# CVE-2025-40303: btrfs: ensure no dirty metadata is written back for an fs with errors

> A use-after-free in the Linux kernel btrfs filesystem can cause crashes or potential code execution when dirty metadata is written back after worker threads are stopped.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 8 Dec 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-40303, DEBIAN-CVE-2025-40303, GHSA-5q2h-f93q-8fv2, ROOT-OS-DEBIAN-11-CVE-2025-40303, ROOT-OS-DEBIAN-12-CVE-2025-40303, ROOT-OS-DEBIAN-13-CVE-2025-40303, ROOT-OS-UBUNTU-2204-CVE-2025-40303, ROOT-OS-UBUNTU-2404-CVE-2025-40303, UBUNTU-CVE-2025-40303

## What is CVE-2025-40303?

CVE-2025-40303 is a high-severity use after free vulnerability in Linux, published on 8 Dec 2025. 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: ensure no dirty metadata is written back for an fs with errors

[BUG]
During development of a minor feature (make sure all btrfs_bio::end_io()
is called in task context), I noticed a crash in generic/388, where
metadata writes triggered new works after btrfs_stop_all_workers().

It turns out that it can even happen without any code modification, just
using RAID5 for metadata and the same workload from generic/388 is going
to trigger the use-after-free.

[CAUSE]
If btrfs hits an error, the fs is marked as error, no new
transaction is allowed thus metadata is in a frozen state.

But there are some metadata modifications before that error, and they are
still in the btree inode page cache.

Since there will be no real transaction commit, all those dirty folios
are just kept as is in the page cache, and they can not be invalidated
by invalidate_inode_pages2() call inside close_ctree(), because they are
dirty.

And finally after btrfs_stop_all_workers(), we call iput() on btree
inode, which triggers writeback of those dirty metadata.

And if the fs is using RAID56 metadata, this will trigger RMW and queue
new works into rmw_workers, which is already stopped, causing warning
from queue_work() and use-after-free.

[FIX]
Add a special handling for write_one_eb(), that if the fs is already in
an error state, immediately mark the bbio as failure, instead of really
submitting them.

Then during close_ctree(), iput() will just discard all those dirty
tree blocks without really writing them back, thus no more new jobs for
already stopped-and-freed workqueues.

The extra discard in write_one_eb() also acts as an extra safenet.
E.g. the transaction abort is triggered by some extent/free space
tree corruptions, and since extent/free space tree is already corrupted
some tree blocks may be allocated where they shouldn't be (overwriting
existing tree blocks). In that case writing them back will further
corrupting the fs.

## Which versions are affected by CVE-2025-40303?


## How do I fix CVE-2025-40303?

Apply the vendor's patch.

## Is CVE-2025-40303 being exploited?

There is no known exploitation of CVE-2025-40303 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40303 and fix it if it is affected.

What it is: CVE-2025-40303 is a high-severity use after free vulnerability in Linux, published on 8 Dec 2025. 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-2025-40303 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-2025-40303/ (Markdown: https://insights.sbomify.com/CVE-2025-40303/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40303
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40303
- OSV: https://osv.dev/vulnerability/CVE-2025-40303

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40303/
Summaries are generated and may contain mistakes; the official description above is the record.