# CVE-2026-89917: KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping

> A race condition in Linux KVM arm64 VNCR handling allows local attackers to trigger kernel crashes or potentially escalate privileges.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **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-2026-89917, DEBIAN-CVE-2026-89917, GHSA-crq7-3pj4-2vvg, UBUNTU-CVE-2026-89917

## What is CVE-2026-89917?

CVE-2026-89917 is a 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: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping

While VNCR TLB invalidation always occurs under the MMU lock,
vcpu_put() doesn't, while it unmaps the VNCR page.

The problem is that the invalidation evaluates vncr_tlb::cpu to
decide whether an unmapping needs to take place (cpu != -1) before
performing it. On the other hand, this_cpu_reset_vncr_fixmap()
unconditionally unmaps if L1_VNCR_MAPPED is set.

These two obviously can race, with a TOCTOU pattern on the TLBI
path, and a BUG_ON() on the vcpu_put() path. And the two can end-up
calling vncr_fixmap(-1), with extra lethal effects.

Move the reset of vncr_tlb::cpu to -1 to a common function, and make
this update atomic so that only a single thread can reset the field
and perform the corresponding unmap. The vcpu_put() still need to
unconditionally unmap the current VNCR to close another ugly race.

Finally, the assignment of vncr_tlb::cpu is moved to be kept in sync
with the actual mapping, similar to L1_VNCR_MAPPED being set.

## Which versions are affected by CVE-2026-89917?


## How do I fix CVE-2026-89917?

Apply the vendor's patch.

## Is CVE-2026-89917 being exploited?

There is no known exploitation of CVE-2026-89917 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89917 and fix it if it is affected.

What it is: CVE-2026-89917 is a 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-2026-89917 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-2026-89917/ (Markdown: https://insights.sbomify.com/CVE-2026-89917/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89917
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89917
- OSV: https://osv.dev/vulnerability/CVE-2026-89917

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89917/
Summaries are generated and may contain mistakes; the official description above is the record.