CVE-2021-20317: Linux Kernel Timer Tree DOS Vulnerability

CVE-2021-20317 Overview

A flaw was found in the Linux kernel affecting the timer subsystem. A corrupted timer tree caused the task wakeup to be missing in the timerqueue_add function in lib/timerqueue.c. This vulnerability allows a local attacker with special user privileges to cause a denial of service condition, progressively slowing and eventually stopping the system while running OSP (OpenStack Platform).

Critical Impact

Local attackers with elevated privileges can exploit this timer tree corruption to cause system-wide denial of service, leading to complete system unavailability.

Affected Products

• Linux Kernel (multiple versions including 5.4)
• Debian Linux 9.0
• Debian Linux 10.0

Discovery Timeline

• 2021-09-27 - CVE-2021-20317 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20317

Vulnerability Analysis

This vulnerability stems from improper initialization (CWE-665) within the Linux kernel's timer queue implementation. The flaw resides in the timerqueue_add function located in lib/timerqueue.c, which is responsible for managing the kernel's timer tree data structure. When the timer tree becomes corrupted, the expected task wakeup mechanism fails to execute properly.

The vulnerability requires local access to the system and high privileges to exploit. While it does not impact confidentiality or integrity, it can severely affect system availability by causing the system to progressively degrade in performance until it becomes completely unresponsive.

Root Cause

The root cause of CVE-2021-20317 is improper initialization (CWE-665) in the timer queue subsystem. The timerqueue_add function fails to properly handle certain edge cases when managing the timer tree structure. This improper handling can lead to a corrupted timer tree state where scheduled task wakeups are missed, preventing time-critical kernel operations from executing properly.

Attack Vector

The attack vector is local, requiring an attacker to have authenticated access to the target system with elevated privileges. The exploitation scenario involves:

1. An attacker with special user privileges on a Linux system running vulnerable kernel versions
2. Triggering conditions that lead to timer tree corruption through the timerqueue_add function
3. The corrupted state causes missed task wakeups, progressively degrading system performance
4. Continued exploitation results in complete system denial of service

The vulnerability is particularly impactful in OpenStack Platform (OSP) environments where timer reliability is critical for virtualization and orchestration operations.

Detection Methods for CVE-2021-20317

Indicators of Compromise

• Unexplained system slowdowns that progressively worsen over time
• Kernel log messages indicating timer-related errors or warnings in lib/timerqueue.c
• Missed scheduled tasks or delayed cron jobs without apparent cause
• System becoming unresponsive without high CPU or memory utilization

Detection Strategies

• Monitor kernel logs (dmesg) for timer subsystem anomalies or error messages
• Implement system performance baseline monitoring to detect gradual degradation patterns
• Deploy kernel auditing to track privileged operations that may trigger the vulnerability
• Use SentinelOne Singularity platform to monitor for abnormal kernel behavior patterns

Monitoring Recommendations

• Enable kernel tracing for timer-related functions to detect corruption attempts
• Configure alerts for system performance degradation that follows the characteristic slowdown pattern
• Monitor for unusual privileged user activity on systems running vulnerable kernel versions
• Implement continuous kernel integrity monitoring through endpoint detection and response solutions

How to Mitigate CVE-2021-20317

Immediate Actions Required

• Update the Linux kernel to patched versions that address the timer tree corruption issue
• Review and restrict user accounts with elevated privileges that could exploit this vulnerability
• Apply vendor-specific patches from Debian (DSA-5096) and other distribution maintainers
• Prioritize patching on systems running OpenStack Platform due to heightened impact

Patch Information

The Linux kernel team has released a fix for this vulnerability. The patch is available through the Linux Kernel Commit Update (commit 511885d7061eda3eb1faf3f57dcc936ff75863f1).

Distribution-specific patches are available:

• Debian Security Advisory DSA-5096
• Debian LTS Announcement December 2021
• Debian LTS Announcement March 2022

Additional technical details can be found in the Red Hat Bug Report and the Siemens Security Certification.

Workarounds

• Implement strict access controls to limit users with elevated privileges on vulnerable systems
• Monitor privileged user sessions for suspicious timer-related activity
• Consider temporary isolation of critical systems running vulnerable kernel versions until patches can be applied
• Enable kernel watchdog mechanisms to automatically recover from system hangs caused by this vulnerability

# Check current kernel version for vulnerability assessment
uname -r

# Apply kernel updates on Debian-based systems
sudo apt update && sudo apt upgrade linux-image-$(uname -r)

# Verify the patch has been applied by checking kernel version
dpkg -l | grep linux-image