CVE-2021-20317

What it actually does

A corrupted timer tree in timerqueue_add prevents proper task wakeup, resulting in progressive system slowdown and eventual denial of service under local high-privilege conditions.

Why it matters to the business

Systems may become unresponsive during operation, leading to service outages, lost productivity, and the need for manual restarts or kernel upgrades across Linux deployments.

How an attack unfolds

An attacker with special user privileges crafts input that corrupts the timer tree structure; when timerqueue_add is invoked the missing wakeup causes accumulating timer failures that degrade and halt system operation.

How to fix it

Update the Linux kernel to the fixed commit 219d54332a09e8d8741c1e1982f5eae56099de85 or later stable releases that contain the timerqueue fix; rebuild and reboot affected systems.

How to confirm the fix worked

Confirm the running kernel contains commit 219d54332a09e8d8741c1e1982f5eae56099de85 via uname or source inspection; test timerqueue behavior under load to verify absence of the corruption path.

Typical attack path

1. Gain access with special user privileges on a Linux system
2. Trigger conditions that corrupt the timer tree data structure
3. Invoke code paths reaching timerqueue_add in lib/timerqueue.c
4. Observe progressive slowdown culminating in full denial of service

How to detect it

Review kernel logs for timerqueue anomalies or stalled task wakeups; correlate with high-privilege process activity around periods of system slowdown.

What your security team should do

• Prioritize kernel upgrades on all exposed Linux hosts to the fixed commit
• Review systems requiring special user privileges and apply least-privilege controls
• Monitor for availability anomalies consistent with timer-related stalls
• Include the fix in standard patch and reboot cycles