CVE-2023-6531: Linux Kernel Use-After-Free Vulnerability

CVE-2023-6531 Overview

A use-after-free vulnerability has been identified in the Linux Kernel stemming from a race condition in the Unix socket garbage collector. The flaw occurs when the garbage collector's deletion of socket buffer (SKB) structures races with unix_stream_read_generic() on the socket that the SKB is queued on. This race condition can lead to use-after-free scenarios where memory is accessed after it has been freed, potentially allowing a local attacker with low privileges to achieve code execution or cause system instability.

Critical Impact

Local attackers with low privileges can exploit this race condition to potentially achieve privilege escalation, execute arbitrary code, or cause denial of service conditions on affected Linux systems.

Affected Products

• Linux Kernel (versions prior to 6.7)
• Linux Kernel 6.7 RC1 through RC4
• Red Hat Enterprise Linux 9.0

Discovery Timeline

• 2024-01-21 - CVE-2023-6531 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2023-6531

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition). The flaw resides in the Unix domain socket implementation within the Linux kernel, specifically in the interaction between the garbage collector and socket read operations. Unix domain sockets use a garbage collector mechanism to handle file descriptor passing and cleanup of orphaned socket buffers. However, a synchronization issue exists where the garbage collector may free an SKB structure while unix_stream_read_generic() is still accessing it.

The race window occurs during the deletion phase of the garbage collection process. When an SKB containing passed file descriptors is being processed for deletion, there's a timing window where a concurrent read operation on the same socket can still reference the SKB being freed. This creates a classic use-after-free condition where the read path accesses memory that has been returned to the kernel's memory allocator.

Root Cause

The root cause is insufficient synchronization between the Unix garbage collector's SKB deletion path and the unix_stream_read_generic() function. The garbage collector operates asynchronously to clean up socket buffers containing passed file descriptors, but the locking mechanism does not adequately prevent concurrent access from the read path. This Time-of-Check Time-of-Use (TOCTOU) issue allows a race condition where memory state changes between validation and use.

Attack Vector

The attack vector is local, requiring an attacker to have low-privileged access to the target system. Exploitation involves triggering the race condition by performing specific sequences of operations on Unix domain sockets while the garbage collector is active. The attacker would need to:

1. Create Unix domain sockets and pass file descriptors between them
2. Trigger garbage collection conditions while simultaneously performing read operations
3. Win the race condition to access freed memory

Due to the high attack complexity required to reliably trigger this race condition, exploitation is considered difficult but feasible on systems with predictable timing characteristics or through repeated exploitation attempts.

The vulnerability can lead to high impacts on confidentiality, integrity, and availability if successfully exploited. An attacker could potentially read sensitive kernel memory, corrupt kernel data structures, or cause system crashes. For detailed technical analysis, refer to the Kernel Mailing List Discussion and the Packet Storm Security Exploit Report.

Detection Methods for CVE-2023-6531

Indicators of Compromise

• Unexpected kernel panics or system crashes involving Unix socket operations
• Kernel oops messages referencing unix_stream_read_generic or Unix garbage collector functions
• Suspicious local processes with high CPU usage performing intensive Unix socket operations
• Memory corruption indicators in kernel logs related to Unix domain socket subsystem

Detection Strategies

• Monitor kernel logs for oops or panic messages involving af_unix.c, unix_stream_read_generic, or garbage collector functions
• Implement kernel address sanitizer (KASAN) in test environments to detect use-after-free access patterns
• Use SentinelOne's kernel-level behavioral monitoring to detect anomalous Unix socket activity patterns
• Deploy Linux audit rules to track intensive Unix domain socket file descriptor passing operations

Monitoring Recommendations

• Enable kernel crash dump analysis for post-incident investigation of potential exploitation attempts
• Configure system monitoring to alert on repeated Unix socket operations combined with process privilege changes
• Implement SentinelOne Singularity XDR for comprehensive endpoint detection and automated threat response
• Monitor for processes attempting to manipulate Unix socket garbage collection timing through rapid socket creation and destruction

How to Mitigate CVE-2023-6531

Immediate Actions Required

• Update Linux kernel to version 6.7 or later where the race condition has been addressed
• Apply vendor-specific patches from Red Hat (RHSA-2024:2394) or Debian as applicable
• Limit local user access on critical systems until patches can be applied
• Consider restricting access to Unix domain socket creation for untrusted users where feasible

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. Red Hat has released Security Advisory RHSA-2024:2394 containing the fix for Enterprise Linux 9. Debian has also released fixes as documented in the Debian LTS Announcement. The fix improves synchronization between the garbage collector and socket read paths to prevent the race condition. Organizations should consult Red Hat Bug #2253034 for additional technical details on the patch.

Workarounds

• Restrict local user access to minimize the attack surface until kernel updates can be applied
• Monitor system logs for indicators of exploitation attempts targeting Unix socket subsystem
• Consider implementing additional access controls for Unix domain socket operations using security modules like SELinux or AppArmor
• Use containerization with restricted capabilities to limit the impact of potential exploitation

# Check current kernel version and update
uname -r
# For Red Hat/CentOS systems
sudo yum update kernel
# For Debian/Ubuntu systems
sudo apt update && sudo apt upgrade linux-image-$(uname -r)
# Reboot to apply the new kernel
sudo reboot