CVE-2024-43856: Linux Kernel Race Condition Vulnerability

CVE-2024-43856 Overview

CVE-2024-43856 is a race condition vulnerability in the Linux kernel's Direct Memory Access (DMA) subsystem. The flaw exists in the dmam_free_coherent() function, where an incorrect call order creates a window for concurrent tasks to cause resource tracking corruption. Specifically, the function frees a DMA allocation before removing its tracking entry from the devres list, allowing another task to potentially allocate memory at the same virtual address and register it in the devres list before the original entry is destroyed.

This timing issue can result in the wrong devres entry being freed, triggering warning conditions in dmam_match() and potentially leading to system instability or denial of service conditions on affected Linux systems.

Critical Impact

Local attackers with low privileges can exploit this race condition to cause denial of service by corrupting DMA resource tracking, potentially leading to kernel warnings and system instability.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux distributions (per LTS announcements)
• Systems utilizing DMA managed coherent memory allocations

Discovery Timeline

• August 17, 2024 - CVE-2024-43856 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2024-43856

Vulnerability Analysis

The vulnerability stems from a Time-of-Check Time-of-Use (TOCTOU) race condition in the kernel's managed DMA allocation subsystem. The dmam_free_coherent() function performs two operations in sequence: freeing the DMA memory allocation and then destroying the devres tracking entry. This ordering creates a critical race window.

When the DMA allocation is freed, the associated virtual address (vaddr) becomes immediately available for reuse by other kernel components. If a concurrent task requests a DMA allocation during this window, the kernel may assign the same vaddr to the new allocation and add a corresponding entry to the devres list. When the original devres_destroy() call executes, it searches for an entry matching the vaddr but may incorrectly match and destroy the new entry instead of the original one.

This condition triggers the WARN_ON() in dmam_match(), indicating resource tracking inconsistency. The practical impact includes kernel warnings, potential memory management corruption, and denial of service through system destabilization.

Root Cause

The root cause is improper operation ordering in dmam_free_coherent(). The function performs memory deallocation before resource tracking cleanup, violating the principle of removing tracking metadata before releasing the underlying resource. This creates a TOCTOU vulnerability where the state of the system can change between the free operation and the devres destruction.

The correct approach, implemented in the fix, is to destroy the devres entry first (while it still uniquely identifies the allocation being freed) and only then release the DMA memory. This ensures that no concurrent allocation can register a conflicting entry before the original is properly cleaned up.

Attack Vector

The attack vector requires local access to the system with the ability to trigger concurrent DMA allocations and deallocations. An attacker with low privileges could potentially exploit this by:

1. Triggering rapid DMA allocation/deallocation cycles through device drivers or kernel interfaces
2. Exploiting the race window to cause devres list corruption
3. Inducing kernel warnings or panics depending on system configuration

The vulnerability requires precise timing to exploit successfully. The attacker must trigger a new DMA allocation that receives the same vaddr during the brief window between dma_free_coherent() and devres_destroy() calls. While challenging, this can be achieved through repeated attempts or by leveraging predictable allocation patterns.

The fix reverses the operation order in dmam_free_coherent(), ensuring the devres entry is destroyed before the DMA memory is freed. This eliminates the race window entirely.

Detection Methods for CVE-2024-43856

Indicators of Compromise

• Kernel warning messages containing WARN_ON() triggered from dmam_match() function
• Unexpected entries in kernel logs related to DMA resource management failures
• System instability or crashes associated with DMA-intensive operations
• Multiple devres entries appearing for the same virtual address in debug output

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for warnings from the DMA subsystem, particularly dmam_match() warnings
• Implement kernel tracing using ftrace or eBPF to monitor dmam_free_coherent() and devres_destroy() call patterns
• Use SentinelOne Singularity platform to detect anomalous kernel behavior and DMA subsystem errors
• Deploy kernel auditing rules to track DMA allocation and deallocation sequences

Monitoring Recommendations

• Enable kernel warning logging at an appropriate verbosity level to capture DMA-related warnings
• Configure automated alerting for kernel oops or panic events related to memory management
• Regularly review system stability metrics on servers with high DMA utilization
• Use SentinelOne's behavioral AI to identify patterns consistent with race condition exploitation attempts

How to Mitigate CVE-2024-43856

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the fix for CVE-2024-43856
• Review and apply patches from vendor security advisories, including Debian LTS announcements
• Prioritize patching on systems with high DMA utilization or those exposed to local user access
• Monitor kernel logs for any signs of exploitation attempts while awaiting patch deployment

Patch Information

The fix has been merged into the stable Linux kernel branches. The patch corrects the operation order in dmam_free_coherent() by calling devres_destroy() before dma_free_coherent(), eliminating the race condition window. Multiple patch commits are available across kernel versions:

• Kernel patch 1fe97f68fce1
• Kernel patch 22094f5f52e7
• Kernel patch 257193083e8f
• Kernel patch 28e8b7406d3a

Distribution-specific updates are available through standard package management channels. Debian users should reference the Debian LTS security announcements for applicable updates.

Workarounds

• Limit local user access on systems where kernel updates cannot be immediately applied
• Reduce DMA allocation/deallocation frequency on critical systems where possible until patching is complete
• Monitor systems closely for kernel warnings indicating potential exploitation
• Consider implementing additional access controls on kernel interfaces that trigger DMA operations

# Check current kernel version and update on Debian-based systems
uname -r
sudo apt update && sudo apt upgrade linux-image-$(uname -r | cut -d'-' -f1-2)

# On RHEL/CentOS systems
sudo yum update kernel

# Verify patch application by checking kernel version
uname -r