CVE-2026-64172: Linux Kernel KVM SVM DOS Vulnerability

CVE-2026-64172 Overview

CVE-2026-64172 is a Linux kernel vulnerability affecting the Kernel-based Virtual Machine (KVM) Secure Virtual Machine (SVM) subsystem on Hygon Family 18h processors. The flaw resides in the Advanced Virtual Interrupt Controller (AVIC) Inter-Processor Interrupt (IPI) virtualization path. Hygon Family 18h CPUs derive from AMD Family 17h (Zen1) silicon and share erratum #1235, which was previously mitigated only on AMD parts. Hardware may read a stale IsRunning=1 bit during Interrupt Command Register (ICR) write emulation and fail to generate the expected AVIC_IPI_FAILURE_TARGET_NOT_RUNNING VM-Exit. The missing exit prevents KVM from waking blocked target vCPUs, causing hung vCPUs and stalled guest execution.

Critical Impact
A local attacker within a guest can trigger unbounded delays and hang virtual CPUs on affected Hygon Family 18h hosts, producing a denial-of-service condition on virtualized workloads.

Affected Products

Linux kernel KVM SVM subsystem when running on Hygon Family 18h CPUs
Hypervisor hosts using AVIC IPI virtualization on affected Hygon silicon
Guest virtual machines scheduled on impacted Hygon Family 18h hardware

Discovery Timeline

2026-07-19 - CVE-2026-64172 published to the National Vulnerability Database
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-64172

Vulnerability Analysis

The flaw is a hardware erratum exposed through the KVM SVM AVIC IPI virtualization path. AVIC allows the CPU to accelerate guest IPI delivery without exiting to the hypervisor when the target vCPU has IsRunning=1 in the Physical ID table. On Hygon Family 18h silicon, the processor may observe a stale IsRunning=1 value during ICR write emulation. Because the hardware sees the target as running, it skips the AVIC_IPI_FAILURE_TARGET_NOT_RUNNING VM-Exit that KVM relies on to wake a blocked vCPU. The sending vCPU proceeds as if the IPI was delivered, but the receiver remains halted, causing hung vCPUs and progress stalls in the guest.

Root Cause

The root cause is missing detection logic for Hygon Family 18h in the KVM SVM erratum handler. The existing workaround for AMD Family 17h erratum #1235 disables AVIC IPI virtualization on affected AMD parts but did not enumerate Hygon Family 18h, which shares the same Zen1 silicon defect. As a result, avic_ipiv_enabled remained true on Hygon hosts and the racy hardware path was exercised.

Attack Vector

Exploitation requires a local attacker with the ability to execute code inside a guest running on an affected Hygon Family 18h host. A malicious or misbehaving guest can repeatedly issue IPIs that race with vCPU state transitions, causing the stale IsRunning read and suppressing the required VM-Exit. The vulnerability is scoped to availability impact and produces guest-level or host-level scheduling stalls in virtualized environments.

No verified proof-of-concept code is publicly available. The upstream fix is documented in the Linux kernel commits referenced in the kernel.org stable tree.

Detection Methods for CVE-2026-64172

Indicators of Compromise

Guest vCPUs entering prolonged blocked or halted states with no forward progress on Hygon Family 18h hosts
Host dmesg messages referencing AVIC, IPI delivery failures, or soft lockups within guests
KVM statistics showing missing avic_incomplete_ipi VM-Exits despite active IPI traffic

Detection Strategies

Inventory hypervisor hosts and identify those running on Hygon Family 18h CPUs via /proc/cpuinfo vendor and family fields
Compare running kernel versions against upstream stable trees containing the erratum #1235 extension for Hygon
Alert on repeated guest soft lockup or rcu_sched stall messages originating from KVM guests on Hygon hardware

Monitoring Recommendations

Collect and centralize host kernel logs and KVM tracepoints such as kvm_avic_incomplete_ipi for anomaly analysis
Track per-guest vCPU runtime and blocked-time metrics to identify stalled workloads
Correlate hypervisor CPU vendor telemetry with kernel patch level to detect unpatched Hygon Family 18h hosts

How to Mitigate CVE-2026-64172

Immediate Actions Required

Identify all Linux hypervisor hosts running Hygon Family 18h processors
Apply the upstream Linux kernel patches extending the AMD Family 17h erratum #1235 workaround to Hygon Family 18h
Reboot affected hosts after installing the patched kernel to activate the disabled AVIC IPI virtualization path
Prioritize multi-tenant virtualization hosts where guest hangs would impact production workloads

Patch Information

The fix extends the existing AMD Family 17h erratum #1235 workaround to Hygon Family 18h, disabling AVIC IPI virtualization on affected silicon. With IsRunning=1 never set in the Physical ID table, every non-self IPI generates a VM-Exit and is correctly emulated by KVM. Patched commits are available in the stable kernel tree: 94ade38f317e, 9560e6fee887, and 9a12fa5213cf.

Workarounds

Disable AVIC on affected hosts by loading the kvm_amd module with avic=0 until the kernel patch can be deployed
Migrate critical guest workloads to hypervisor hosts not based on Hygon Family 18h CPUs
Restrict untrusted tenants from executing on unpatched Hygon Family 18h hardware

bash

# Disable KVM AVIC as a temporary workaround on affected hosts
modprobe -r kvm_amd
modprobe kvm_amd avic=0

# Persist the setting across reboots
echo 'options kvm_amd avic=0' > /etc/modprobe.d/kvm-amd-avic.conf