# CVE-2026-98538: writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes

> A local attacker can cause a denial of service via soft lockups in the Linux kernel writeback subsystem by triggering quadratic time complexity during inode cleanup.

- **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:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-108635, BELL-CVE-2026-98538, DEBIAN-CVE-2026-98538, GHSA-m444-q4wx-jpmh, UBUNTU-CVE-2026-98538

## What is CVE-2026-98538?

CVE-2026-98538 is a denial of service vulnerability in Linux, published on 18 Oct 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:

writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes

cleanup_offline_cgwb() prepares at most WB_MAX_INODES_PER_ISW inodes
per call and is called again until the dying wb is drained, but every
call walks wb->b_attached and then wb->b_dirty_time from the same end.
Inodes already prepared (they stay on the list with I_WB_SWITCH set
until the switch worker runs) and inodes that cannot be switched
(I_FREEING, I_WILL_FREE, !SB_ACTIVE, DAX, already on the target wb)
stay where they are, so each pass rescans a growing run of them under
wb->list_lock and a full drain is quadratic in the number of inodes on
the list. With ~17M inodes attached to one dying cgwb we saw this end
in soft lockups, with CPUs reported stuck for 21-48s.

Walk both lists from the oldest end and move every scanned inode to
the newest end, so the next pass starts where the previous one stopped
and the drain becomes linear. b_attached is unordered, so nobody sees
the reorder there. b_dirty_time is ordered by dirtied_when, but the
oldest unscanned inode stays at the end move_expired_inodes() picks
from, sync takes the whole list regardless of order, and prepared
inodes leave the list as soon as the switch work runs and get a new
dirtied_time_when on the new wb anyway, so the only inodes left out of
order are the ones that can never switch (DAX), and only on the dying
wb.

## Which versions are affected by CVE-2026-98538?


## How do I fix CVE-2026-98538?

Apply the vendor's patch.

## Is CVE-2026-98538 being exploited?

There is no known exploitation of CVE-2026-98538 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98538 and fix it if it is affected.

What it is: CVE-2026-98538 is a denial of service vulnerability in Linux, published on 18 Oct 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-2026-98538 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-2026-98538/ (Markdown: https://insights.sbomify.com/CVE-2026-98538/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98538
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98538
- OSV: https://osv.dev/vulnerability/CVE-2026-98538

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98538/
Summaries are generated and may contain mistakes; the official description above is the record.