CVE-2021-47094 is a vulnerability in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically within the x86 architecture's memory management unit (MMU) handling code. The flaw arises from improper iterator advancement in the TDP (Two-Dimensional Paging) MMU iterator during shadow page table entry (SPTE) management. When the iterator is incorrectly advanced after a restart due to yielding, it causes the skipping of top-level SPTEs and their children. This skipping is critical because it leads to unprocessed SPTEs that should have been zapped (invalidated). The failure to zap these SPTEs results in either shadow page leaks or use-after-free conditions. The leaked shadow pages cause warnings and potential resource exhaustion, while the use-after-free can lead to marking freed pages as dirty or accessed, triggering kernel warnings and potentially causing data corruption or kernel instability. The vulnerability is triggered during VM destruction or MMU teardown, where the iterator logic incorrectly handles yielding and rescheduling, leading to these unsafe states. The issue was introduced by changes that moved calls to tdp_mmu_iter_cond_resched() but did not fully address iterator advancement logic. The vulnerability requires local privileges (PR:L) and no user interaction (UI:N), with low attack complexity (AC:L). It impacts confidentiality and availability, as it can cause data corruption and kernel crashes, but does not directly affect integrity. The CVSS score is 7.1 (high severity). No known exploits are reported in the wild yet. The vulnerability affects Linux kernel versions containing the specified commit hashes prior to the fix.

For European organizations, this vulnerability poses a significant risk to environments running Linux-based virtualization infrastructure using KVM, which is common in cloud providers, data centers, and enterprise virtualization setups. Exploitation could lead to denial of service through kernel crashes or data corruption affecting virtual machines, potentially disrupting critical services and workloads. Confidentiality impact arises from potential memory leaks or unintended information exposure due to improper SPTE handling. Organizations relying on KVM for multi-tenant virtualization could face stability issues, impacting service availability and reliability. Given the local privilege requirement, attackers would need some level of access to the host or guest environment, which elevates the importance of internal threat monitoring and access controls. The vulnerability could also complicate incident response and forensic analysis due to kernel warnings and instability. Overall, the threat could affect cloud service providers, hosting companies, and enterprises with Linux virtualization infrastructure across Europe, potentially impacting sectors like finance, telecommunications, and government services where uptime and data integrity are critical.

1. Apply the official Linux kernel patches that address CVE-2021-47094 as soon as possible. Monitor vendor advisories for backported fixes in enterprise Linux distributions such as Red Hat, SUSE, and Ubuntu. 2. For environments where immediate patching is not feasible, consider temporarily disabling KVM or limiting access to KVM-enabled hosts to trusted administrators only to reduce the attack surface. 3. Implement strict local access controls and monitoring to detect any unusual activity or attempts to exploit kernel vulnerabilities. 4. Use kernel hardening features such as Kernel Address Space Layout Randomization (KASLR) and Kernel Page Table Isolation (KPTI) to reduce exploitation likelihood. 5. Regularly audit and restrict privileged user accounts and container/VM escape vectors to prevent local privilege escalation. 6. Employ comprehensive logging and alerting on kernel warnings related to KVM and MMU operations to detect early signs of exploitation or instability. 7. Test patches in staging environments to ensure stability before production deployment, as kernel updates can impact system behavior. 8. Coordinate with cloud providers or virtualization platform vendors to confirm patch status and mitigation strategies if using managed services.