CVE-2021-46977: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46977 affects the Linux kernel's KVM (Kernel Virtual Machine) VMX functionality. The vulnerability is related to the handling of user return MSRs (Model Specific Registers) in the VMX module. The issue was discovered when probing user return MSRs via RDMSR/WRMSR operations, where a race condition could occur if KVM is preempted between these operations (NVD).

The vulnerability occurs when probing a user return MSR via RDSMR/WRMSR operations. If the MSR holds different values per logical CPU, the WRMSR operation could corrupt the host's value if KVM is preempted between the RDMSR and WRMSR operations and then rescheduled on a different CPU. The issue was introduced in commit 4be534102624 'KVM: VMX: Initialize vmx->guest_msrs[] right after allocation'. The vulnerability has a CVSS v3.1 Base Score of 5.5 (MEDIUM) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (NVD).

The vulnerability could lead to host MSR value corruption if exploited, potentially affecting system stability and reliability. The impact is primarily focused on availability, as indicated by the CVSS metrics showing no impact on confidentiality or integrity, but high impact on availability (NVD).

The vulnerability requires local access with low privileges and no user interaction to exploit. The attack complexity is low, making it relatively straightforward to exploit if an attacker has the necessary local access (NVD).

The vulnerability was fixed by introducing preemption disabling when probing user return MSRs. The fix involves adding a new helper function kvm_probe_user_return_msr() that properly handles the MSR operations with preemption disabled. The patch was implemented in the Linux kernel and is available through various stable kernel releases (Kernel Patch).