# CVE-2025-40209: btrfs: fix memory leak of qgroup_list in btrfs_add_qgroup_relation

> An unprivileged user can cause a kernel memory leak in btrfs by triggering invalid qgroup relations, potentially leading to denial of service.

- **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:** 21 Nov 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40209, DEBIAN-CVE-2025-40209, GHSA-2833-8jvf-3588, ROOT-OS-DEBIAN-13-CVE-2025-40209, UBUNTU-CVE-2025-40209

## What is CVE-2025-40209?

CVE-2025-40209 is a memory corruption vulnerability in Linux, published on 21 Nov 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: fix memory leak of qgroup_list in btrfs_add_qgroup_relation

When btrfs_add_qgroup_relation() is called with invalid qgroup levels
(src >= dst), the function returns -EINVAL directly without freeing the
preallocated qgroup_list structure passed by the caller. This causes a
memory leak because the caller unconditionally sets the pointer to NULL
after the call, preventing any cleanup.

The issue occurs because the level validation check happens before the
mutex is acquired and before any error handling path that would free
the prealloc pointer. On this early return, the cleanup code at the
'out' label (which includes kfree(prealloc)) is never reached.

In btrfs_ioctl_qgroup_assign(), the code pattern is:

    prealloc = kzalloc(sizeof(*prealloc), GFP_KERNEL);
    ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst, prealloc);
    prealloc = NULL;  // Always set to NULL regardless of return value
    ...
    kfree(prealloc);  // This becomes kfree(NULL), does nothing

When the level check fails, 'prealloc' is never freed by either the
callee or the caller, resulting in a 64-byte memory leak per failed
operation. This can be triggered repeatedly by an unprivileged user
with access to a writable btrfs mount, potentially exhausting kernel
memory.

Fix this by freeing prealloc before the early return, ensuring prealloc
is always freed on all error paths.

## Which versions are affected by CVE-2025-40209?


## How do I fix CVE-2025-40209?

Apply the vendor's patch.

## Is CVE-2025-40209 being exploited?

There is no known exploitation of CVE-2025-40209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40209 and fix it if it is affected.

What it is: CVE-2025-40209 is a memory corruption vulnerability in Linux, published on 21 Nov 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-2025-40209 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-2025-40209/ (Markdown: https://insights.sbomify.com/CVE-2025-40209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40209
- OSV: https://osv.dev/vulnerability/CVE-2025-40209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40209/
Summaries are generated and may contain mistakes; the official description above is the record.