# CVE-2026-68421: sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

> A logic error in the Linux kernel scheduler causes false warnings during core scheduling, potentially leading to system instability or denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Aug 2026
- **Updated:** 21 Oct 2026
- **Also known as:** BELL-CVE-2026-68421, DEBIAN-CVE-2026-68421, GHSA-wg3q-w98j-244c, UBUNTU-CVE-2026-68421

## What is CVE-2026-68421?

CVE-2026-68421 is a denial of service vulnerability in Linux, published on 10 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

put_prev_task_scx() warns when a runnable task drops to a lower sched_class
without SCX_OPS_ENQ_LAST, on the assumption that balance_one() would have
kept it running. Core scheduling breaks that: a forced-idle SMT sibling
reschedules through the core_pick fast path in pick_next_task(), which skips
pick_task_scx() and thus balance_one(), so a runnable task can drop to idle
with ENQ_LAST unset.

Gate the warning on sched_cpu_cookie_match(): a cookie mismatch means core
scheduling forced the idle, while a match (or core scheduling off) still
catches a genuine missing-ENQ_LAST drop.

## Which versions are affected by CVE-2026-68421?


## How do I fix CVE-2026-68421?

Apply the vendor's patch.

## Is CVE-2026-68421 being exploited?

There is no known exploitation of CVE-2026-68421 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-68421 and fix it if it is affected.

What it is: CVE-2026-68421 is a denial of service vulnerability in Linux, published on 10 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-68421 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-2026-68421/ (Markdown: https://insights.sbomify.com/CVE-2026-68421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-68421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-68421
- OSV: https://osv.dev/vulnerability/CVE-2026-68421

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-68421/
Summaries are generated and may contain mistakes; the official description above is the record.