CVE-2026-23177: Linux Kernel Race Condition Vulnerability

CVE-2026-23177 Overview

A race condition vulnerability has been identified in the Linux kernel's memory management subsystem, specifically within the shared memory (shmem) component. The flaw exists in the shmem_free_swap() function during truncate operations on large swap entries, where improper index handling can lead to an infinite loop condition.

When truncating a large swap entry, shmem_free_swap() returns 0 when the entry's index doesn't match the given index due to lookup alignment. A specific scenario occurs when the index points to the middle of a large swap entry that doesn't cross the end boundary, causing find_get_entries() to repeatedly return the same entry, resulting in an infinite loop that can hang the system.

Critical Impact

System availability can be compromised through denial of service when truncate operations trigger the infinite loop condition in the kernel's shared memory subsystem.

Affected Products

• Linux Kernel (mm/shmem subsystem)
• Systems utilizing shared memory with large swap entries
• Linux distributions with vulnerable kernel versions

Discovery Timeline

• 2026-02-14 - CVE-2026-23177 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-23177

Vulnerability Analysis

The vulnerability resides in the shared memory truncation logic within the Linux kernel. When processing truncate operations on large swap entries, the kernel uses find_get_entries() to locate entries for removal. The function returns entries with their indices, and shmem_free_swap() validates that the entry's base index matches the requested index before proceeding with the free operation.

The race condition manifests when the truncate operation's index points to a position within a large swap entry (not at its base). In this scenario, find_get_entries() returns the large swap entry with indices[0] set to the requested index, but the actual base index of the entry is smaller. The shmem_free_swap() function performs a "base < index" check, which fails because the base is indeed smaller than the requested index, causing the function to return 0 without freeing the swap entry.

The failure fallback path then calls shmem_confirm_swap() to get the order and checks if the entry crosses the END boundary. If it doesn't cross the boundary, the code retries with the same index value. Since nothing has changed, the next iteration finds the same entry at the same index with the same conditions, creating an infinite loop.

Root Cause

The root cause is improper handling of index alignment when truncating large swap entries. The code fails to account for the scenario where the truncation index falls within the middle of a large swap entry that doesn't extend beyond the truncation end boundary. The retry logic uses the same unmodified index, causing the loop to repeat indefinitely without making progress.

Attack Vector

This vulnerability can be triggered through normal file system operations involving shared memory truncation. An attacker with local access could potentially craft specific truncate operations targeting large swap entries at particular index offsets to trigger the infinite loop condition, resulting in a denial of service through kernel hang.

The vulnerability requires:

1. A large swap entry to be present in the shared memory mapping
2. A truncate operation targeting an index within the middle of that entry
3. The swap entry must not cross the truncation end boundary

The fix addresses this by retrying with a round-down index and aborting if the calculated index falls below the truncate range, preventing the infinite loop condition.

Detection Methods for CVE-2026-23177

Indicators of Compromise

• Unexplained system hangs during file system operations
• Kernel processes stuck in D (uninterruptible sleep) state related to shmem operations
• High CPU utilization in kernel space without corresponding user-space activity
• System unresponsiveness following truncate operations on shared memory files

Detection Strategies

• Monitor for kernel soft lockup warnings in system logs (dmesg) referencing shmem or truncate operations
• Implement watchdog monitoring for system responsiveness degradation
• Use kernel tracing tools (ftrace, perf) to monitor shmem_free_swap() and shmem_confirm_swap() function call patterns
• Deploy SentinelOne Singularity platform for real-time kernel-level behavioral analysis

Monitoring Recommendations

• Enable kernel lockup detection (softlockup and hardlockup detectors)
• Configure alerting for elevated kernel CPU time without corresponding system activity
• Monitor /proc/vmstat for anomalous swap-related metrics
• Implement proactive kernel log analysis for truncation-related warnings

How to Mitigate CVE-2026-23177

Immediate Actions Required

• Update the Linux kernel to a patched version containing the fix
• Review systems with heavy shared memory usage for unexplained hangs
• Consider temporarily limiting access to shared memory operations on critical systems until patching is complete
• Monitor affected systems closely for signs of exploitation

Patch Information

The vulnerability has been addressed in the Linux kernel stable branches. The fix modifies the truncation retry logic to use a round-down index and adds boundary checking to abort if the index falls below the truncation range.

Patches are available from the following kernel git commits:

• Kernel Git Commit 2030dddf
• Kernel Git Commit 7b6a0f12
• Kernel Git Commit dfc3ab6b

Workarounds

• Limit local user access to systems where shared memory truncation is critical
• Implement resource limits (ulimit) on shared memory segment sizes to reduce exposure
• Consider using memory cgroups to isolate and monitor shared memory usage
• Deploy monitoring to detect and automatically restart hung processes

# Check current kernel version for vulnerability assessment
uname -r

# Monitor for shmem-related kernel warnings
dmesg -w | grep -i "shmem\|truncate\|lockup"

# Enable kernel soft lockup detector if not already active
echo 1 > /proc/sys/kernel/soft_watchdog