# CVE-2025-37856: btrfs: harden block_group::bg_list against list_del() races

> A race condition in the Linux kernel's btrfs filesystem can cause a refcount underflow and kernel warning, potentially leading to denial of service.

- **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:** 9 May 2025
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-62738, AZL-70141, BELL-CVE-2025-37856, DEBIAN-CVE-2025-37856, GHSA-ppp2-86mr-9vh3, ROOT-OS-DEBIAN-11-CVE-2025-37856, ROOT-OS-DEBIAN-12-CVE-2025-37856, ROOT-OS-UBUNTU-2204-CVE-2025-37856, ROOT-OS-UBUNTU-2404-CVE-2025-37856, UBUNTU-CVE-2025-37856

## What is CVE-2025-37856?

CVE-2025-37856 is a high-severity vulnerability in Linux kernel, published on 9 May 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: harden block_group::bg_list against list_del() races

As far as I can tell, these calls of list_del_init() on bg_list cannot
run concurrently with btrfs_mark_bg_unused() or btrfs_mark_bg_to_reclaim(),
as they are in transaction error paths and situations where the block
group is readonly.

However, if there is any chance at all of racing with mark_bg_unused(),
or a different future user of bg_list, better to be safe than sorry.

Otherwise we risk the following interleaving (bg_list refcount in parens)

T1 (some random op)                       T2 (btrfs_mark_bg_unused)
                                        !list_empty(&bg->bg_list); (1)
list_del_init(&bg->bg_list); (1)
                                        list_move_tail (1)
btrfs_put_block_group (0)
                                        btrfs_delete_unused_bgs
                                             bg = list_first_entry
                                             list_del_init(&bg->bg_list);
                                             btrfs_put_block_group(bg); (-1)

Ultimately, this results in a broken ref count that hits zero one deref
early and the real final deref underflows the refcount, resulting in a WARNING.

## Which versions are affected by CVE-2025-37856?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-37856?

Apply the vendor's patch.

## Is CVE-2025-37856 being exploited?

There is no known exploitation of CVE-2025-37856 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-37856 and fix it if it is affected.

What it is: CVE-2025-37856 is a high-severity vulnerability in Linux kernel, published on 9 May 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-2025-37856 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2025-37856/ (Markdown: https://insights.sbomify.com/CVE-2025-37856/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-37856
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-37856
- OSV: https://osv.dev/vulnerability/CVE-2025-37856

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-37856/
Summaries are generated and may contain mistakes; the official description above is the record.