CVE-2023-37454: Linux Kernel Use-After-Free Vulnerability

CVE-2023-37454 Overview

A use-after-free write vulnerability has been identified in the Linux kernel through version 6.4.2. The flaw exists in the UDF (Universal Disk Format) filesystem implementation, specifically within the udf_put_super and udf_close_lvid functions located in fs/udf/super.c. When a specially crafted UDF filesystem image is processed, it can trigger a use-after-free condition that results in a write operation to freed memory.

Critical Impact

Local attackers with low privileges can craft malicious UDF filesystem images to trigger a use-after-free condition, potentially causing system crashes and denial of service on affected Linux systems.

Affected Products

• Linux Kernel through version 6.4.2
• All Linux distributions running vulnerable kernel versions
• Systems that process untrusted UDF filesystem images

Discovery Timeline

• 2023-07-06 - CVE-2023-37454 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-37454

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) occurs during the cleanup operations of the UDF filesystem driver. When unmounting a UDF filesystem or closing a logical volume, the kernel executes udf_put_super and udf_close_lvid functions. Under certain conditions triggered by a maliciously crafted UDF image, memory that has already been freed is subsequently written to, leading to memory corruption.

The vulnerability was identified through automated fuzzing using the Syzkaller kernel fuzzer, which detected the improper memory access patterns. Multiple related bug reports were generated, indicating the issue manifested across different code paths within the UDF subsystem.

Note: SUSE has documented a different perspective on this vulnerability in their bug tracking system, suggesting some disagreement about the severity or exploitability of this issue.

Root Cause

The root cause lies in improper memory lifecycle management within the UDF filesystem superblock handling code. The udf_put_super function, which is responsible for releasing resources when a UDF filesystem is unmounted, and the udf_close_lvid function, which handles logical volume identifier cleanup, contain a race condition or ordering issue that allows memory to be accessed after it has been deallocated.

This type of vulnerability typically occurs when:

• Memory is freed prematurely while still referenced
• Callback functions access stale pointers
• Resource cleanup ordering is incorrect
• Proper locking mechanisms are not implemented

Attack Vector

Exploitation requires local access to the target system with the ability to mount filesystem images. An attacker would need to:

1. Create a maliciously crafted UDF filesystem image with specific structural elements designed to trigger the vulnerable code path
2. Cause the system to mount the crafted image (requires appropriate permissions)
3. Trigger the unmount or cleanup operation to activate the use-after-free condition

The attack is limited to local exploitation since it requires mounting a filesystem, which typically requires elevated privileges or access to removable media automounting features. The primary impact is denial of service through kernel crash, though memory corruption vulnerabilities can sometimes be escalated to achieve code execution.

The vulnerability was discovered and documented through multiple Syzkaller Bug Report #26873 instances, with additional reports available at Syzkaller Bug Report #60864 and Syzkaller Bug Report #61564.

Detection Methods for CVE-2023-37454

Indicators of Compromise

• Kernel panic or oops messages referencing udf_put_super or udf_close_lvid functions
• System crashes occurring during UDF filesystem unmount operations
• KASAN (Kernel Address Sanitizer) reports indicating use-after-free in fs/udf/super.c
• Unexpected system reboots when processing UDF optical media or disk images

Detection Strategies

• Enable KASAN in development or testing environments to detect memory access violations in the UDF subsystem
• Monitor kernel logs for oops/panic messages containing references to fs/udf/super.c or related UDF functions
• Deploy endpoint detection solutions capable of monitoring for kernel exploitation attempts
• Use SentinelOne's behavioral AI to detect anomalous filesystem operations preceding system instability

Monitoring Recommendations

• Configure syslog forwarding to capture kernel messages for centralized analysis
• Implement alerting on kernel panic events, particularly those involving filesystem code paths
• Monitor for unusual UDF filesystem mount activities, especially from untrusted sources
• Review automount configurations to prevent automatic mounting of untrusted removable media

How to Mitigate CVE-2023-37454

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the fix (commit 6f861765464f43a71462d52026fbddfc858239a5)
• Disable UDF filesystem support if not required by adding blacklist udf to /etc/modprobe.d/blacklist.conf
• Restrict mounting privileges to trusted administrators only
• Disable automounting of removable media on security-sensitive systems

Patch Information

A fix has been committed to the upstream Linux kernel. The patch is available in the Linux Kernel Commit 6f86176. Users should update to kernel versions that include this commit, or apply the patch manually if running custom kernel builds.

For distribution-specific guidance, consult your Linux distribution's security advisories. SUSE users can reference the SUSE Bug Report CVE-2023-37454 for additional context and their specific assessment of this issue.

Workarounds

• Blacklist the UDF kernel module to prevent loading: echo "blacklist udf" >> /etc/modprobe.d/blacklist-udf.conf
• Remove the UDF module if currently loaded: modprobe -r udf
• Configure system policies to prevent mounting of untrusted filesystem images
• Implement mandatory access control (SELinux/AppArmor) policies to restrict filesystem mounting operations

# Configuration example - Disable UDF filesystem support
# Add UDF module to blacklist
echo "blacklist udf" | sudo tee /etc/modprobe.d/blacklist-udf.conf

# Remove module if currently loaded
sudo modprobe -r udf

# Prevent manual loading by alias
echo "install udf /bin/false" | sudo tee -a /etc/modprobe.d/blacklist-udf.conf

# Update initramfs to apply changes at boot
sudo update-initramfs -u