# CVE-2026-89981: arm64: Don't read GMID_EL1 when MTE is disabled

> A kernel panic occurs in pKVM environments when MTE is disabled and a CPU is hotplugged, causing a local denial of service.

- **Severity:** Not scored
- **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:** DEBIAN-CVE-2026-89981, GHSA-x8h3-pr46-6w2j, UBUNTU-CVE-2026-89981

## What is CVE-2026-89981?

CVE-2026-89981 is a 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:

arm64: Don't read GMID_EL1 when MTE is disabled

__cpuinfo_store_cpu() gates the GMID_EL1 read on the raw
ID_AA64PFR1_EL1, so it reads the register even when the kernel has
disabled MTE (CONFIG_ARM64_MTE=n or arm64.nomte). KVM sets HCR_EL2.TID5
in that case, and pKVM injects an UNDEF the host cannot handle:

  Internal error: Oops - Undefined instruction: 0000000002000000 [#1]  SMP
  pc : __cpuinfo_store_cpu+0xf4/0x264
  Kernel panic - not syncing: Attempted to kill the idle task!

Only pKVM reaches it, and only after a CPU is offlined and brought back
online: its CPU_ON relay sets the host HCR before the CPU enters EL1,
while plain nVHE sets it at CPUHP_AP_KVM_ONLINE.

Gate the read on the CPU's own ID_AA64PFR1_EL1 with the command-line
override applied, and on CONFIG_ARM64_MTE, which no register reflects.
The boot CPU stores its registers before init_cpu_features() strips an
unsafe override, so clamp against the hardware value here too.

## Which versions are affected by CVE-2026-89981?


## How do I fix CVE-2026-89981?

Apply the vendor's patch.

## Is CVE-2026-89981 being exploited?

There is no known exploitation of CVE-2026-89981 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89981 and fix it if it is affected.

What it is: CVE-2026-89981 is a 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-2026-89981 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-2026-89981/ (Markdown: https://insights.sbomify.com/CVE-2026-89981/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89981
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89981
- OSV: https://osv.dev/vulnerability/CVE-2026-89981

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89981/
Summaries are generated and may contain mistakes; the official description above is the record.