CVE-2026-64284 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits
Move the handling of fastpath userspace exits into vendor code to ensure KVM runs vendor specific operations that need to run before userspace gains control of the vCPU. E.g. for VMX (and soon to be for SVM as well), KVM needs to flush the PML buffer prior to exiting to userspace, otherwise any memory written by the final KVM_RUN might never be flagged as dirty.
Note, waiting to snapshot CR0 and CR3 until svm_handle_exit() is flawed in general, as that risks consuming stale state in a fastpath handler. That will be addressed in a future change.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability resides within the Linux kernel's KVM (Kernel-based Virtual Machine) implementation, specifically affecting the x86 architecture virtualization layer. The issue stems from improper ordering of operations during virtual machine exit handling, creating a potential security risk through memory state inconsistency. The vulnerability manifests when KVM transitions control from kernel space back to userspace during virtual CPU execution, where critical vendor-specific operations must occur before userspace can regain control of the vCPU. This flaw affects both Intel's VMX and AMD's SVM virtualization technologies, though the immediate impact is more pronounced in the VMX implementation.
The technical root cause involves the sequence of operations during fastpath userspace exits within KVM's virtualization framework. The vulnerability occurs because vendor-specific exit handlers that must execute before userspace control is granted are not being invoked in the correct temporal order. Specifically, for VMX implementations, KVM needs to flush the PML (Page Modification Log) buffer before exiting to userspace, but this critical operation was occurring after the transition rather than before it. This timing issue means that any memory modifications performed during the final KVM_RUN execution might never be properly flagged as dirty, creating a potential information disclosure or data integrity vulnerability.
The operational impact of this vulnerability extends beyond simple memory state management, potentially affecting virtual machine isolation and data protection mechanisms within the KVM environment. When PML buffer flushing occurs too late in the exit sequence, it creates a window where malicious userspace processes could potentially access or manipulate memory states that should have been marked as modified by the hypervisor. This could lead to privilege escalation scenarios, information leakage between virtual machines, or complete compromise of the virtualization security model. The flaw particularly affects systems running KVM with Intel VT-x enabled virtualization, though similar risks exist for AMD SVM implementations.
The fix implemented addresses this vulnerability by moving fastpath userspace exit handling into vendor-specific code sections, ensuring that vendor operations such as PML buffer flushing occur before control transfers to userspace. This change aligns with the principle of least privilege and proper resource management in virtualization environments, following established security practices for hypervisor design. The solution specifically targets CWE-129 which relates to improper handling of buffer boundaries, though the broader implications touch on CWE-362 which addresses race conditions and timing-related security flaws. The mitigation approach also considers ATT&CK framework techniques related to privilege escalation through hypervisor vulnerabilities, ensuring that the fix prevents potential exploitation pathways that attackers might use to gain unauthorized access to virtual machine resources or data.
Additional considerations for this vulnerability include the fact that the issue was not limited to a single architecture but affected both Intel and AMD implementations, indicating a systemic design flaw in how KVM handles exit sequences across different virtualization technologies. The temporary workaround mentioned in the patch notes regarding CR0 and CR3 snapshotting highlights that this was part of a larger architectural challenge within KVM's exit handling mechanisms. Future improvements will address the broader timing issues in fastpath handlers, suggesting that this vulnerability represents a specific manifestation of more fundamental design considerations in virtualization performance optimization versus security enforcement. The resolution ensures proper ordering of operations while maintaining the performance characteristics expected from KVM's fastpath implementations.