
echo 2 | sudo tee /proc/sys/kernel/io_uring_disabled