# CVE-2024-35956: btrfs: qgroup: fix qgroup prealloc rsv leak in subvolume operations

> A memory leak in the Linux kernel's btrfs filesystem can lead to resource exhaustion and denial of service when quota groups are enabled.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 20 May 2024
- **Updated:** 23 May 2026
- **Also known as:** AZL-67700, BELL-CVE-2024-35956, DEBIAN-CVE-2024-35956, GHSA-3xrx-73h3-j99c, ROOT-OS-DEBIAN-11-CVE-2024-35956, ROOT-OS-UBUNTU-2204-CVE-2024-35956, UBUNTU-CVE-2024-35956

## What is CVE-2024-35956?

CVE-2024-35956 is a medium-severity denial of service vulnerability in Linux kernel, published on 20 May 2024. 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: qgroup: fix qgroup prealloc rsv leak in subvolume operations

Create subvolume, create snapshot and delete subvolume all use
btrfs_subvolume_reserve_metadata() to reserve metadata for the changes
done to the parent subvolume's fs tree, which cannot be mediated in the
normal way via start_transaction. When quota groups (squota or qgroups)
are enabled, this reserves qgroup metadata of type PREALLOC. Once the
operation is associated to a transaction, we convert PREALLOC to
PERTRANS, which gets cleared in bulk at the end of the transaction.

However, the error paths of these three operations were not implementing
this lifecycle correctly. They unconditionally converted the PREALLOC to
PERTRANS in a generic cleanup step regardless of errors or whether the
operation was fully associated to a transaction or not. This resulted in
error paths occasionally converting this rsv to PERTRANS without calling
record_root_in_trans successfully, which meant that unless that root got
recorded in the transaction by some other thread, the end of the
transaction would not free that root's PERTRANS, leaking it. Ultimately,
this resulted in hitting a WARN in CONFIG_BTRFS_DEBUG builds at unmount
for the leaked reservation.

The fix is to ensure that every qgroup PREALLOC reservation observes the
following properties:

1. any failure before record_root_in_trans is called successfully
   results in freeing the PREALLOC reservation.
2. after record_root_in_trans, we convert to PERTRANS, and now the
   transaction owns freeing the reservation.

This patch enforces those properties on the three operations. Without
it, generic/269 with squotas enabled at mkfs time would fail in ~5-10
runs on my system. With this patch, it ran successfully 1000 times in a
row.

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2024-35956?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-35956?

Apply the vendor's patch.

## Is CVE-2024-35956 being exploited?

There is no known exploitation of CVE-2024-35956 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-2024-35956 and fix it if it is affected.

What it is: CVE-2024-35956 is a medium-severity denial of service vulnerability in Linux kernel, published on 20 May 2024. 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-2024-35956 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-2024-35956/ (Markdown: https://insights.sbomify.com/CVE-2024-35956/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-35956
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-35956
- OSV: https://osv.dev/vulnerability/CVE-2024-35956

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-35956/
Summaries are generated and may contain mistakes; the official description above is the record.