CVE-2021-46977 is a vulnerability identified in the Linux kernel's KVM (Kernel-based Virtual Machine) module, specifically related to the VMX (Virtual Machine Extensions) functionality on x86 architectures. The issue arises during the handling of Model-Specific Registers (MSRs) when probing user return MSRs via the RDMSR (Read MSR) and WRMSR (Write MSR) instructions. The vulnerability occurs because preemption is not disabled during this probing process. If the MSR value differs across logical CPUs, a context switch (preemption) between the RDMSR and WRMSR instructions could cause the WRMSR to write an incorrect value to the host's MSR, potentially corrupting the host's state. This is due to the KVM being rescheduled on a different CPU where the MSR value is different, leading to inconsistent or corrupted MSR values. The fix involves disabling preemption during this critical section to ensure atomicity of the read-modify-write sequence. The vulnerability affects Linux kernel versions containing the specified commit hashes and is relevant to environments running KVM with VMX support. The description also notes that the helper function used for this fix is opportunistically landed in common x86 code, with plans to extend support to SVM (Secure Virtual Machine) in future updates. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, the impact of this vulnerability primarily concerns environments that utilize Linux-based virtualization infrastructure, especially those leveraging KVM for virtual machine management on x86 hardware. The vulnerability could lead to corruption of host MSR values, which may cause instability or unpredictable behavior in the host system, potentially leading to denial of service or compromised integrity of the virtualization environment. This could disrupt critical services hosted on virtual machines, affecting availability and operational continuity. Confidentiality impact is limited as the vulnerability does not directly enable data leakage, but integrity and availability of virtualized workloads could be compromised. Organizations running multi-tenant or cloud environments with KVM virtualization are particularly at risk. Given the absence of known exploits, immediate widespread attacks are unlikely, but the vulnerability represents a latent risk that could be exploited if combined with other vulnerabilities or in targeted attacks. The complexity of exploitation is moderate, requiring specific conditions related to CPU scheduling and MSR handling, which may limit the scope but does not eliminate risk.

To mitigate this vulnerability, European organizations should prioritize updating their Linux kernel to the latest patched versions that include the fix for CVE-2021-46977. Kernel updates should be applied promptly in all environments running KVM with VMX support. Additionally, organizations should audit their virtualization infrastructure to identify affected hosts and ensure that preemption-related kernel patches are applied. Monitoring for unusual host behavior or instability in virtualized environments can help detect potential exploitation attempts. For environments where immediate patching is not feasible, consider isolating critical virtual machines or limiting access to the host systems to reduce attack surface. Implementing strict CPU affinity and scheduling policies may reduce the likelihood of preemption-related issues but is not a substitute for patching. Regularly review and update virtualization security best practices, including minimizing privileged access and ensuring secure configuration of KVM and related components.