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


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 23 Oct 2026
- **Also known as:** BELL-CVE-2026-99085, DEBIAN-CVE-2026-99085, GHSA-7qjf-qv2c-rr8m, UBUNTU-CVE-2026-99085

## What is CVE-2026-99085?

CVE-2026-99085 is a vulnerability in Linux, published on 23 Oct 2026. 

### Official 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.

## Which versions are affected by CVE-2026-99085?


## How do I fix CVE-2026-99085?

Upgrade to a fixed version.

## Is CVE-2026-99085 being exploited?

There is no known exploitation of CVE-2026-99085 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99085 and fix it if it is affected.

What it is: CVE-2026-99085 is a vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99085 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-99085/ (Markdown: https://insights.sbomify.com/CVE-2026-99085/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99085
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99085
- OSV: https://osv.dev/vulnerability/CVE-2026-99085

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99085/
Summaries are generated and may contain mistakes; the official description above is the record.