# CVE-2026-72412: s390/mm: Fix handling of _PAGE_UNUSED pte bit

> A flaw in the Linux kernel s390 memory management allows guest memory corruption and potential denial of service due to improper handling of the _PAGE_UNUSED bit.

- **Severity:** Critical 9.3 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-72412, GHSA-94jh-p9fj-qxf8, UBUNTU-CVE-2026-72412

## What is CVE-2026-72412?

CVE-2026-72412 is a critical memory corruption vulnerability in Linux, published on 15 Aug 2026. 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:

s390/mm: Fix handling of _PAGE_UNUSED pte bit

The _PAGE_UNUSED softbit should not really be lying around. Its sole
purpose is to signal to try_to_unmap_one() and try_to_migrate_one()
that the page can be discarded instead of being moved / swapped.

KVM has no way to know why a page is being unmapped, so it sets the bit
on userspace ptes corresponding to unused guest pages every time they
get unmapped. KVM has no reasonable way to clear the bit once the page
is in use again.

While set_ptes() checks and clears the bit, other paths that set new
ptes did not. This led to used pages being thrown out as if they were
unused, causing guest corruption.

Fix the issue by clearing the _PAGE_UNUSED bit for present ptes in
set_pte(), i.e. whenever a present pte is getting set. The check in
set_ptes() is then redundant and can be removed.

Also fix gmap_helper_try_set_pte_unused() to only set the bit if the
pte is present; the _PAGE_UNUSED bit is only defined for present ptes
and thus should not be set for non-present ptes.

## Which versions are affected by CVE-2026-72412?


## How do I fix CVE-2026-72412?

Apply the vendor's patch.

## Is CVE-2026-72412 being exploited?

There is no known exploitation of CVE-2026-72412 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72412 and fix it if it is affected.

What it is: CVE-2026-72412 is a critical memory corruption vulnerability in Linux, published on 15 Aug 2026. 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-2026-72412 in the wild and no public exploit code.

Affected product: Linux. 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-2026-72412/ (Markdown: https://insights.sbomify.com/CVE-2026-72412/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72412
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72412
- OSV: https://osv.dev/vulnerability/CVE-2026-72412

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72412/
Summaries are generated and may contain mistakes; the official description above is the record.