CVE-2018-20784

What it actually does

The vulnerability in kernel/sched/fair.c allows an attacker to trigger an infinite loop in update_blocked_averages by inducing high load on leaf cfs_rq structures, resulting in denial of service or potential unspecified impact.

Why it matters to the business

Systems may become unresponsive or crash under load, disrupting services and requiring reboots; widespread in Linux environments but mitigated by updates released years ago with no active exploitation campaigns noted.

How an attack unfolds

An attacker crafts workloads to force repeated scheduling events on affected cfs_rq structures, causing the kernel function to loop indefinitely and degrade or halt system operation.

How to fix it

Upgrade the Linux kernel to a version containing commit c40f7d74c741a907cfaeb73a7697081881c497d0 or later; apply vendor-supplied kernel packages and reboot affected systems.

How to confirm the fix worked

Verify the running kernel version is at or beyond the fixed commit; review scheduler-related source changes and test under high-load conditions to confirm absence of the loop.

Typical attack path

1. Gain ability to run processes or submit workloads on the target system
2. Induce sustained high CPU or scheduling load through application or synthetic traffic
3. Trigger the mishandled leaf cfs_rq path in the scheduler
4. Observe infinite loop leading to DoS or system instability

How to detect it

Examine kernel logs for repeated scheduler entries or hangs during high load; look for processes stuck in update_blocked_averages and correlate with workload spikes.

What your security team should do

• Prioritize kernel upgrades on all exposed Linux systems using the listed fixed commit
• Monitor for unusual scheduling or CPU behavior in production workloads
• Review and test high-load scenarios after patching to validate stability