Linux Kernel Free CPU Management Vulnerability in Deadline Scheduler

A vulnerability exists in the Linux kernel's deadline scheduling that improperly manages the free CPU mask for CPUs with offline runqueues. This issue can cause deadline tasks to be incorrectly scheduled on powered-down CPUs, preventing them from running. The vulnerability arises from changes made in previous commits that removed essential checks and introduced functions to manipulate the free CPU mask based on the runqueue's online state. Exploitation can occur when a CPU is disconnected from the default root domain after being offlined, or when tasks are migrated from an offline CPU, leaving the free CPU bit set erroneously.

Impact

The vulnerability can lead to improper task scheduling in the deadline scheduler, causing tasks to be sent to CPUs that are offline or powered down, and preventing them from executing as intended.

Reproduction

The vulnerability can be reproduced by creating a scenario where a CPU is offlined and then unplugged, while still having its free_cpus bit set in the deadline scheduler. This can happen when a deadline task is migrated from a CPU with an offlined runqueue, or when a CPU is transitioning through the default root domain after being offlined, pushing a deadline task to a powered-down CPU.

Remediation

The vulnerability has been addressed in the Linux kernel. Users should upgrade to the latest version where this issue has been fixed.