# CVE-2025-21771: sched_ext: Fix incorrect autogroup migration detection

> A logic error in the Linux kernel's sched_ext subsystem can cause incorrect task migration handling, leading to kernel warnings and potential system instability.

- **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:** 27 Feb 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-21771, DEBIAN-CVE-2025-21771, GHSA-49m4-j2fj-6qvh, UBUNTU-CVE-2025-21771

## What is CVE-2025-21771?

CVE-2025-21771 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Feb 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:

sched_ext: Fix incorrect autogroup migration detection

scx_move_task() is called from sched_move_task() and tells the BPF scheduler
that cgroup migration is being committed. sched_move_task() is used by both
cgroup and autogroup migrations and scx_move_task() tried to filter out
autogroup migrations by testing the destination cgroup and PF_EXITING but
this is not enough. In fact, without explicitly tagging the thread which is
doing the cgroup migration, there is no good way to tell apart
scx_move_task() invocations for racing migration to the root cgroup and an
autogroup migration.

This led to scx_move_task() incorrectly ignoring a migration from non-root
cgroup to an autogroup of the root cgroup triggering the following warning:

  WARNING: CPU: 7 PID: 1 at kernel/sched/ext.c:3725 scx_cgroup_can_attach+0x196/0x340
  ...
  Call Trace:
  <TASK>
    cgroup_migrate_execute+0x5b1/0x700
    cgroup_attach_task+0x296/0x400
    __cgroup_procs_write+0x128/0x140
    cgroup_procs_write+0x17/0x30
    kernfs_fop_write_iter+0x141/0x1f0
    vfs_write+0x31d/0x4a0
    __x64_sys_write+0x72/0xf0
    do_syscall_64+0x82/0x160
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix it by adding an argument to sched_move_task() that indicates whether the
moving is for a cgroup or autogroup migration. After the change,
scx_move_task() is called only for cgroup migrations and renamed to
scx_cgroup_move_task().

## Which versions are affected by CVE-2025-21771?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21771?

Apply the vendor's patch.

## Is CVE-2025-21771 being exploited?

There is no known exploitation of CVE-2025-21771 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-2025-21771 and fix it if it is affected.

What it is: CVE-2025-21771 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Feb 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-2025-21771 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-2025-21771/ (Markdown: https://insights.sbomify.com/CVE-2025-21771/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21771
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21771
- OSV: https://osv.dev/vulnerability/CVE-2025-21771

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21771/
Summaries are generated and may contain mistakes; the official description above is the record.