# CVE-2026-64211: srcu: Don't queue workqueue handlers to never-online CPUs

> A flaw in the Linux kernel's SRCU implementation can cause system hangs on s390 architectures by queuing work on never-online CPUs.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Jul 2026
- **Updated:** 24 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-64211, GHSA-9gr3-vwh2-w2vv, ROOT-OS-UBUNTU-2204-CVE-2026-64211, UBUNTU-CVE-2026-64211

## What is CVE-2026-64211?

CVE-2026-64211 is a medium-severity denial of service vulnerability in Linux kernel, published on 24 Jul 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:

srcu: Don't queue workqueue handlers to never-online CPUs

While an srcu_struct structure is in the midst of switching from CPU-0
to all-CPUs state, it can attempt to invoke callbacks for CPUs that
have never been online.  Worse yet, it can attempt in invoke callbacks
for CPUs that never will be online, even including imaginary CPUs not in
cpu_possible_mask.  This can cause hangs on s390, which is not set up to
deal with workqueue handlers being scheduled on such CPUs.  This commit
therefore causes Tree SRCU to refrain from queueing workqueue handlers
on CPUs that have not yet (and might never) come online.

Because callbacks are not invoked on CPUs that have not been
online, it is an error to invoke call_srcu(), synchronize_srcu(), or
synchronize_srcu_expedited() on a CPU that is not yet fully online.
However, it turns out to be less code to redirect the callbacks
from too-early invocations of call_srcu() than to warn about such
invocations.  This commit therefore also redirects callbacks queued on
not-yet-fully-online CPUs to the boot CPU.

## Which versions are affected by CVE-2026-64211?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64211?

Apply the vendor's patch.

## Is CVE-2026-64211 being exploited?

There is no known exploitation of CVE-2026-64211 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64211 and fix it if it is affected.

What it is: CVE-2026-64211 is a medium-severity denial of service vulnerability in Linux kernel, published on 24 Jul 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-2026-64211 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-64211/ (Markdown: https://insights.sbomify.com/CVE-2026-64211/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64211
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64211
- OSV: https://osv.dev/vulnerability/CVE-2026-64211

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64211/
Summaries are generated and may contain mistakes; the official description above is the record.