CVE-2026-31556: Linux Kernel Race Condition Vulnerability

CVE-2026-31556 Overview

A vulnerability has been identified in the Linux kernel's XFS filesystem quota scrub functionality. The flaw exists in the xchk_quota_item function, which can return early after calling xchk_fblock_process_error. When that helper function returns false, the code path returned immediately without properly releasing dq->q_qlock, resulting in a dquot lock being left in a held state. This improper lock management can lead to lock leaks or deadlocks during subsequent quota operations.

Critical Impact
Local attackers with low privileges can trigger a denial of service condition by exploiting this lock leak vulnerability, potentially causing system hangs or deadlocks in XFS quota operations.

Affected Products

Linux Kernel versions prior to patched releases
Linux Kernel version 6.8
Linux Kernel version 7.0 (RC1 through RC7)

Discovery Timeline

April 24, 2026 - CVE-2026-31556 published to NVD
April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31556

Vulnerability Analysis

This vulnerability represents a race condition and deadlock scenario in the Linux kernel's XFS online filesystem scrub feature. The XFS scrub mechanism is designed to validate and repair filesystem metadata, including quota information stored in dquot (disk quota) structures.

The core issue lies in the error handling path of xchk_quota_item. When the function calls xchk_fblock_process_error and that helper returns false (indicating an error condition), the code immediately returns without executing the cleanup code that would normally release the dq->q_qlock spinlock. This creates a situation where the lock remains held indefinitely.

The impact of this vulnerability is primarily availability-focused. When the lock is leaked, subsequent quota operations that require the same lock will block indefinitely, potentially causing:

System hangs during quota enforcement checks
Deadlocks when multiple quota operations compete for the same resource
Denial of service for processes performing XFS quota operations

Root Cause

The root cause is a missing lock release operation in an early return path within the xchk_quota_item function. The function acquires dq->q_qlock before processing quota items but fails to release it when xchk_fblock_process_error returns false, triggering an early exit. Proper error handling should ensure all acquired locks are released before any function return, regardless of the exit path taken.

Attack Vector

The vulnerability requires local access to the system with the ability to trigger XFS quota scrub operations. An attacker with low privileges could potentially trigger this condition by:

Initiating filesystem scrub operations on XFS-mounted partitions with quotas enabled
Creating error conditions that cause xchk_fblock_process_error to return false
Repeatedly triggering the vulnerable code path to accumulate leaked locks

The fix implemented by the kernel developers ensures that dq->q_qlock is properly unlocked before any early return in the xchk_quota_item function, preventing the lock from being leaked.

Detection Methods for CVE-2026-31556

Indicators of Compromise

Unexplained system hangs or freezes during XFS quota operations
Kernel log messages indicating lock contention or deadlock warnings in XFS subsystem
Process stuck states (D state in ps output) related to XFS or quota operations

Detection Strategies

Monitor kernel logs (dmesg, /var/log/kern.log) for XFS-related warnings or errors mentioning quota or dquot locks
Use lockdep kernel debugging feature to detect potential lock ordering issues or held locks
Implement monitoring for processes in uninterruptible sleep state that are blocked on XFS operations

Monitoring Recommendations

Enable XFS tracing via ftrace to monitor quota scrub operations and lock acquisition/release patterns
Configure alerting on sustained lock contention in the XFS subsystem
Monitor system call latencies for quota-related syscalls to detect anomalous blocking behavior

How to Mitigate CVE-2026-31556

Immediate Actions Required

Update the Linux kernel to a patched version that includes the fix for this vulnerability
If immediate patching is not possible, consider temporarily disabling XFS quota scrub operations on affected systems
Monitor affected systems for signs of lock contention or deadlocks in XFS operations

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix ensures proper unlocking of dq->q_qlock before any early return in the xchk_quota_item function. Patches are available through the following kernel git commits:

Organizations should apply kernel updates from their Linux distribution vendors as soon as they become available.

Workarounds

Avoid running XFS scrub operations (xfs_scrub) on production systems until the kernel is patched
If quotas are not required, consider disabling XFS quota functionality on affected filesystems
Implement process monitoring to automatically detect and alert on stuck quota operations

bash

# Check current kernel version
uname -r

# Verify XFS quota status on mounted filesystems
xfs_quota -x -c "state" /mount/point

# Temporarily disable quota enforcement (if acceptable for your environment)
xfs_quota -x -c "disable -ugp" /mount/point