# CVE-2021-47454: powerpc/smp: do not decrement idle task preempt count in CPU offline

> A kernel bug in PowerPC CPU offline handling causes a scheduling while atomic crash, leading to denial of service when CPUs are hotplugged.

- **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:** 22 May 2024
- **Updated:** 23 May 2026
- **Also known as:** DEBIAN-CVE-2021-47454, GHSA-rcm4-5vxv-3g8q, UBUNTU-CVE-2021-47454

## What is CVE-2021-47454?

CVE-2021-47454 is a medium-severity denial of service vulnerability in Linux kernel, published on 22 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:

powerpc/smp: do not decrement idle task preempt count in CPU offline

With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we
get:

BUG: scheduling while atomic: swapper/1/0/0x00000000
no locks held by swapper/1/0.
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
Call Trace:
 dump_stack_lvl+0xac/0x108
 __schedule_bug+0xac/0xe0
 __schedule+0xcf8/0x10d0
 schedule_idle+0x3c/0x70
 do_idle+0x2d8/0x4a0
 cpu_startup_entry+0x38/0x40
 start_secondary+0x2ec/0x3a0
 start_secondary_prolog+0x10/0x14

This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:
Re-enable preemption before cpu_die()"), specifically "start_secondary()
expects a preempt_count() of 0."

However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle
task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:
Initialize the idle task with preemption disabled"), that justification no
longer holds.

The idle task isn't supposed to re-enable preemption, so remove the
vestigial preempt_enable() from the CPU offline path.

Tested with pseries and powernv in qemu, and pseries on PowerVM.

## Which versions are affected by CVE-2021-47454?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47454?

Apply the vendor's patch.

## Is CVE-2021-47454 being exploited?

There is no known exploitation of CVE-2021-47454 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-2021-47454 and fix it if it is affected.

What it is: CVE-2021-47454 is a medium-severity denial of service vulnerability in Linux kernel, published on 22 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-2021-47454 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-2021-47454/ (Markdown: https://insights.sbomify.com/CVE-2021-47454/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47454
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47454
- OSV: https://osv.dev/vulnerability/CVE-2021-47454

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47454/
Summaries are generated and may contain mistakes; the official description above is the record.