KVM: nVMX: Fix kernel info-leak when enabling KVM_CAP_HYPERV_ENLIGHTE…

…NED_VMCS more than once

Consider the case that userspace enables KVM_CAP_HYPERV_ENLIGHTENED_VMCS twice:
1) kvm_vcpu_ioctl_enable_cap() is called to enable
KVM_CAP_HYPERV_ENLIGHTENED_VMCS which calls nested_enable_evmcs().
2) nested_enable_evmcs() sets enlightened_vmcs_enabled to true and fills
vmcs_version which is then copied to userspace.
3) kvm_vcpu_ioctl_enable_cap() is called again to enable
KVM_CAP_HYPERV_ENLIGHTENED_VMCS which calls nested_enable_evmcs().
4) This time nested_enable_evmcs() just returns 0 as
enlightened_vmcs_enabled is already true. *Without filling
vmcs_version*.
5) kvm_vcpu_ioctl_enable_cap() continues as usual and copies
*uninitialized* vmcs_version to userspace which leads to kernel info-leak.

Fix this issue by simply changing nested_enable_evmcs() to always fill
vmcs_version output argument. Even when enlightened_vmcs_enabled is
already set to true.

Note that SVM's nested_enable_evmcs() should not be modified because it
always returns a non-zero value (-ENODEV) which results in
kvm_vcpu_ioctl_enable_cap() skipping the copy of vmcs_version to
userspace (as it should).

Fixes: 57b119d ("KVM: nVMX: add KVM_CAP_HYPERV_ENLIGHTENED_VMCS capability")
Reported-by: [email protected]
Reviewed-by: Krish Sadhukhan <[email protected]>
Reviewed-by: Vitaly Kuznetsov <[email protected]>
Signed-off-by: Liran Alon <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

arch/x86/kvm/vmx.c

@@ -1611,12 +1611,6 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	/* We don't support disabling the feature for simplicity. */
-	if (vmx->nested.enlightened_vmcs_enabled)
-		return 0;
-
-	vmx->nested.enlightened_vmcs_enabled = true;
-
 	/*
 	 * vmcs_version represents the range of supported Enlightened VMCS
 	 * versions: lower 8 bits is the minimal version, higher 8 bits is the
@@ -1626,6 +1620,12 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
 	if (vmcs_version)
 		*vmcs_version = (KVM_EVMCS_VERSION << 8) | 1;
 
+	/* We don't support disabling the feature for simplicity. */
+	if (vmx->nested.enlightened_vmcs_enabled)
+		return 0;
+
+	vmx->nested.enlightened_vmcs_enabled = true;
+
 	vmx->nested.msrs.pinbased_ctls_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
 	vmx->nested.msrs.entry_ctls_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
 	vmx->nested.msrs.exit_ctls_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;