---
id: "CVE-2026-98713"
kind: "vulnerability"
aliases: ["CVE-2026-98713"]
title: "Linux KVM arm64: fine-grained UNDEF traps applied without FEAT_FGT hardware"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["arm64", "infrastructure", "kvm", "linux-kernel", "self-hosted"]
not_affected_if: ["no-kvm", "x86-hosts"]
published_at: "2026-10-18T16:17:24+00:00"
first_seen_at: "2026-10-23T21:57:16+00:00"
updated_at: "2026-10-25T05:17:31+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98713", "first_seen_at": "2026-10-23T21:57:16+00:00"}]
---

# Linux KVM arm64: fine-grained UNDEF traps applied without FEAT_FGT hardware

The Linux kernel's arm64 KVM hypervisor now injects fine-grained UNDEF traps even on hardware without FEAT_FGT, fixing a spurious kernel WARN when a guest runs a TLBI OS operation. The sources describe a correctness fix and do not state an attacker-controlled impact.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- Update the Linux kernel to a release that includes the upstream KVM arm64 fine-grained UNDEF commit, as listed in the NVD entry for CVE-2026-98713.

## Timeline

- 2026-10-23T21:57:16+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98713

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

FGUs don't actually require hardware support, so don't gate fgt trap
config insertion on FGT. This is the only action required to allow FGUs
always, as kvm_calculate_traps() already computes the FGU bits with or
without FEAT_FGT.

This also fixes a spurious WARN when a TLBI OS is run on a guest in vEL1
without FEAT_TLBIOS on non FEAT_FGT hardware. In this configuration the
course-grained HCR_EL2.TTLBOS trap reaches the handler handle_tlbi_el1(),
which expects an EL1 TLBI only from vEL2. With FGUs enabled the UNDEF
will be injected without needing the specific handler.

[oliver: Take Wei-Lin's suggested changelog]

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_