CVE-2021-47092 is a vulnerability identified in the Linux kernel's KVM (Kernel-based Virtual Machine) module, specifically affecting the VMX (Intel Virtualization Technology) nested virtualization implementation. The issue arises from improper handling of the vmx->fail flag during emulation-required conditions when the virtual CPU (vCPU) is operating in L2 (nested guest) mode. The vulnerability stems from a recent change that incorrectly set the vmx->fail flag if emulation was required for the vCPU in L2. This behavior is fundamentally flawed because setting vmx->fail and synthesizing a VM-Exit are mutually exclusive operations; a VM-Fail cannot coexist with a VM-Exit, and the VMCS (Virtual Machine Control Structure) exit reason is not updated on VM-Fail events. Emulation-required conditions pertain to guest state checks that always trigger VM-Exits, not VM-Fails. In practice, KVM handles emulation-required conditions before nested VM exits, so setting vmx->fail has no immediate effect but can cause warnings and instability during VM teardown, particularly in nested virtualization scenarios. This can lead to kernel warnings (WARNs) and potentially unstable behavior or crashes when nested VMs are destroyed. The vulnerability does not appear to be exploitable for remote code execution or privilege escalation directly but can cause denial of service conditions due to kernel warnings and instability in nested virtualization environments. The issue was resolved by reverting the problematic change to ensure vmx->fail is cleared appropriately during emulation-required conditions, maintaining correct VM exit semantics and preventing erroneous kernel warnings.

For European organizations, the impact of CVE-2021-47092 primarily concerns environments utilizing nested virtualization on Linux hosts with Intel VT-x support. Enterprises running cloud infrastructure, virtualized data centers, or development/testing environments that rely on nested KVM virtualization could experience instability or denial of service conditions when the vulnerability is triggered. This may affect service availability, particularly in scenarios where nested VMs are frequently created and destroyed, such as CI/CD pipelines, multi-tenant cloud platforms, or complex virtualized network functions. Although the vulnerability does not enable direct code execution or privilege escalation, the resulting kernel warnings and potential crashes could disrupt critical workloads, leading to downtime and increased operational costs. Organizations using nested virtualization for security sandboxing or multi-layered virtualization strategies should be particularly cautious. The vulnerability's impact is mitigated if nested virtualization is not in use or if affected kernel versions are patched promptly.

To mitigate CVE-2021-47092, European organizations should: 1) Apply the latest Linux kernel updates that include the fix reverting the incorrect vmx->fail handling in the KVM VMX nested virtualization code. This is the definitive solution to prevent the vulnerability from manifesting. 2) Audit virtualization infrastructure to identify hosts running nested KVM virtualization with Intel VT-x and prioritize patching these systems. 3) Where possible, limit or disable nested virtualization if it is not required, reducing the attack surface and exposure to this vulnerability. 4) Monitor kernel logs for WARN messages related to nested_vmx_vmexit or vmx->fail conditions as indicators of potential exploitation or instability. 5) Implement robust VM lifecycle management to avoid excessive creation and destruction of nested VMs until patches are applied. 6) Coordinate with cloud service providers to confirm that underlying hypervisors are patched if nested virtualization services are consumed externally.