CVE-2026-89927 Linux CVE debrief

Who should care

Linux KVM administrators and security teams responsible for virtualized environments using Hyper-V synthetic timers should assess their exposure and apply patches to mitigate this vulnerability. They should also monitor for suspicious timer programming attempts and review compensating controls for exposed systems.

Why it matters

A vulnerability in Linux KVM's handling of Hyper-V synthetic timers could cause CPU livelock, requiring patching and monitoring.

• Possible CPU livelock in KVM hypervisor
• Required patching and verification of Linux kernel versions
• Monitoring for suspicious timer programming attempts

Technical summary

The Linux kernel's KVM hypervisor for x86 did not properly handle Hyper-V synthetic timer deadlines, allowing for an integer overflow that could cause a CPU livelock. The vulnerability arises from the improper translation of COUNT to an absolute monotonic deadline, which can result in a deadline in the past being programmed. This issue can be triggered by both the guest and the host through the HV_X64_MSR_STIMERi_CONFIG and HV_X64_MSR_STIMERi_COUNT MSRs. The fix involves clamping the stimer deadline to prevent livelock conditions.

Defensive priority

Assess exposure and apply patches for Linux KVM deployments using Hyper-V synthetic timers.

Recommended defensive actions

• Inventory Linux KVM deployments using Hyper-V synthetic timers
• Verify and apply patches from Linux kernel maintainers
• Monitor for guest or userspace attempts to program timer deadlines
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and Linux kernel source references indicate a fix for the integer overflow issue in KVM's handling of Hyper-V synthetic timers. According to the Linux kernel maintainers, the fix involves clamping the stimer deadline to prevent livelock conditions. The CVE record was published on 2026-09-16T10:32:19.473Z and has not been modified since then. The Linux kernel source code review indicates that the fix was applied to the KVM hypervisor for x86. The vulnerability is tracked as CVE-2026-89927.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.