CVE-2021-47112 is a vulnerability identified in the Linux kernel's x86 KVM (Kernel-based Virtual Machine) subsystem related to the teardown of paravirtualized (PV) features on the boot CPU during system resume from hibernation. Paravirtualized features such as Async Page Fault (Async PF), PV End Of Interrupt (PV EOI), and steal time rely on memory shared between the guest kernel and the hypervisor. When a system resumes from hibernation, the kernel must properly teardown these PV features to prevent the hypervisor from writing to stale memory locations that were valid before hibernation but may now be repurposed or contain sensitive data. The vulnerability arises because, prior to the fix, the teardown process was performed correctly for secondary CPUs via the kvm_cpu_down_prepare() function, but not for the boot CPU. This inconsistency could allow the hypervisor to interact with stale memory regions on the boot CPU after resume, potentially leading to undefined behavior or information leakage. The patch introduces syscore operations to ensure that the boot CPU also properly tears down PV features during resume, aligning its behavior with secondary CPUs and closing the gap. Although no known exploits are reported in the wild, this vulnerability affects Linux kernel versions identified by the commit hash fd10cde9294f73eeccbc16f3fec1ae6cde7b800c and similar builds. The issue is particularly relevant in virtualized environments where KVM is used, especially those leveraging paravirtualized features for performance optimization. The vulnerability does not have an assigned CVSS score but is recognized by CISA and has been published as of March 15, 2024.

For European organizations, the impact of CVE-2021-47112 primarily concerns environments running Linux-based virtualized infrastructure using KVM with paravirtualized features enabled. Potential impacts include unauthorized information disclosure due to stale memory access by the hypervisor, which could lead to leakage of sensitive data from the guest kernel memory. Additionally, improper teardown could cause instability or unpredictable behavior in virtual machines after resuming from hibernation, potentially affecting availability of critical services. Organizations relying heavily on virtualization for cloud services, private data centers, or hybrid cloud deployments may face increased risk if they have not applied the patch. Given the widespread use of Linux in enterprise servers and cloud infrastructure across Europe, the vulnerability could affect sectors such as finance, healthcare, government, and telecommunications, where data confidentiality and service availability are paramount. However, the absence of known exploits in the wild and the requirement for specific virtualization configurations somewhat limit the immediate risk. Still, the vulnerability represents a latent threat that could be leveraged in targeted attacks or insider threat scenarios, especially where hypervisor control is compromised or shared infrastructure is used.

European organizations should prioritize updating their Linux kernel to versions that include the patch for CVE-2021-47112. Specifically, system administrators should: 1) Identify all Linux hosts running KVM with paravirtualized features enabled, especially those that utilize hibernation or suspend/resume cycles. 2) Apply the latest kernel updates from trusted Linux distributions that incorporate the fix for this vulnerability. 3) Review and harden hypervisor configurations to limit access and control to trusted administrators only, reducing the risk of hypervisor-level attacks. 4) Implement monitoring for unusual hypervisor or VM behavior post-resume, which could indicate exploitation attempts. 5) Where possible, disable hibernation on critical virtualized hosts if not required, to reduce the attack surface. 6) Conduct security audits and penetration testing focused on virtualization components to detect potential misuse of paravirtualized features. 7) Maintain strict separation and isolation policies for multi-tenant environments to prevent lateral movement if the vulnerability is exploited. These steps go beyond generic patching advice by emphasizing environment-specific controls and operational best practices tailored to virtualization security.