# CVE-2026-99205: sched/core: Call wakeup_preempt() for queued tasks during class change


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-99205, GHSA-2frc-h2g3-59hw, UBUNTU-CVE-2026-99205

## What is CVE-2026-99205?

CVE-2026-99205 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

sched/core: Call wakeup_preempt() for queued tasks during class change

Paul reported recent RCU changes exposed a lost resched where CPU idles
with a rcutorture thread waiting while being completely runnable.

Dumping the rq state at the time of idling with runnable tasks showed
the following:

  rq->next_class = &fair_sched_class
  rq->nr_running = 1
  rq->cfs.h_nr_queued = 0
  rq->rt.rt_nr_running = 1

Although the waiting task was an RT task, "rq->next_class" did not point
to rt_sched_class and was stuck at the lower priority fair_sched_class.

Inspecting sched_change_end() revealed the following race with
__sched_setscheduler() for queued, but non-running tasks:

    CPU0                                   CPU1
    ====                                   ====

  __schedule()

    /*
     * rq->donor is a fair task that blocks.
     * rq->next_class = &fair_sched_class
     */

    rq_lock(rq0)

    pick_next_task()
      /* nr_running = 0 */
      pick_task_fair()
        sched_balance_newidle()
          raw_spin_rq_unlock(rq0)
                                    rq_lock(rq0)

                                    activate_task(rq0, p)
                                      /* p is a fair task */
                                      rq0->nr_running++;
                                      rq0->cfs.h_nr_queued++;
                                      wakeup_preempt(rq, p)
                                        rq->next_class = &fair_sched_class
                                        resched_curr()

                                    rq_unlock(rq0)

                                    __sched_setscheduler(p)
                                      /* p is moved to SCHED_RR */
                                      task_rq_lock(p)
                                      sched_change_begin()

				      rq0->cfs.h_nr_queued--;
				      rq0->rt.rt_nr_running++;

                                      sched_change_end()
                                        /*
                                         * p->on_rq = 1 (ctx->queued)
                                         * but p is not rq->donor
                                         * (!ctx->running) hence
                                         * resched_curr() is and
                                         * rq->next_class upgrade is
                                         * skipped.
                                         */

                                       task_rq_unlock()

          raw_spin_rq_lock(rq0)

          /*
           * rq->nr_running = 1
           * rq->cfs.h_nr_queued = 0
           * rq->rt.rt_nr_running = 1
           * rq->next_class = &fair_sched_class
           */

           if (rq->cfs.h_nr_queued && ...)
             /* Above is false */

           if (rq_modified_above(this_rq, &fair_sched_class))
             /* Also false since rq->next_class is still fair */

      /*
       * sched_balance_newidle() is considered to
       * have failed to pull any fair tasks and no
       * other wakeups were detected.
       */
       return NULL; /* pick_task_fair() */

    pick_task_idle()
      return rq->idle;

    next = rq->idle;

    /* Preemption indicators are lost */
    clear_tsk_need_resched(prev);
    clear_preempt_need_resched();

    rq->curr = rq->idle;

  !!! rq->idle switches in with rq->running == 1 but !need_resched() !!!

In simple words, enqueue, followed by a class change of the newly queued
task goes unnoticed in rq->next_class indicator and
sched_balance_newidle() sees no fair task, nor any higher priority task
enqueued.

The above sequence will lead to CPU idling with a runnable task on the
CPU.

Call wakeup_preempt() for ctx->queued cases in sched_change_end() to
re-evaluate rq->next_class after class change (ENQUEUE_CLASS) and set
the NEED_RESCHED indicator accordingly.

Newidle balance will correctly see an enqueue of a higher priority
t
---truncated---

## Which versions are affected by CVE-2026-99205?


## How do I fix CVE-2026-99205?

Upgrade to a fixed version.

## Is CVE-2026-99205 being exploited?

There is no known exploitation of CVE-2026-99205 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99205 and fix it if it is affected.

What it is: CVE-2026-99205 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99205 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-2026-99205/ (Markdown: https://insights.sbomify.com/CVE-2026-99205/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99205
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99205
- OSV: https://osv.dev/vulnerability/CVE-2026-99205

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99205/
Summaries are generated and may contain mistakes; the official description above is the record.