Code: static int handle_vmon(struct kvm_vcpu *vcpu)
{
	int ret;
	gpa_t vmptr;
	struct page *page;
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	const u64 VMXON_NEEDED_FEATURES = FEATURE_CONTROL_LOCKED
		| FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX;

	/*
	 * The Intel VMX Instruction Reference lists a bunch of bits that are
	 * prerequisite to running VMXON, most notably cr4.VMXE must be set to
	 * 1 (see vmx_set_cr4() for when we allow the guest to set this).
	 * Otherwise, we should fail with #UD.  But most faulting conditions
	 * have already been checked by hardware, prior to the VM-exit for
	 * VMXON.  We do test guest cr4.VMXE because processor CR4 always has
	 * that bit set to 1 in non-root mode.
	 */
	if (!kvm_read_cr4_bits(vcpu, X86_CR4_VMXE)) {
		kvm_queue_exception(vcpu, UD_VECTOR);
 		return 1;
 	}
 
 	if (vmx->nested.vmxon) {
 		nested_vmx_failValid(vcpu, VMXERR_VMXON_IN_VMX_ROOT_OPERATION);
 		return kvm_skip_emulated_instruction(vcpu);
	}

	if ((vmx->msr_ia32_feature_control & VMXON_NEEDED_FEATURES)
			!= VMXON_NEEDED_FEATURES) {
		kvm_inject_gp(vcpu, 0);
		return 1;
	}

	if (nested_vmx_get_vmptr(vcpu, &vmptr))
		return 1;

	/*
	 * SDM 3: 24.11.5
	 * The first 4 bytes of VMXON region contain the supported
	 * VMCS revision identifier
	 *
	 * Note - IA32_VMX_BASIC[48] will never be 1 for the nested case;
	 * which replaces physical address width with 32
	 */
	if (!PAGE_ALIGNED(vmptr) || (vmptr >> cpuid_maxphyaddr(vcpu))) {
		nested_vmx_failInvalid(vcpu);
		return kvm_skip_emulated_instruction(vcpu);
	}

	page = kvm_vcpu_gpa_to_page(vcpu, vmptr);
	if (is_error_page(page)) {
		nested_vmx_failInvalid(vcpu);
		return kvm_skip_emulated_instruction(vcpu);
	}
	if (*(u32 *)kmap(page) != VMCS12_REVISION) {
		kunmap(page);
		kvm_release_page_clean(page);
		nested_vmx_failInvalid(vcpu);
		return kvm_skip_emulated_instruction(vcpu);
	}
	kunmap(page);
	kvm_release_page_clean(page);

	vmx->nested.vmxon_ptr = vmptr;
	ret = enter_vmx_operation(vcpu);
	if (ret)
		return ret;

	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>