---
id: "CVE-2026-98653"
kind: "vulnerability"
aliases: ["CVE-2026-98653"]
title: "Linux KVM on s390: nested guest shadow fault returns wrong error"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["infrastructure", "kvm", "linux-kernel", "s390", "self-hosted"]
not_affected_if: ["no-nested-guests", "non-s390-architecture"]
published_at: "2026-10-18T16:17:18+00:00"
first_seen_at: "2026-10-23T21:57:16+00:00"
updated_at: "2026-10-25T05:19:37+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98653", "first_seen_at": "2026-10-23T21:57:16+00:00"}]
---

# Linux KVM on s390: nested guest shadow fault returns wrong error

In the Linux kernel's KVM for IBM s390, a nested guest's memory page that is missing from the parent guest mapping is reported as an error and propagated to userspace instead of being mapped. The sources describe this as a correctness bug fixed by linking the missing mapping, and do not state an attacker gain.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` on s390 hosts to see the kernel version, then compare against the fixed kernel release in the upstream commit.
- Check whether the host runs KVM nested virtualization on s390 (unknown from the sources).

## Fix

- Update the Linux kernel to a release that includes the upstream fix for _gaccess_shadow_fault() (the exact version is unknown from the sources).

## Timeline

- 2026-10-23T21:57:16+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98653

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

KVM: s390: Fix _gaccess_shadow_fault()

In some circumstances, it is possible that the page of nested guest
memory that is being shadowed is not present at all in the parent guest
gmap. dat_entry_walk() will not find any leaf entry and return with
-ENOENT, which will erroneously be propagated all the way to userspace.

Fix by manually calling gmap_link() on the memory of the nested guest
that is being shadowed if the mapping was not already present.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_