# CVE-2026-90261: btrfs: zoned: flush active metadata block group at btree_writepages() start

> A flaw in the Linux kernel's btrfs zoned filesystem can cause a deadlock and force the filesystem read-only during metadata-heavy operations, requiring local access.

- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102258, BELL-CVE-2026-90261, DEBIAN-CVE-2026-90261, GHSA-6683-vc4x-94r5, UBUNTU-CVE-2026-90261

## What is CVE-2026-90261?

CVE-2026-90261 is a denial of service vulnerability in Linux, published on 17 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:

btrfs: zoned: flush active metadata block group at btree_writepages() start

btree_writepages() writes the btree inode's dirty metadata in ascending
logical address order. On a zoned filesystem only one metadata and one
system block group is active for writing at a time, and
check_bg_is_active() (via btrfs_check_meta_write_pointer()) pivots the
active block group as writeback moves from one block group to the next.

If the active block group sits at a higher logical address than another
block group that also holds dirty metadata, the ascending walk reaches
the lower one first and, to write it, has to finish the active block
group and activate the lower one. It cannot finish a block group that
still has unsent IO, and during WB_SYNC_ALL && !for_sync (commit)
writeback it deliberately refuses to wait for that IO under
fs_info->zoned_meta_io_lock, as that can deadlock. The pivot thus cannot
issue the submission itself either, so it gives up:
btrfs_check_meta_write_pointer() returns -EAGAIN, which
btrfs_write_and_wait_transaction() treats as fatal and aborts the
transaction, forcing the filesystem read-only. This happens
intermittently under metadata-heavy relocation (e.g. fstests btrfs/187).

Flush the active metadata and system block groups at the start of
btree_writepages(), under the fs_info->zoned_meta_io_lock it already
holds, so they have no unsent IO left and the later pivot can finish
them and make forward progress.

## Which versions are affected by CVE-2026-90261?


## How do I fix CVE-2026-90261?

Apply the vendor's patch.

## Is CVE-2026-90261 being exploited?

There is no known exploitation of CVE-2026-90261 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90261 and fix it if it is affected.

What it is: CVE-2026-90261 is a denial of service vulnerability in Linux, published on 17 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-2026-90261 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-2026-90261/ (Markdown: https://insights.sbomify.com/CVE-2026-90261/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90261
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90261
- OSV: https://osv.dev/vulnerability/CVE-2026-90261

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90261/
Summaries are generated and may contain mistakes; the official description above is the record.