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CVE-2025-21839 is a debug-register state-handling vulnerability in the Linux kernel's x86 KVM implementation. When debug-register interception is disabled, a guest can modify hardware DR6 directly. The affected implementation loads the guest's saved DR6 value inside the core vCPU run loop, while saving guest register state occurs outside that loop. A fastpath VM-Exit can therefore cause KVM to reload stale vcpu->arch.dr6 state before preserving the guest's updated value, overwriting the actual DR6 value. The issue is particularly apparent in nested VMX configurations because nested execution substantially enlarges the affected timing window.
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A Linux kernel KVM x86 flaw can overwrite a guest's actual DR6 debug-register value with stale state following a fastpath VM exit. The issue primarily manifests in nested VMX environments. The reference rates it Medium, with a CVSS v3 score of 5.5, indicating local exploitation requiring low privileges and a high availability impact, without confidentiality or integrity impact.
A Linux kernel KVM x86 vulnerability causes a stale saved DR6 debug-register value to overwrite the guest's actual hardware value following a fastpath VM-Exit. The issue primarily manifests with nested VMX virtualization. The advisory rates it Medium, with CVSS v3 severity 5.5 and high availability impact. Echo's fix is to update linux and related packages to version 6.1.140-1 or later.
Linux kernel KVM x86 guest debug-register loading flaw.
A medium-severity Linux kernel KVM/x86 availability vulnerability involving stale debug-register DR6 state. A fast-path VM exit can overwrite the guest's DR6 value, potentially causing guest malfunction; nested VMX configurations have a substantially wider exposure window.
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Cross-references every affected SKU, including bundled OEM variants.
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