CVE-2023-40791: Linux Kernel Use-After-Free Vulnerability

CVE-2023-40791 Overview

A memory management vulnerability exists in the Linux kernel's extract_user_to_sg function located in lib/scatterlist.c. In versions prior to 6.4.12, the function fails to properly unpin pages under certain conditions, leading to memory leaks and potential system instability. This issue is demonstrated by a WARNING message generated for try_grab_page, indicating improper handling of pinned user pages in the scatterlist memory management subsystem.

Critical Impact

Local attackers with low privileges could exploit this page unpinning failure to cause integrity and availability impacts through memory exhaustion or corruption scenarios.

Affected Products

• Linux Kernel (versions prior to 6.4.12)
• NetApp H300S/H300S Firmware
• NetApp H500S/H500S Firmware
• NetApp H700S/H700S Firmware
• NetApp H410S/H410S Firmware

Discovery Timeline

• 2023-10-16 - CVE CVE-2023-40791 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-40791

Vulnerability Analysis

The vulnerability resides in the kernel's scatterlist memory management code, specifically within the extract_user_to_sg function. This function is responsible for handling user-space memory pages and mapping them into scatter-gather lists for efficient I/O operations. The flaw occurs when the function encounters an error condition but fails to properly release (unpin) the memory pages that were previously pinned.

When user pages are pinned for DMA or other I/O operations, the kernel increases a reference count to prevent those pages from being freed or swapped out. If these pages are not properly unpinned when an error occurs, the reference counts remain elevated, preventing proper memory reclamation. Over time, this can lead to memory exhaustion as pages accumulate without being properly released.

The vulnerability requires local access and elevated complexity to exploit, but an authenticated attacker could potentially trigger the error condition repeatedly to exhaust system memory resources or cause data integrity issues.

Root Cause

The root cause stems from incomplete error handling in the extract_user_to_sg function. When constructing a scatterlist from user-space memory, the function pins pages into memory to ensure they remain available during I/O operations. However, in certain failure paths, the cleanup code does not properly iterate through and unpin all previously pinned pages before returning an error. This results in orphaned page pins that persist beyond their intended lifetime, manifesting as the try_grab_page warning observed in kernel logs.

Attack Vector

The attack vector is local, requiring an authenticated user with low privileges to interact with kernel subsystems that utilize scatterlist operations. The attacker would need to craft specific operations that trigger the vulnerable code path in extract_user_to_sg, causing the function to fail after pinning pages but before the scatterlist is fully constructed. By repeatedly triggering this condition, an attacker could gradually exhaust system memory or cause integrity issues.

The vulnerability mechanism involves:

1. A local user initiates an operation that requires scatterlist construction from user memory
2. The extract_user_to_sg function begins pinning user pages
3. An error condition is encountered mid-operation
4. The error handling path fails to unpin the already-pinned pages
5. Memory reference counts remain elevated, preventing proper page reclamation

For detailed technical analysis, refer to the Linux Kernel Commit f443fd5 and the Linux Crypto Discussion Thread.

Detection Methods for CVE-2023-40791

Indicators of Compromise

• Kernel warning messages related to try_grab_page in system logs
• Unexplained memory pressure or gradual memory exhaustion on systems
• Elevated page reference counts visible in memory debugging tools
• System instability or performance degradation over time without apparent cause

Detection Strategies

• Monitor kernel logs (dmesg or /var/log/kern.log) for WARNING messages containing try_grab_page or extract_user_to_sg
• Implement memory monitoring to detect abnormal pinned page accumulation over time
• Use kernel memory debugging tools such as kmemleak to identify unreleased page references
• Deploy SentinelOne Singularity platform for real-time kernel-level threat detection and anomaly monitoring

Monitoring Recommendations

• Enable kernel memory debugging options during testing to identify page leak patterns
• Configure alerting for unusual memory consumption patterns on affected systems
• Regularly review kernel version deployment across infrastructure to identify unpatched systems
• Implement centralized log aggregation to correlate try_grab_page warnings across multiple hosts

How to Mitigate CVE-2023-40791

Immediate Actions Required

• Update Linux kernel to version 6.4.12 or later on all affected systems
• Review and update NetApp H-series firmware to patched versions as per NetApp Security Advisory ntap-20231110-0009
• Prioritize patching for systems running crypto or I/O-intensive workloads that heavily utilize scatterlist operations
• Monitor affected systems for signs of memory exhaustion until patches are applied

Patch Information

The vulnerability is resolved in Linux kernel version 6.4.12 through commit f443fd5af5dbd531f880d3645d5dd36976cf087f. This patch ensures that all pinned pages are properly unpinned during error conditions in the extract_user_to_sg function. The fix implements proper cleanup logic that iterates through pinned pages and releases them before returning error conditions.

For detailed patch information, see the Linux Kernel Change Log 6.4.12 and the Linux Kernel Commit f443fd5.

Workarounds

• Limit local user access to systems running vulnerable kernel versions where possible
• Implement resource quotas and memory limits through cgroups to contain potential memory exhaustion attacks
• Increase system monitoring and alerting thresholds for memory-related anomalies
• Consider isolating workloads that heavily utilize scatter-gather I/O operations

# Check current kernel version
uname -r

# Verify if system is running patched kernel (6.4.12 or later)
if [[ $(uname -r | cut -d. -f1-3) < "6.4.12" ]]; then
    echo "WARNING: System may be vulnerable to CVE-2023-40791"
    echo "Current kernel: $(uname -r)"
    echo "Recommended: Update to kernel 6.4.12 or later"
fi