CVE-2021-47255 is a vulnerability identified in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically related to the Local Advanced Programmable Interrupt Controller (LAPIC) emulation. The issue arises from improper handling of APIC register accesses within the kvm_lapic_reg_read function. According to the Intel Software Developer's Manual (SDM), any access that touches bytes 4 through 15 of an APIC register is undefined behavior and must be prevented. However, a recent kernel commit (01402cf81051) inadvertently removed a guard that disallowed such illegal accesses, leading to potential exposure of kernel stack contents. This means that when a guest virtual machine performs an illegal read on certain APIC register bytes, it may cause leakage of sensitive kernel memory data, which could include critical information such as kernel pointers, credentials, or other sensitive data residing on the kernel stack. The vulnerability does not appear to require user interaction or authentication within the guest VM context, but exploitation would require the ability to execute code or instructions within a virtualized environment that uses KVM with LAPIC emulation. The vulnerability was addressed by restoring the previously removed guard to prevent illegal APIC register accesses, thereby mitigating the risk of kernel stack information leakage. No known public exploits have been reported in the wild as of the published date (May 21, 2024).

For European organizations, this vulnerability poses a risk primarily to environments running Linux-based virtualized infrastructure using KVM, which is common in cloud service providers, data centers, and enterprise virtualization platforms. The leakage of kernel stack contents could allow attackers with guest VM access to gain insights into kernel memory layout, potentially facilitating further privilege escalation or escape from the virtualized environment. This could compromise confidentiality and integrity of host systems and other guest VMs sharing the same physical hardware. Organizations relying on multi-tenant cloud environments or hosting sensitive workloads on KVM-based virtualization platforms could face increased risk of data breaches or unauthorized access. The impact is heightened in sectors with strict data protection regulations such as finance, healthcare, and government institutions across Europe. However, since exploitation requires guest VM code execution and no known active exploits exist, the immediate risk is moderate but warrants prompt patching to prevent future attacks.

European organizations should prioritize applying the Linux kernel patch that restores the guard preventing illegal APIC register accesses in KVM LAPIC emulation. This involves updating to the latest stable Linux kernel versions that include the fix for CVE-2021-47255. Virtualization administrators should audit their KVM environments to ensure all hosts are patched and verify that guest VMs are running trusted code to reduce the risk of malicious exploitation. Additionally, organizations should implement strict isolation policies between guest VMs, monitor for unusual VM behavior indicative of exploitation attempts, and employ kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR) and stack canaries to reduce the impact of potential memory disclosures. Regular vulnerability scanning and penetration testing focused on virtualization layers can help detect exploitation attempts. For cloud providers, offering customers transparency about patch status and encouraging timely updates is critical. Finally, maintaining comprehensive logging and alerting on hypervisor and guest VM activities can aid in early detection of exploitation attempts.