# CVE-2024-42103: btrfs: fix adding block group to a reclaim list and the unused list during reclaim

> A race condition in the Linux kernel's Btrfs filesystem can cause list corruption and a kernel panic, 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:** 30 Jul 2024
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2024-42103, DEBIAN-CVE-2024-42103, GHSA-qj52-rm77-v6ch, UBUNTU-CVE-2024-42103

## What is CVE-2024-42103?

CVE-2024-42103 is a high-severity vulnerability in Linux kernel, published on 30 Jul 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: fix adding block group to a reclaim list and the unused list during reclaim

There is a potential parallel list adding for retrying in
btrfs_reclaim_bgs_work and adding to the unused list. Since the block
group is removed from the reclaim list and it is on a relocation work,
it can be added into the unused list in parallel. When that happens,
adding it to the reclaim list will corrupt the list head and trigger
list corruption like below.

Fix it by taking fs_info->unused_bgs_lock.

  [177.504][T2585409] BTRFS error (device nullb1): error relocating ch= unk 2415919104
  [177.514][T2585409] list_del corruption. next->prev should be ff1100= 0344b119c0, but was ff11000377e87c70. (next=3Dff110002390cd9c0)
  [177.529][T2585409] ------------[ cut here ]------------
  [177.537][T2585409] kernel BUG at lib/list_debug.c:65!
  [177.545][T2585409] Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
  [177.555][T2585409] CPU: 9 PID: 2585409 Comm: kworker/u128:2 Tainted: G        W          6.10.0-rc5-kts #1
  [177.568][T2585409] Hardware name: Supermicro SYS-520P-WTR/X12SPW-TF, BIOS 1.2 02/14/2022
  [177.579][T2585409] Workqueue: events_unbound btrfs_reclaim_bgs_work[btrfs]
  [177.589][T2585409] RIP: 0010:__list_del_entry_valid_or_report.cold+0x70/0x72
  [177.624][T2585409] RSP: 0018:ff11000377e87a70 EFLAGS: 00010286
  [177.633][T2585409] RAX: 000000000000006d RBX: ff11000344b119c0 RCX:0000000000000000
  [177.644][T2585409] RDX: 000000000000006d RSI: 0000000000000008 RDI:ffe21c006efd0f40
  [177.655][T2585409] RBP: ff110002e0509f78 R08: 0000000000000001 R09:ffe21c006efd0f08
  [177.665][T2585409] R10: ff11000377e87847 R11: 0000000000000000 R12:ff110002390cd9c0
  [177.676][T2585409] R13: ff11000344b119c0 R14: ff110002e0508000 R15:dffffc0000000000
  [177.687][T2585409] FS:  0000000000000000(0000) GS:ff11000fec880000(0000) knlGS:0000000000000000
  [177.700][T2585409] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [177.709][T2585409] CR2: 00007f06bc7b1978 CR3: 0000001021e86005 CR4:0000000000771ef0
  [177.720][T2585409] DR0: 0000000000000000 DR1: 0000000000000000 DR2:0000000000000000
  [177.731][T2585409] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:0000000000000400
  [177.742][T2585409] PKRU: 55555554
  [177.748][T2585409] Call Trace:
  [177.753][T2585409]  <TASK>
  [177.759][T2585409]  ? __die_body.cold+0x19/0x27
  [177.766][T2585409]  ? die+0x2e/0x50
  [177.772][T2585409]  ? do_trap+0x1ea/0x2d0
  [177.779][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.788][T2585409]  ? do_error_trap+0xa3/0x160
  [177.795][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.805][T2585409]  ? handle_invalid_op+0x2c/0x40
  [177.812][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.820][T2585409]  ? exc_invalid_op+0x2d/0x40
  [177.827][T2585409]  ? asm_exc_invalid_op+0x1a/0x20
  [177.834][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.843][T2585409]  btrfs_delete_unused_bgs+0x3d9/0x14c0 [btrfs]

There is a similar retry_list code in btrfs_delete_unused_bgs(), but it is
safe, AFAICS. Since the block group was in the unused list, the used bytes
should be 0 when it was added to the unused list. Then, it checks
block_group->{used,reserved,pinned} are still 0 under the
block_group->lock. So, they should be still eligible for the unused list,
not the reclaim list.

The reason it is safe there it's because because we're holding
space_info->groups_sem in write mode.

That means no other task can allocate from the block group, so while we
are at deleted_unused_bgs() it's not possible for other tasks to
allocate and deallocate extents from the block group, so it can't be
added to the unused list or the reclaim list by anyone else.

The bug can be reproduced by btrfs/166 after a few rounds. In practice
this can be hit when relocation cannot find more chunk space and ends
with ENOSPC.

## Which versions are affected by CVE-2024-42103?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-42103?

Apply the vendor's patch.

## Is CVE-2024-42103 being exploited?

There is no known exploitation of CVE-2024-42103 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-2024-42103 and fix it if it is affected.

What it is: CVE-2024-42103 is a high-severity vulnerability in Linux kernel, published on 30 Jul 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-2024-42103 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-2024-42103/ (Markdown: https://insights.sbomify.com/CVE-2024-42103/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-42103
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-42103
- OSV: https://osv.dev/vulnerability/CVE-2024-42103

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-42103/
Summaries are generated and may contain mistakes; the official description above is the record.