# CVE-2026-89835: f2fs: avoid NULL checkpoint thread access in sysfs

> A local attacker with write access to sysfs can trigger a NULL pointer dereference in the Linux kernel f2fs module, causing a system crash.

- **Severity:** Not scored
- **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:** 16 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-101379, BELL-CVE-2026-89835, DEBIAN-CVE-2026-89835, GHSA-2hjf-mqww-7576, UBUNTU-CVE-2026-89835

## What is CVE-2026-89835?

CVE-2026-89835 is a vulnerability in Linux, published on 16 Sep 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:

f2fs: avoid NULL checkpoint thread access in sysfs

checkpoint_merge can be enabled even when no checkpoint merge thread is
running. A read-only mount is one case: f2fs does not start
f2fs_issue_ckpt there, but ckpt_thread_ioprio is still writable through
sysfs.

The ckpt_thread_ioprio store path updates the saved ioprio value and,
when checkpoint_merge is enabled, calls set_task_ioprio() for the
checkpoint thread. If cprc->f2fs_issue_ckpt is NULL, that dereferences a
NULL task pointer.

Protect ckpt_thread_ioprio sysfs writes with s_umount as well, so the
checkpoint thread cannot disappear under the store path while updating
its ioprio.

## Which versions are affected by CVE-2026-89835?


## How do I fix CVE-2026-89835?

Apply the vendor's patch.

## Is CVE-2026-89835 being exploited?

There is no known exploitation of CVE-2026-89835 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89835 and fix it if it is affected.

What it is: CVE-2026-89835 is a vulnerability in Linux, published on 16 Sep 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-2026-89835 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-2026-89835/ (Markdown: https://insights.sbomify.com/CVE-2026-89835/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89835
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89835
- OSV: https://osv.dev/vulnerability/CVE-2026-89835

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89835/
Summaries are generated and may contain mistakes; the official description above is the record.