KVM: x86: Add a helper to retrieve userspace address from kvm_device_attr

Add a helper to handle converting the u64 userspace address embedded in
struct kvm_device_attr into a userspace pointer, it's all too easy to
forget the intermediate "unsigned long" cast as well as the truncation
check.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -4332,7 +4332,15 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		break;
 	}
 	return r;
+}
+
+static inline void __user *kvm_get_attr_addr(struct kvm_device_attr *attr)
+{
+	void __user *uaddr = (void __user*)(unsigned long)attr->addr;
 
+	if ((u64)(unsigned long)uaddr != attr->addr)
+		return ERR_PTR(-EFAULT);
+	return uaddr;
 }
 
 long kvm_arch_dev_ioctl(struct file *filp,
@@ -5025,11 +5033,11 @@ static int kvm_arch_tsc_has_attr(struct kvm_vcpu *vcpu,
 static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
 				 struct kvm_device_attr *attr)
 {
-	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
+	u64 __user *uaddr = kvm_get_attr_addr(attr);
 	int r;
 
-	if ((u64)(unsigned long)uaddr != attr->addr)
-		return -EFAULT;
+	if (IS_ERR(uaddr))
+		return PTR_ERR(uaddr);
 
 	switch (attr->attr) {
 	case KVM_VCPU_TSC_OFFSET:
@@ -5048,12 +5056,12 @@ static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
 static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
 				 struct kvm_device_attr *attr)
 {
-	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
+	u64 __user *uaddr = kvm_get_attr_addr(attr);
 	struct kvm *kvm = vcpu->kvm;
 	int r;
 
-	if ((u64)(unsigned long)uaddr != attr->addr)
-		return -EFAULT;
+	if (IS_ERR(uaddr))
+		return PTR_ERR(uaddr);
 
 	switch (attr->attr) {
 	case KVM_VCPU_TSC_OFFSET: {