# CVE-2021-47623: powerpc/fixmap: Fix VM debug warning on unmap

> A bug in the Linux kernel's powerpc fixmap handling triggers a VM debug warning during unmap, potentially causing a denial of service.

- **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:** 16 Jul 2024
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2021-47623, GHSA-cm9w-2x7j-gc65, UBUNTU-CVE-2021-47623

## What is CVE-2021-47623?

CVE-2021-47623 is a medium-severity denial of service vulnerability in Linux kernel, published on 16 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:

powerpc/fixmap: Fix VM debug warning on unmap

Unmapping a fixmap entry is done by calling __set_fixmap()
with FIXMAP_PAGE_CLEAR as flags.

Today, powerpc __set_fixmap() calls map_kernel_page().

map_kernel_page() is not happy when called a second time
for the same page.

	WARNING: CPU: 0 PID: 1 at arch/powerpc/mm/pgtable.c:194 set_pte_at+0xc/0x1e8
	CPU: 0 PID: 1 Comm: swapper Not tainted 5.16.0-rc3-s3k-dev-01993-g350ff07feb7d-dirty #682
	NIP:  c0017cd4 LR: c00187f0 CTR: 00000010
	REGS: e1011d50 TRAP: 0700   Not tainted  (5.16.0-rc3-s3k-dev-01993-g350ff07feb7d-dirty)
	MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 42000208  XER: 00000000

	GPR00: c0165fec e1011e10 c14c0000 c0ee2550 ff800000 c0f3d000 00000000 c001686c
	GPR08: 00001000 b00045a9 00000001 c0f58460 c0f50000 00000000 c0007e10 00000000
	GPR16: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
	GPR24: 00000000 00000000 c0ee2550 00000000 c0f57000 00000ff8 00000000 ff800000
	NIP [c0017cd4] set_pte_at+0xc/0x1e8
	LR [c00187f0] map_kernel_page+0x9c/0x100
	Call Trace:
	[e1011e10] [c0736c68] vsnprintf+0x358/0x6c8 (unreliable)
	[e1011e30] [c0165fec] __set_fixmap+0x30/0x44
	[e1011e40] [c0c13bdc] early_iounmap+0x11c/0x170
	[e1011e70] [c0c06cb0] ioremap_legacy_serial_console+0x88/0xc0
	[e1011e90] [c0c03634] do_one_initcall+0x80/0x178
	[e1011ef0] [c0c0385c] kernel_init_freeable+0xb4/0x250
	[e1011f20] [c0007e34] kernel_init+0x24/0x140
	[e1011f30] [c0016268] ret_from_kernel_thread+0x5c/0x64
	Instruction dump:
	7fe3fb78 48019689 80010014 7c630034 83e1000c 5463d97e 7c0803a6 38210010
	4e800020 81250000 712a0001 41820008 <0fe00000> 9421ffe0 93e1001c 48000030

Implement unmap_kernel_page() which clears an existing pte.

## Which versions are affected by CVE-2021-47623?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47623?

Apply the vendor's patch.

## Is CVE-2021-47623 being exploited?

There is no known exploitation of CVE-2021-47623 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-2021-47623 and fix it if it is affected.

What it is: CVE-2021-47623 is a medium-severity denial of service vulnerability in Linux kernel, published on 16 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-2021-47623 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-2021-47623/ (Markdown: https://insights.sbomify.com/CVE-2021-47623/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47623
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47623
- OSV: https://osv.dev/vulnerability/CVE-2021-47623

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47623/
Summaries are generated and may contain mistakes; the official description above is the record.