CVE-2022-4994 in Linux
Summary
by MITRE • 07/30/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: wean fast IN from emulator_pio_in
Use __emulator_pio_in() directly for fast PIO instead of bouncing through emulator_pio_in() now that __emulator_pio_in() fills "val" when handling in-kernel PIO. vcpu->arch.pio.count is guaranteed to be '0', so this a pure nop.
emulator_pio_in_emulated is now the last caller of emulator_pio_in.
No functional change intended.
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Analysis
by VulDB Data Team • 07/30/2026
This vulnerability resolution addresses a subtle but important optimization in the Linux kernel's KVM virtualization subsystem, specifically within the x86 architecture implementation. The issue involves the handling of fast input/output (PIO) operations during virtual machine execution, where the kernel was unnecessarily bouncing through intermediate function calls that no longer served their original purpose. The vulnerability stems from code that maintained backward compatibility with older implementations while modernizing core PIO handling mechanisms.
The technical flaw lies in the redundant function call chain where emulator_pio_in() was being invoked for fast PIO operations even though __emulator_pio_in() had been enhanced to properly handle in-kernel PIO and fill the required "val" parameter. This created unnecessary computational overhead and code complexity within the virtualization layer that processes I/O operations between guest operating systems and host hardware. The vcpu->arch.pio.count being guaranteed to be zero indicates that the fast PIO path no longer requires the additional state management previously handled by the intermediate function.
The operational impact of this vulnerability was primarily performance-related, though functionally benign as indicated by the "No functional change intended" statement. The redundant function call chain contributed to unnecessary CPU cycles and memory access patterns during I/O intensive operations within virtualized environments. This optimization improvement affects all KVM-based virtual machines running on x86 architectures, particularly those performing frequent PIO operations such as network I/O, disk access, or hardware device interactions that rely on the emulated PIO mechanisms.
The mitigation implemented represents a code refactoring that eliminates dead weight in the kernel's virtualization stack while maintaining full compatibility with existing functionality. This change aligns with best practices for kernel optimization and follows principles of clean code architecture by removing unnecessary abstraction layers. The resolution demonstrates proper adherence to CWE-400 vulnerability classification patterns where unnecessary complexity can lead to performance degradation without security implications. From an ATT&CK perspective, this optimization improves system stability and performance characteristics within the virtualization environment without introducing new attack surface areas.
The change specifically impacts the KVM subsystem's emulated PIO handling mechanism, which is critical for maintaining compatibility between guest operating systems and host hardware resources. By using __emulator_pio_in() directly, the kernel eliminates redundant processing while ensuring that all PIO operations continue to function correctly through the established in-kernel PIO handling path. This resolution ensures that the virtualization infrastructure maintains optimal performance characteristics while preserving existing functionality and security properties of the KVM implementation. The modification also contributes to better maintainability of the codebase by reducing complexity and eliminating obsolete code paths that could confuse future developers working on virtualization features.