CVE-2024-55881 is a vulnerability identified in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically affecting the x86 architecture. The issue arises in the function complete_hypercall_exit(), which handles the completion of hypercalls made by virtual machines (VMs) running on KVM. The vulnerability is due to improper detection of 64-bit hypercalls. The kernel code incorrectly uses is_64_bit_mode() to detect 64-bit hypercalls, but for guests with protected states such as SEV-ES (Secure Encrypted Virtualization - Encrypted State) and SEV-SNP (Secure Nested Paging), the vCPU state required to determine 64-bit mode is unavailable. The correct approach is to use is_64_bit_hypercall(), which assumes hypercalls are made in 64-bit mode for these protected guests. This flaw can cause the kernel to trigger warnings and potentially unstable behavior when handling hypercalls, as demonstrated by the sev_smoke_test selftest generating a KVM_HC_MAP_GPA_RANGE hypercall via VMGEXIT that triggers a kernel warning and trace. While no known exploits are currently reported in the wild, the vulnerability could lead to denial of service or instability in virtualized environments using KVM with SEV-ES or SEV-SNP enabled guests. The issue affects Linux kernel versions prior to the patch and is relevant for environments leveraging AMD SEV technologies for VM protection. The vulnerability was published on January 11, 2025, and no CVSS score has been assigned yet.

For European organizations, especially those utilizing virtualization technologies with AMD SEV-ES or SEV-SNP for enhanced VM security, this vulnerability poses a risk of hypervisor instability or denial of service. Organizations running cloud infrastructure, data centers, or private clouds with Linux KVM hypervisors on AMD hardware could experience unexpected VM crashes or degraded service availability. This could disrupt critical services, impact business continuity, and increase operational costs due to troubleshooting and remediation efforts. Confidentiality and integrity impacts appear limited as the vulnerability relates primarily to hypercall handling and does not directly expose guest memory or allow privilege escalation. However, the availability impact on virtualized workloads could be significant, particularly for sectors relying on high availability such as finance, healthcare, and government services. Additionally, the complexity of SEV-ES and SEV-SNP environments means that failure to patch could undermine trust in hardware-based VM protections, potentially affecting compliance with data protection regulations like GDPR if service disruptions lead to data unavailability.

1. Apply the Linux kernel patch that corrects the hypercall detection logic by replacing is_64_bit_mode() with is_64_bit_hypercall() in complete_hypercall_exit(). Ensure all KVM-related kernel modules are updated accordingly. 2. Conduct thorough testing of virtualized environments with SEV-ES and SEV-SNP enabled guests after patching to verify stability and correct hypercall handling. 3. Monitor kernel logs for WARN messages related to complete_hypercall_exit() or KVM hypercalls as indicators of potential exploitation or instability. 4. For organizations using custom or older Linux kernels, backport the patch or upgrade to a supported kernel version that includes the fix. 5. Coordinate with hardware vendors and cloud providers to confirm that firmware and microcode updates complement the kernel patch for SEV technologies. 6. Implement robust VM monitoring and automated recovery mechanisms to minimize downtime in case of hypervisor crashes. 7. Limit access to hypervisor management interfaces and restrict VM configurations to trusted administrators to reduce risk of triggering the vulnerability through crafted hypercalls.