CVE-2023-4622: Linux Kernel Use-After-Free Vulnerability

CVE-2023-4622 Overview

A use-after-free vulnerability exists in the Linux kernel's af_unix component that can be exploited to achieve local privilege escalation. The vulnerability stems from the unix_stream_sendpage() function attempting to add data to the last skb in the peer's recv queue without properly locking the queue. This creates a race condition where unix_stream_sendpage() could access an skb locklessly while it is being released by garbage collection, resulting in a use-after-free condition.

This vulnerability requires local access and involves a race condition, making exploitation more complex but still achievable by a skilled attacker with low privileges on the system.

Critical Impact

Successful exploitation allows local attackers to escalate privileges to root, potentially gaining complete control over affected Linux systems.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux 10.0
• Debian Linux 12.0

Discovery Timeline

• September 6, 2023 - CVE-2023-4622 published to NVD
• February 13, 2025 - Last updated in NVD database

Technical Details for CVE-2023-4622

Vulnerability Analysis

This use-after-free vulnerability in the Linux kernel's Unix domain socket implementation (af_unix) presents a serious local privilege escalation risk. The vulnerability arises from improper synchronization in the unix_stream_sendpage() function when handling socket buffers (skb) in the peer's receive queue.

The core issue is a Time-of-Check Time-of-Use (TOCTOU) race condition. When sending data through a Unix stream socket, the unix_stream_sendpage() function attempts to append data to the last skb in the peer's receive queue. However, this operation is performed without holding the appropriate queue lock. Concurrently, the kernel's garbage collection mechanism may be freeing the same skb, leading to the use-after-free scenario.

If an attacker can win the race condition, they can manipulate the freed memory region, potentially corrupting kernel data structures in a way that leads to arbitrary code execution with kernel privileges.

Root Cause

The root cause is a missing lock acquisition in the unix_stream_sendpage() function before accessing the peer's receive queue. The function attempts to optimize performance by accessing queue elements locklessly, but this creates a window where the skb can be freed by garbage collection while still being accessed. Proper synchronization using the receive queue lock should have been implemented to prevent concurrent access and garbage collection from causing a use-after-free condition.

Attack Vector

The attack vector is local, requiring an attacker to have low-privileged access to the target system. The exploitation involves:

1. Creating Unix domain stream sockets
2. Triggering the unix_stream_sendpage() code path while simultaneously triggering garbage collection
3. Winning the race condition to cause the use-after-free
4. Leveraging the freed memory to corrupt kernel structures and achieve privilege escalation

The complexity is high due to the race condition requirement, but techniques such as heap spraying and precise timing can increase exploitation success rates.

The vulnerability mechanism involves the unix_stream_sendpage() function accessing socket buffer data in the peer's receive queue without proper locking. When garbage collection runs concurrently and frees the skb being accessed, the function operates on freed memory. Technical details of the fix can be found in the Linux Kernel Commit Update.

Detection Methods for CVE-2023-4622

Indicators of Compromise

• Unusual kernel crashes or system instability related to Unix domain sockets
• Unexpected privilege escalation events or processes running with elevated privileges without proper authorization
• Anomalous activity in /var/log/kern.log or dmesg output showing use-after-free errors in af_unix subsystem
• Signs of heap spray attempts or memory corruption targeting kernel structures

Detection Strategies

• Monitor for kernel oops or panic events related to unix_stream_sendpage() or af_unix components
• Deploy kernel-level monitoring tools to detect suspicious socket operations and potential race condition exploitation attempts
• Utilize SentinelOne's Behavioral AI engine to identify anomalous process behaviors indicative of privilege escalation

Monitoring Recommendations

• Enable kernel audit logging for Unix domain socket operations using auditd
• Monitor for processes attempting rapid socket creation and data transfer patterns that could indicate race condition exploitation
• Implement runtime kernel integrity monitoring to detect unauthorized modifications to kernel memory structures

How to Mitigate CVE-2023-4622

Immediate Actions Required

• Update the Linux kernel to a version containing commit 790c2f9d15b594350ae9bca7b236f2b1859de02c or later
• For Debian systems, apply the patches from Debian Security Advisory DSA-5492
• Restrict local system access to trusted users only until patches are applied
• Consider deploying kernel live patching solutions as referenced in the Packet Storm Security Notice

Patch Information

The vulnerability has been fixed in the Linux kernel through commit 790c2f9d15b594350ae9bca7b236f2b1859de02c. The fix adds proper locking to the unix_stream_sendpage() function to prevent the race condition with garbage collection.

Debian has released security advisories addressing this vulnerability:

• Debian Security Advisory DSA-5492
• Debian LTS Announcement October 2023
• Debian LTS Announcement January 2024

Workarounds

• Limit local user access to only essential personnel until patches can be applied
• Implement strict user privilege separation and monitoring to detect exploitation attempts
• Consider using security modules like SELinux or AppArmor in enforcing mode to limit the impact of privilege escalation

# Check current kernel version
uname -r

# Update kernel on Debian-based systems
sudo apt update && sudo apt upgrade linux-image-$(uname -r)

# Verify the patch is applied by checking kernel version
# Ensure your version includes the security fix for CVE-2023-4622