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

> A race condition in KVM arm64 TLB invalidation logic can cause missed invalidations, potentially leading to denial of service.

- **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:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89915, DEBIAN-CVE-2026-89915, GHSA-crqv-c33h-f779, UBUNTU-CVE-2026-89915

## What is CVE-2026-89915?

CVE-2026-89915 is a critical vulnerability in Linux, published on 16 Sep 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:

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.

## Which versions are affected by CVE-2026-89915?


## How do I fix CVE-2026-89915?

Apply the vendor's patch.

## Is CVE-2026-89915 being exploited?

There is no known exploitation of CVE-2026-89915 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89915 and fix it if it is affected.

What it is: CVE-2026-89915 is a critical vulnerability in Linux, published on 16 Sep 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-2026-89915 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-2026-89915/ (Markdown: https://insights.sbomify.com/CVE-2026-89915/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89915
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89915
- OSV: https://osv.dev/vulnerability/CVE-2026-89915

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89915/
Summaries are generated and may contain mistakes; the official description above is the record.