# CVE-2026-89775: KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation

> A flaw in Linux kernel KVM arm64 TLB invalidation logic causes incorrect size calculations when the MMU is disabled, 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-2026-89775, DEBIAN-CVE-2026-89775, GHSA-4h2c-gw77-f478, UBUNTU-CVE-2026-89775

## What is CVE-2026-89775?

CVE-2026-89775 is a critical denial of service 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 negative S1 walk levels in VNCR TLB size evaluation

Computing the effects of a TLB invalidation involves looking at
the size of the mapping cached by the TLB. For S1 mappings such as
VNCR, this is deducted from the combination of the base granule size
and the mapping level.

However, this implies that the S1 MMU is *on*. When the MMU is off,
we indicate this with the level being set to a "creative" value of
-127 (S1_MMU_DISABLED).

This ends-up being misinterpreted by pgshift_level_to_ttl() as it
doesn't handle negative levels at all (the level is immediately cast
to a u8 and only the bottom two bits considered), leading to an
invalidation size of 0. Not helpful.

Tidy-up pgshift_level_to_ttl() to handle these negative levels, and
ttl_to_size() to always return SZ_1G when no valid TTL is present.
This allows the removal of open-coded checks for similar situations.

Note that the check for a negative value not explicitely checking for
S1_MMU_DISABLED is deliberate, so that actual negative levels introduced
with LVA2 and D128 can take the same path if we ever support them.

## Which versions are affected by CVE-2026-89775?


## How do I fix CVE-2026-89775?

Apply the vendor's patch.

## Is CVE-2026-89775 being exploited?

There is no known exploitation of CVE-2026-89775 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89775 and fix it if it is affected.

What it is: CVE-2026-89775 is a critical denial of service 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-2026-89775 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-2026-89775/ (Markdown: https://insights.sbomify.com/CVE-2026-89775/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89775
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89775
- OSV: https://osv.dev/vulnerability/CVE-2026-89775

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89775/
Summaries are generated and may contain mistakes; the official description above is the record.