CVE-2023-30456: Linux Kernel Privilege Escalation Flaw

CVE-2023-30456 Overview

CVE-2023-30456 is a kernel vulnerability in the Linux nested VMX (nVMX) implementation on x86_64 systems. The flaw resides in arch/x86/kvm/vmx/nested.c and affects Linux kernel versions prior to 6.2.8. The nVMX code lacks consistency checks for the CR0 and CR4 control registers when processing nested virtualization state, allowing a local guest with virtualization privileges to trigger host-side issues. The vulnerability is classified under [CWE-754] (Improper Check for Unusual or Exceptional Conditions). Successful exploitation impacts availability on the host, potentially causing denial-of-service conditions affecting other tenants on the same physical machine.

Critical Impact
A local attacker with the ability to run a nested virtual machine can trigger a high-impact availability failure on the KVM host, with scope change affecting workloads beyond the attacking guest.

Affected Products

Linux kernel versions before 6.2.8
Linux kernel 6.3-rc1
Linux kernel 6.3-rc2

Discovery Timeline

2023-04-10 - CVE-2023-30456 published to NVD
2025-03-19 - Last updated in NVD database

Technical Details for CVE-2023-30456

Vulnerability Analysis

The vulnerability resides in the Kernel-based Virtual Machine (KVM) nested VMX subsystem, which enables running a hypervisor inside a guest virtual machine. When KVM processes a VM-entry from an L1 hypervisor to an L2 guest, it must validate the virtual machine control structure (VMCS) state, including the CR0 and CR4 control register values. The Intel Software Developer's Manual mandates specific consistency checks on these registers during nested VM-entry.

The affected arch/x86/kvm/vmx/nested.c code omits portions of these required checks. As a result, an L1 guest can supply CR0 or CR4 values that violate architectural constraints without KVM rejecting the VM-entry. This leads to inconsistent processor state in the nested guest context and can destabilize host execution. The CVSS scope change reflects that impact extends from the guest to the host environment.

Root Cause

The root cause is missing input validation on guest-controlled control register state during nested VM-entry processing. Intel VMX architecture requires the hypervisor to verify that CR0 and CR4 values comply with VMX operation requirements, including fixed-bit constraints defined by IA32_VMX_CR0_FIXED0/FIXED1 and IA32_VMX_CR4_FIXED0/FIXED1 MSRs. KVM's nVMX implementation accepted invalid values, violating the architectural contract.

Attack Vector

Exploitation requires local access with low privileges on a system running KVM with nested virtualization enabled. The attacker must operate as an L1 hypervisor guest that can construct a malicious VMCS and attempt VM-entry into an L2 guest. No user interaction is required. The vulnerability cannot be triggered remotely or without the ability to execute nested VMX instructions.

The fix is contained in commit 112e66017bff7f2837030f34c2bc19501e9212d5, which adds the missing consistency checks. See the Linux Commit by Torvalds for the source-level changes.

Detection Methods for CVE-2023-30456

Indicators of Compromise

Unexpected KVM host crashes, hangs, or kernel panics traced to functions within arch/x86/kvm/vmx/nested.c
Kernel log entries referencing VM-entry failures, VMCS consistency errors, or nested virtualization faults
Guest virtual machines becoming unresponsive when running nested hypervisors

Detection Strategies

Audit kernel package versions across hypervisor hosts and flag systems running Linux kernels older than 6.2.8 with KVM nested virtualization enabled
Review dmesg and /var/log/messages for KVM warnings, WARN_ON traces, or panics originating in nested VMX code paths
Inventory hosts where /sys/module/kvm_intel/parameters/nested returns Y or 1 to scope exposure

Monitoring Recommendations

Monitor host stability metrics on hypervisors that permit guest-initiated nested virtualization
Alert on repeated KVM-related kernel oops or BUG messages on multi-tenant virtualization hosts
Track kernel package update compliance through configuration management and vulnerability scanning tooling

How to Mitigate CVE-2023-30456

Immediate Actions Required

Upgrade affected systems to Linux kernel 6.2.8 or later, or apply the distribution-provided patch backport
Apply Debian LTS updates referenced in Debian LTS Advisory May 2023-1 and Debian LTS Advisory May 2023-2
Review the NetApp Security Advisory NTAP-20230511-0007 for affected appliance guidance
Where live patching is supported, apply the Canonical Livepatch fix described in the Packet Storm Security Notice

Patch Information

The upstream fix landed in Linux 6.2.8. Refer to the Linux ChangeLog 6.2.8 and the source commit 112e66017bff7f2837030f34c2bc19501e9212d5 for details. Distribution-specific backports are available from Debian, Ubuntu (via Livepatch), and NetApp.

Workarounds

Disable nested virtualization on KVM hosts where it is not required by setting nested=0 on the kvm_intel module
Restrict the ability to launch nested hypervisor workloads to trusted tenants only
On multi-tenant hosts, isolate untrusted workloads from systems that require nested KVM support

bash

# Configuration example
# Disable nested virtualization for Intel KVM until patching is complete
sudo modprobe -r kvm_intel
sudo modprobe kvm_intel nested=0

# Persist the setting across reboots
echo "options kvm_intel nested=0" | sudo tee /etc/modprobe.d/kvm-nested.conf

# Verify nested virtualization is disabled
cat /sys/module/kvm_intel/parameters/nested