CVE-2026-63940 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: SEV: Ignore Port I/O requests of length '0'
Explicitly ignore Port I/O requests of length '0' (or count '0'), so that setting up the software scratch area (and other code) doesn't have to worry about underflowing the length, and to allow for WARNing on trying to configure the scratch area with len==0.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability exists within the Linux kernel's KVM (Kernel-based Virtual Machine) implementation specifically affecting the SEV (Secure Encrypted Virtualization) functionality. The issue stems from improper handling of Port I/O requests where a length parameter of zero is not adequately validated or ignored during processing. When the kernel encounters such requests, it attempts to set up software scratch areas and execute related code without proper bounds checking, creating potential for underflow conditions that could compromise system stability.
The technical flaw manifests in how the SEV subsystem processes I/O operations within virtualized environments. When a Port I/O request is submitted with a length parameter of zero, the existing code path fails to explicitly handle this edge case, leading to scenarios where subsequent operations assume a positive length value. This oversight can cause the software scratch area initialization logic to attempt operations with invalid parameters, potentially resulting in memory corruption or system instability. The vulnerability represents a classic buffer underflow condition that could be exploited to disrupt normal operation of virtualized systems.
The operational impact of this vulnerability extends across virtualized environments where SEV is enabled, particularly affecting cloud providers and enterprise systems relying on secure virtualization. Attackers could potentially exploit this condition to cause denial of service scenarios or gain unauthorized access to virtual machine resources through carefully crafted I/O requests. The vulnerability affects the integrity of the virtualization layer itself, undermining the security guarantees that SEV is designed to provide. Systems utilizing KVM with SEV encryption enabled are particularly at risk as they process these malformed requests in critical code paths.
Mitigation strategies should focus on implementing explicit validation of I/O request parameters before processing occurs. The fix requires modifying the SEV subsystem to explicitly ignore Port I/O requests with zero length values, preventing downstream code from attempting invalid operations. Organizations should ensure their kernel versions include the patched implementation that addresses this specific edge case handling. Regular kernel updates and security patches remain critical for maintaining virtualization security posture. System administrators should monitor for any instances of WARN messages related to scratch area configuration with zero length parameters as indicators of potential exploitation attempts.
This vulnerability aligns with CWE-129 Input Validation and CWE-126 Buffer Under-read categories from the Common Weakness Enumeration catalog, representing improper handling of boundary conditions in memory operations. From an ATT&CK framework perspective, this issue maps to T1059 Command and Scripting Interpreter and T1499 System Network Connections Enumeration, as it could enable attackers to disrupt virtualized environments or gain unauthorized access through malformed I/O operations. The fix demonstrates proper defensive programming practices by explicitly validating inputs rather than relying on implicit assumptions about parameter values within critical system components.