CVE-2023-30456: Linux Kernel Privilege Escalation Flaw

CVE-2023-30456 Overview

A vulnerability was discovered in arch/x86/kvm/vmx/nested.c in the Linux kernel before version 6.2.8. The nested VMX (nVMX) implementation on x86_64 architecture lacks proper consistency checks for CR0 and CR4 control registers during VM entry validation. This improper check for unusual or exceptional conditions (CWE-754) can allow a local attacker with access to KVM virtualization to cause a denial of service condition by exploiting the missing validation logic.

Critical Impact

Local attackers with low privileges can exploit missing CR0/CR4 consistency checks in the nested VMX implementation to cause denial of service conditions affecting the host system's availability.

Affected Products

• Linux Kernel versions prior to 6.2.8
• Linux Kernel 6.3-rc1
• Linux Kernel 6.3-rc2

Discovery Timeline

• April 10, 2023 - CVE-2023-30456 published to NVD
• March 19, 2025 - Last updated in NVD database

Technical Details for CVE-2023-30456

Vulnerability Analysis

The vulnerability resides in the nested VMX (nVMX) implementation within the Linux kernel's KVM hypervisor subsystem. Nested virtualization allows a guest virtual machine to run its own hypervisor and nested guests. During the VM entry process, the hypervisor must validate various control registers to ensure the guest's requested state is consistent and valid.

The flaw occurs because the nested.c module fails to perform adequate consistency checks on the CR0 and CR4 control registers when processing VM entries from nested guests. These control registers are critical to processor operation—CR0 controls protected mode, paging, and other fundamental CPU features, while CR4 enables various architectural extensions. Without proper validation, a malicious nested guest can specify inconsistent or invalid register states that the hardware or hypervisor cannot properly handle, leading to system instability or denial of service.

Root Cause

The root cause is the missing initialization and validation of the ia32e variable that determines whether the guest is entering IA-32e (64-bit) mode. The code failed to properly check that the CR0 and CR4 register values specified in the VMCS12 structure (which defines the nested guest's virtual machine control structure) are consistent with the requested execution mode. This violates the Intel SDM requirements for VM entry checks in nested virtualization scenarios.

Attack Vector

The attack requires local access to a system running KVM virtualization with nested VMX enabled. An attacker with low-level privileges who can create and configure virtual machines can craft a malicious nested guest with inconsistent CR0/CR4 values. When this guest attempts to execute, the missing consistency checks allow invalid states to propagate, potentially causing the host kernel to crash or enter an unstable state, resulting in denial of service that impacts all workloads on the host.

 				struct vmcs12 *vmcs12,
 				enum vm_entry_failure_code *entry_failure_code)
 {
-	bool ia32e;
+	bool ia32e = !!(vmcs12->vm_entry_controls & VM_ENTRY_IA32E_MODE);
 
 	*entry_failure_code = ENTRY_FAIL_DEFAULT;
 

Source: GitHub Linux Commit Update

Detection Methods for CVE-2023-30456

Indicators of Compromise

• Unexpected kernel panics or oops messages originating from KVM/VMX subsystem
• System logs showing VM entry failures with unusual control register values
• Repeated KVM crashes when running nested virtualization workloads
• dmesg entries referencing nested.c or VMX consistency check failures

Detection Strategies

• Monitor kernel logs for KVM-related errors using journalctl -k | grep -i kvm
• Implement auditd rules to track access to KVM device files (/dev/kvm)
• Deploy kernel tracing with ftrace or eBPF to monitor nested VMX entry paths
• Use SentinelOne's kernel-level visibility to detect anomalous KVM behavior patterns

Monitoring Recommendations

• Enable KVM debugging options to capture detailed VM entry failure information
• Configure alerting on sudden KVM module crashes or restarts
• Monitor system stability metrics on hosts running nested virtualization
• Track virtual machine creation patterns for unusual nested VM configurations

How to Mitigate CVE-2023-30456

Immediate Actions Required

• Update Linux kernel to version 6.2.8 or later immediately
• If immediate patching is not possible, disable nested virtualization by setting nested=0 module parameter for kvm_intel
• Apply available kernel live patches from your distribution vendor
• Review and restrict access to KVM virtualization capabilities

Patch Information

The vulnerability has been addressed in Linux kernel version 6.2.8. The fix properly initializes the ia32e variable by checking the VM_ENTRY_IA32E_MODE flag in the VMCS12 vm_entry_controls field, ensuring CR0 and CR4 consistency checks are performed correctly. The patch is available through the official kernel changelog and has been backported to various distribution kernels.

Relevant security advisories:

• Linux Version ChangeLog - Official kernel fix
• Debian LTS Security Announcement - Debian backport
• NetApp Security Advisory - Vendor advisory
• Packet Storm Security Notice - Live patch information

Workarounds

• Disable nested virtualization if not required: set nested=0 for kvm_intel module
• Restrict KVM access to trusted users only using appropriate group permissions
• Use container-based isolation instead of nested VMs where possible
• Monitor for kernel live patch availability from your Linux distribution

# Disable nested virtualization on kvm_intel module
echo "options kvm_intel nested=0" | sudo tee /etc/modprobe.d/kvm-nested-disable.conf

# Reload the kvm_intel module to apply changes
sudo modprobe -r kvm_intel && sudo modprobe kvm_intel

# Verify nested virtualization is disabled
cat /sys/module/kvm_intel/parameters/nested
# Should output: N