CVE-2026-23109 Overview
A vulnerability has been discovered in the Linux kernel's filesystem writeback mechanism, specifically in the wait_sb_inodes() function. The issue involves improper handling of AS_NO_DATA_INTEGRITY mappings, which can cause the kernel to wait indefinitely when synchronizing filesystems. This particularly affects FUSE (Filesystem in Userspace) implementations where a faulty FUSE server that does not reply to issued write requests can cause wait_sb_inodes() to hang forever, resulting in a denial of service condition.
Critical Impact
A faulty or malicious FUSE server can cause system hangs by not responding to write requests, leading to indefinite waiting in the writeback subsystem during filesystem sync operations.
Affected Products
- Linux kernel (versions prior to the security patch)
- Systems using FUSE (Filesystem in Userspace)
- Systems performing filesystem sync operations with non-data-integrity mappings
Discovery Timeline
- 2026-02-04 - CVE CVE-2026-23109 published to NVD
- 2026-02-05 - Last updated in NVD database
Technical Details for CVE-2026-23109
Vulnerability Analysis
The vulnerability exists in the Linux kernel's fs/writeback subsystem, specifically within the wait_sb_inodes() function. This function is responsible for waiting for all pages under writeback to complete for data integrity purposes during filesystem synchronization operations.
The core issue is that the function was not properly checking for AS_NO_DATA_INTEGRITY mappings before waiting. Filesystems like FUSE traditionally do not have data integrity semantics, meaning sync operations should effectively be no-ops for these filesystems. However, without the proper check, the kernel would attempt to wait for writeback completion on FUSE inodes.
When a FUSE server is faulty or unresponsive and does not reply to issued write requests, this causes the wait_sb_inodes() function to wait indefinitely, leading to a system hang. This represents a user-space regression where legitimate FUSE usage patterns that previously worked correctly now cause system-level hangs.
Root Cause
The root cause is a missing conditional check in the wait_sb_inodes() function that should skip inodes with AS_NO_DATA_INTEGRITY mappings. The function's documented behavior states it must wait for all pages under writeback for data integrity, but this should not apply to mappings that explicitly do not have data integrity semantics.
The fix involves adding a check to skip these mappings entirely, restoring FUSE to its prior behavior where sync operations are effectively no-ops for FUSE filesystems.
Attack Vector
The attack vector involves user-space control over FUSE server behavior. An attacker or faulty implementation can:
- Create a FUSE filesystem that accepts write requests
- Intentionally not reply to issued write requests from the kernel
- Trigger a filesystem sync operation (e.g., via sync() system call)
- The kernel enters wait_sb_inodes() and waits indefinitely for the FUSE writeback to complete
- System becomes unresponsive as the sync operation never completes
This vulnerability requires local access to mount a FUSE filesystem and can be triggered by any process capable of initiating sync operations on the affected filesystem.
Detection Methods for CVE-2026-23109
Indicators of Compromise
- System hangs during filesystem sync operations
- Processes stuck in uninterruptible sleep state (D state) related to writeback operations
- FUSE mount points that are unresponsive
- Kernel stack traces showing wait_sb_inodes() in a waiting state
Detection Strategies
- Monitor for processes in uninterruptible sleep (D state) with stack traces involving wait_sb_inodes()
- Track FUSE server response times and identify servers not responding to write requests
- Implement watchdog timers on critical sync operations
- Review kernel logs for writeback-related warnings or stalls
Monitoring Recommendations
- Set up alerts for abnormally long filesystem sync operations
- Monitor FUSE mount health and server responsiveness
- Track kernel CPU time spent waiting in writeback functions
- Implement system-level timeout monitoring for sync syscalls
How to Mitigate CVE-2026-23109
Immediate Actions Required
- Update the Linux kernel to a patched version containing the fix
- Review and audit any custom FUSE server implementations for proper write request handling
- Consider temporarily avoiding sync operations on FUSE filesystems until patched
- Implement process-level timeouts for sync operations in critical applications
Patch Information
The vulnerability has been resolved in the Linux kernel stable tree. The fix modifies wait_sb_inodes() to skip inodes with AS_NO_DATA_INTEGRITY mappings, restoring FUSE to its prior behavior where syncs are no-ops.
Patches are available via the kernel Git repository:
Workarounds
- Ensure FUSE servers properly respond to all write requests
- Avoid mounting untrusted FUSE filesystems on production systems
- Implement timeout mechanisms at the application level for sync operations
- Consider using alternative filesystems that do not rely on FUSE for critical operations
# Check kernel version and update
uname -r
# Update kernel via package manager (example for Debian/Ubuntu)
sudo apt update && sudo apt upgrade linux-image-$(uname -r)
# Verify FUSE mounts on the system
mount | grep fuse
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
