# CVE-2025-21634: cgroup/cpuset: remove kernfs active break

> A race condition in the Linux kernel's cgroup/cpuset subsystem can trigger a kernel warning and potential denial of service during concurrent cgroup removal and cpuset updates.

- **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:** 19 Jan 2025
- **Updated:** 11 May 2026
- **Also known as:** AZL-56252, AZL-56318, BELL-CVE-2025-21634, CLEANSTART-2026-LS90448, DEBIAN-CVE-2025-21634, GHSA-v62x-6v8m-46vv, ROOT-OS-DEBIAN-11-CVE-2025-21634, ROOT-OS-DEBIAN-12-CVE-2025-21634, ROOT-OS-UBUNTU-2204-CVE-2025-21634, UBUNTU-CVE-2025-21634

## What is CVE-2025-21634?

CVE-2025-21634 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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:

cgroup/cpuset: remove kernfs active break

A warning was found:

WARNING: CPU: 10 PID: 3486953 at fs/kernfs/file.c:828
CPU: 10 PID: 3486953 Comm: rmdir Kdump: loaded Tainted: G
RIP: 0010:kernfs_should_drain_open_files+0x1a1/0x1b0
RSP: 0018:ffff8881107ef9e0 EFLAGS: 00010202
RAX: 0000000080000002 RBX: ffff888154738c00 RCX: dffffc0000000000
RDX: 0000000000000007 RSI: 0000000000000004 RDI: ffff888154738c04
RBP: ffff888154738c04 R08: ffffffffaf27fa15 R09: ffffed102a8e7180
R10: ffff888154738c07 R11: 0000000000000000 R12: ffff888154738c08
R13: ffff888750f8c000 R14: ffff888750f8c0e8 R15: ffff888154738ca0
FS:  00007f84cd0be740(0000) GS:ffff8887ddc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000555f9fbe00c8 CR3: 0000000153eec001 CR4: 0000000000370ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 kernfs_drain+0x15e/0x2f0
 __kernfs_remove+0x165/0x300
 kernfs_remove_by_name_ns+0x7b/0xc0
 cgroup_rm_file+0x154/0x1c0
 cgroup_addrm_files+0x1c2/0x1f0
 css_clear_dir+0x77/0x110
 kill_css+0x4c/0x1b0
 cgroup_destroy_locked+0x194/0x380
 cgroup_rmdir+0x2a/0x140

It can be explained by:
rmdir 				echo 1 > cpuset.cpus
				kernfs_fop_write_iter // active=0
cgroup_rm_file
kernfs_remove_by_name_ns	kernfs_get_active // active=1
__kernfs_remove					  // active=0x80000002
kernfs_drain			cpuset_write_resmask
wait_event
//waiting (active == 0x80000001)
				kernfs_break_active_protection
				// active = 0x80000001
// continue
				kernfs_unbreak_active_protection
				// active = 0x80000002
...
kernfs_should_drain_open_files
// warning occurs
				kernfs_put_active

This warning is caused by 'kernfs_break_active_protection' when it is
writing to cpuset.cpus, and the cgroup is removed concurrently.

The commit 3a5a6d0c2b03 ("cpuset: don't nest cgroup_mutex inside
get_online_cpus()") made cpuset_hotplug_workfn asynchronous, This change
involves calling flush_work(), which can create a multiple processes
circular locking dependency that involve cgroup_mutex, potentially leading
to a deadlock. To avoid deadlock. the commit 76bb5ab8f6e3 ("cpuset: break
kernfs active protection in cpuset_write_resmask()") added
'kernfs_break_active_protection' in the cpuset_write_resmask. This could
lead to this warning.

After the commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug
processing synchronous"), the cpuset_write_resmask no longer needs to
wait the hotplug to finish, which means that concurrent hotplug and cpuset
operations are no longer possible. Therefore, the deadlock doesn't exist
anymore and it does not have to 'break active protection' now. To fix this
warning, just remove kernfs_break_active_protection operation in the
'cpuset_write_resmask'.

Weakness: CWE-667 (Improper Locking)

## Which versions are affected by CVE-2025-21634?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21634?

Apply the vendor's patch.

## Is CVE-2025-21634 being exploited?

There is no known exploitation of CVE-2025-21634 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-2025-21634 and fix it if it is affected.

What it is: CVE-2025-21634 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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-2025-21634 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-2025-21634/ (Markdown: https://insights.sbomify.com/CVE-2025-21634/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21634
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21634
- OSV: https://osv.dev/vulnerability/CVE-2025-21634

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21634/
Summaries are generated and may contain mistakes; the official description above is the record.