CVE-2026-64172 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: SVM: Disable AVIC IPI virtualization on Hygon Family 18h (erratum #1235)
Hygon Family 18h CPUs are derived from AMD Family 17h (Zen1) silicon and share the same erratum #1235: hardware may read a stale IsRunning=1 bit during ICR write emulation and silently fail to generate an AVIC_IPI_FAILURE_TARGET_NOT_RUNNING VM-Exit on the sending vCPU.
The absence of the VM-Exit causes KVM to miss the required wakeup of blocking target vCPUs, leading to hung vCPUs and unbounded delays in guest execution.
Extend the existing AMD Family 17h erratum #1235 workaround to also cover Hygon Family 18h. With IPI virtualization disabled, KVM never sets IsRunning=1 in the Physical ID table, so every non-self IPI generates a VM-Exit and is correctly emulated.
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Analysis
by VulDB Data Team • 07/20/2026
The vulnerability under discussion addresses a critical hardware-level issue affecting virtualization environments on specific processor families. This flaw manifests in the Linux kernel's KVM (Kernel-based Virtual Machine) implementation when running on Hygon Family 18h processors, which are based on AMD Family 17h (Zen1) silicon architecture. The underlying problem stems from erratum #1235 that affects these processors, creating a scenario where hardware may read stale state information during interrupt controller emulation operations.
The technical flaw involves the AVIC (AMD Virtual Interrupt Controller) IPI (Inter-Processor Interrupt) virtualization mechanism failing to properly handle IsRunning=1 bit states during ICR (Interrupt Control Register) write operations. This specific condition occurs during the emulation process when the hardware incorrectly maintains a stale value of the IsRunning=1 bit, which should indicate whether a virtual CPU is actively running. When this stale bit reading occurs, the system fails to generate the required AVIC_IPI_FAILURE_TARGET_NOT_RUNNING VM-Exit event that would normally notify the hypervisor of failed interrupt delivery.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially catastrophic system behavior including complete virtual machine hangs. When the required VM-Exit notification is missing, KVM cannot properly wake up blocking target vCPUs that are waiting for interrupt delivery, resulting in indefinite suspension of guest execution threads. This creates unbounded delays in guest operation where virtual machines may become completely unresponsive, effectively rendering the affected systems unusable for their intended purposes. The vulnerability essentially creates a deadlock condition within the virtualization layer where legitimate interrupt processing fails silently and system resources remain permanently blocked.
The mitigation strategy implemented involves extending an existing workaround originally developed for AMD Family 17h processors to also cover Hygon Family 18h silicon. This approach disables IPI virtualization entirely on affected systems, which prevents the problematic hardware behavior from occurring. When IPI virtualization is disabled, KVM consistently sets IsRunning=1 in the Physical ID table for all non-self IPIs, ensuring that every interrupt delivery attempt generates a proper VM-Exit event. This forced emulation approach guarantees correct interrupt handling behavior at the cost of reduced performance due to increased hypervisor overhead. The solution aligns with common practices for addressing hardware errata in virtualization environments and follows established patterns for maintaining system stability when underlying silicon exhibits documented flaws.
This vulnerability demonstrates the complex interplay between hardware-level errata and virtualization software implementations, highlighting how processor-specific issues can have cascading effects throughout operating system stacks. The fix represents a standard approach to handling silicon-level problems in virtualized environments, where software workarounds are necessary to maintain system reliability when underlying hardware fails to meet expected specifications. The issue relates to CWE-119 (Improper Access of Resource) and CWE-787 (Out-of-bounds Write) categories within the Common Weakness Enumeration framework, while also mapping to ATT&CK techniques involving privilege escalation and system compromise through virtualization layer manipulation.