Code: static int handle_vmread(struct kvm_vcpu *vcpu)
{
	unsigned long field;
	u64 field_value;
	unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
	u32 vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);
	gva_t gva = 0;

	if (!nested_vmx_check_permission(vcpu))
		return 1;

	if (!nested_vmx_check_vmcs12(vcpu))
		return kvm_skip_emulated_instruction(vcpu);

	/* Decode instruction info and find the field to read */
	field = kvm_register_readl(vcpu, (((vmx_instruction_info) >> 28) & 0xf));
	/* Read the field, zero-extended to a u64 field_value */
	if (vmcs12_read_any(vcpu, field, &field_value) < 0) {
		nested_vmx_failValid(vcpu, VMXERR_UNSUPPORTED_VMCS_COMPONENT);
		return kvm_skip_emulated_instruction(vcpu);
	}
	/*
	 * Now copy part of this value to register or memory, as requested.
	 * Note that the number of bits actually copied is 32 or 64 depending
	 * on the guest's mode (32 or 64 bit), not on the given field's length.
	 */
	if (vmx_instruction_info & (1u << 10)) {
		kvm_register_writel(vcpu, (((vmx_instruction_info) >> 3) & 0xf),
			field_value);
	} else {
 		if (get_vmx_mem_address(vcpu, exit_qualification,
 				vmx_instruction_info, true, &gva))
 			return 1;
		/* _system ok, as hardware has verified cpl=0 */
 		kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, gva,
 			     &field_value, (is_long_mode(vcpu) ? 8 : 4), NULL);
 	}

	nested_vmx_succeed(vcpu);
	return kvm_skip_emulated_instruction(vcpu);
}

Vulnerability Type: DoS 
CWE ID: 
Summary: In arch/x86/kvm/vmx.c in the Linux kernel before 4.17.2, when nested virtualization is used, local attackers could cause L1 KVM guests to VMEXIT, potentially allowing privilege escalations and denial of service attacks due to lack of checking of CPL.
Commit Message: kvm: nVMX: Enforce cpl=0 for VMX instructions

VMX instructions executed inside a L1 VM will always trigger a VM exit
even when executed with cpl 3. This means we must perform the
privilege check in software.

Fixes: 70f3aac964ae("kvm: nVMX: Remove superfluous VMX instruction fault checks")
Cc: stable@vger.kernel.org
Signed-off-by: Felix Wilhelm <fwilhelm@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>