# CVE-2024-49867: btrfs: wait for fixup workers before stopping cleaner kthread during umount

> A use-after-free vulnerability in the Linux kernel's btrfs filesystem during unmount can cause a system crash.

- **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:** 21 Oct 2024
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-51498, BELL-CVE-2024-49867, DEBIAN-CVE-2024-49867, GHSA-h7p9-f9r3-6533, UBUNTU-CVE-2024-49867

## What is CVE-2024-49867?

CVE-2024-49867 is a high-severity use after free vulnerability in Linux kernel, published on 21 Oct 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: wait for fixup workers before stopping cleaner kthread during umount

During unmount, at close_ctree(), we have the following steps in this order:

1) Park the cleaner kthread - this doesn't destroy the kthread, it basically
   halts its execution (wake ups against it work but do nothing);

2) We stop the cleaner kthread - this results in freeing the respective
   struct task_struct;

3) We call btrfs_stop_all_workers() which waits for any jobs running in all
   the work queues and then free the work queues.

Syzbot reported a case where a fixup worker resulted in a crash when doing
a delayed iput on its inode while attempting to wake up the cleaner at
btrfs_add_delayed_iput(), because the task_struct of the cleaner kthread
was already freed. This can happen during unmount because we don't wait
for any fixup workers still running before we call kthread_stop() against
the cleaner kthread, which stops and free all its resources.

Fix this by waiting for any fixup workers at close_ctree() before we call
kthread_stop() against the cleaner and run pending delayed iputs.

The stack traces reported by syzbot were the following:

  BUG: KASAN: slab-use-after-free in __lock_acquire+0x77/0x2050 kernel/locking/lockdep.c:5065
  Read of size 8 at addr ffff8880272a8a18 by task kworker/u8:3/52

  CPU: 1 UID: 0 PID: 52 Comm: kworker/u8:3 Not tainted 6.12.0-rc1-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
  Workqueue: btrfs-fixup btrfs_work_helper
  Call Trace:
   <TASK>
   __dump_stack lib/dump_stack.c:94 [inline]
   dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
   print_address_description mm/kasan/report.c:377 [inline]
   print_report+0x169/0x550 mm/kasan/report.c:488
   kasan_report+0x143/0x180 mm/kasan/report.c:601
   __lock_acquire+0x77/0x2050 kernel/locking/lockdep.c:5065
   lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5825
   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
   _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162
   class_raw_spinlock_irqsave_constructor include/linux/spinlock.h:551 [inline]
   try_to_wake_up+0xb0/0x1480 kernel/sched/core.c:4154
   btrfs_writepage_fixup_worker+0xc16/0xdf0 fs/btrfs/inode.c:2842
   btrfs_work_helper+0x390/0xc50 fs/btrfs/async-thread.c:314
   process_one_work kernel/workqueue.c:3229 [inline]
   process_scheduled_works+0xa63/0x1850 kernel/workqueue.c:3310
   worker_thread+0x870/0xd30 kernel/workqueue.c:3391
   kthread+0x2f0/0x390 kernel/kthread.c:389
   ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
   </TASK>

  Allocated by task 2:
   kasan_save_stack mm/kasan/common.c:47 [inline]
   kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
   unpoison_slab_object mm/kasan/common.c:319 [inline]
   __kasan_slab_alloc+0x66/0x80 mm/kasan/common.c:345
   kasan_slab_alloc include/linux/kasan.h:247 [inline]
   slab_post_alloc_hook mm/slub.c:4086 [inline]
   slab_alloc_node mm/slub.c:4135 [inline]
   kmem_cache_alloc_node_noprof+0x16b/0x320 mm/slub.c:4187
   alloc_task_struct_node kernel/fork.c:180 [inline]
   dup_task_struct+0x57/0x8c0 kernel/fork.c:1107
   copy_process+0x5d1/0x3d50 kernel/fork.c:2206
   kernel_clone+0x223/0x880 kernel/fork.c:2787
   kernel_thread+0x1bc/0x240 kernel/fork.c:2849
   create_kthread kernel/kthread.c:412 [inline]
   kthreadd+0x60d/0x810 kernel/kthread.c:765
   ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

  Freed by task 61:
   kasan_save_stack mm/kasan/common.c:47 [inline]
   kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
   kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:579
   poison_slab_object mm/kasan/common.c:247 [inline]
   __kasan_slab_free+0x59/0x70 mm/kasan/common.c:264
   kasan_slab_free include/linux/kasan.h:230 [inline]
   slab_free_h
---truncated---

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2024-49867?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-49867?

Apply the vendor's patch.

## Is CVE-2024-49867 being exploited?

There is no known exploitation of CVE-2024-49867 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-2024-49867 and fix it if it is affected.

What it is: CVE-2024-49867 is a high-severity use after free vulnerability in Linux kernel, published on 21 Oct 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-2024-49867 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-2024-49867/ (Markdown: https://insights.sbomify.com/CVE-2024-49867/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-49867
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-49867
- OSV: https://osv.dev/vulnerability/CVE-2024-49867

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-49867/
Summaries are generated and may contain mistakes; the official description above is the record.