CVE-2023-4732: Linux Kernel DoS Vulnerability

CVE-2023-4732 Overview

A race condition vulnerability was discovered in the pfn_swap_entry_to_page function within the memory management subsystem of the Linux Kernel. This flaw allows an attacker with local user privileges to trigger a denial of service condition through improper handling of memory page operations. The vulnerability is specifically tied to a BUG statement referencing pmd_t x, which can be exploited to crash the system.

Critical Impact

Local attackers with user-level privileges can cause system-wide denial of service by triggering a kernel BUG statement through race condition exploitation in the memory management subsystem.

Affected Products

• Linux Kernel (multiple versions)
• Red Hat Enterprise Linux 8.0 (x86_64, ARM64, IBM Z Systems, Power LE)
• Red Hat Enterprise Linux for Real Time 8.0
• Red Hat Enterprise Linux for Real Time for NFV 8.0
• Red Hat CodeReady Linux Builder 8.0 (x86_64, ARM64, Power LE)

Discovery Timeline

• 2023-10-03 - CVE-2023-4732 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-4732

Vulnerability Analysis

This vulnerability exists in the Linux Kernel's memory management subsystem, specifically within the pfn_swap_entry_to_page function. The flaw is categorized under CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and CWE-366 (Race Condition within a Thread), indicating a fundamental synchronization issue in how the kernel handles page frame number (PFN) to page conversions during swap operations.

The vulnerability requires local access and user-level privileges to exploit. The attack complexity is considered high due to the race condition nature of the flaw, meaning that successful exploitation requires precise timing to trigger the vulnerable code path.

Root Cause

The root cause stems from a race condition in the memory management code path involving the pfn_swap_entry_to_page function. When multiple threads or processes interact with the same memory pages during swap operations, improper synchronization allows a race condition to occur. This race condition can lead to the kernel encountering an unexpected state, triggering a BUG statement that references pmd_t x (page middle directory entry), causing an immediate kernel panic and system crash.

The lack of proper locking or atomic operations around critical sections in the swap entry handling code creates a window where concurrent accesses can produce inconsistent state.

Attack Vector

The attack requires local access to the system with standard user privileges. An attacker must be able to execute code that can manipulate memory operations in a way that triggers the race condition within the kernel's swap handling routines.

The exploitation involves creating conditions where multiple concurrent memory operations compete for access to the same page table entries during swap operations. When the timing is right, the race condition causes the kernel to hit the BUG assertion, resulting in a kernel panic and system crash. This denial of service impact affects system availability, as the entire system becomes unresponsive and requires a reboot.

Detection Methods for CVE-2023-4732

Indicators of Compromise

• Unexpected kernel panics or system crashes with BUG statements referencing pmd_t in the crash dump
• Kernel log messages indicating issues in pfn_swap_entry_to_page or related memory management functions
• Repeated system instability when running memory-intensive workloads with swap enabled

Detection Strategies

• Monitor kernel logs (/var/log/kern.log or dmesg) for BUG statements or oops messages related to memory management subsystems
• Deploy kernel-level monitoring to detect anomalous patterns of memory and swap operations that may indicate exploitation attempts
• Use crash dump analysis tools to identify signatures of this specific vulnerability in kernel panics

Monitoring Recommendations

• Enable kdump or similar crash collection mechanisms to capture kernel panic data for forensic analysis
• Configure alerting on kernel panic events through system monitoring solutions
• Monitor swap activity metrics for unusual patterns that could indicate exploitation attempts

How to Mitigate CVE-2023-4732

Immediate Actions Required

• Apply the latest kernel updates from your Linux distribution vendor, particularly the patches referenced in Red Hat Security Advisories
• Review system configurations for exposure to untrusted local users who could potentially exploit this vulnerability
• Consider restricting swap usage on highly sensitive systems until patches are applied

Patch Information

Red Hat has released several security advisories addressing this vulnerability:

• Red Hat Security Advisory RHSA-2023:6901
• Red Hat Security Advisory RHSA-2023:7077
• Red Hat Security Advisory RHSA-2023:7539
• Red Hat Security Advisory RHSA-2024:0412

For detailed vulnerability information and tracking, refer to the Red Hat CVE Analysis for CVE-2023-4732 and Red Hat Bug Report #2236982.

Workarounds

• Limit local user access to systems where the kernel cannot be immediately patched
• Consider disabling swap temporarily on critical systems if the business impact of a denial of service is severe and patching is not immediately possible
• Implement strict access controls to prevent untrusted users from gaining local shell access to vulnerable systems

# Check current kernel version for vulnerability status
uname -r

# Update kernel packages (RHEL/CentOS)
sudo yum update kernel

# Reboot to apply new kernel
sudo reboot