Fix cgo data conversion for bytes and strings

This commit fixes the conversion of CFStringRef to strings when
CFStringGetCStringPtr fails. It also fixes panics extracting attributes
not present in the key, one example of this is extracting the tag in the
mackms SearchKeys method, if a key doesn't have one.

internal/darwin/security/security_darwin.go

@@ -320,34 +320,22 @@ func SecCopyErrorMessageString(status C.OSStatus) *cf.StringRef {
 
 func GetSecAttrApplicationLabel(v *cf.DictionaryRef) []byte {
 	data := C.CFDataRef(C.CFDictionaryGetValue(C.CFDictionaryRef(v.Value), unsafe.Pointer(C.kSecAttrApplicationLabel)))
-	return C.GoBytes(
-		unsafe.Pointer(C.CFDataGetBytePtr(data)),
-		C.int(C.CFDataGetLength(data)),
-	)
+	return goBytes(data)
 }
 
 func GetSecAttrApplicationTag(v *cf.DictionaryRef) string {
 	data := C.CFDataRef(C.CFDictionaryGetValue(C.CFDictionaryRef(v.Value), unsafe.Pointer(C.kSecAttrApplicationTag)))
-	return string(C.GoBytes(
-		unsafe.Pointer(C.CFDataGetBytePtr(data)),
-		C.int(C.CFDataGetLength(data)),
-	))
+	return string(goBytes(data))
 }
 
-func GetSecAttrLabel(v *cf.DictionaryRef) (label string) {
+func GetSecAttrLabel(v *cf.DictionaryRef) string {
 	ref := C.CFStringRef(C.CFDictionaryGetValue(C.CFDictionaryRef(v.Value), unsafe.Pointer(C.kSecAttrLabel)))
-	if cstr := C.CFStringGetCStringPtr(ref, C.kCFStringEncodingUTF8); cstr != nil {
-		label = C.GoString(cstr)
-	}
-	return label
+	return goString(ref)
 }
 
-func GetSecAttrTokenID(v *cf.DictionaryRef) (tokenID string) {
+func GetSecAttrTokenID(v *cf.DictionaryRef) string {
 	ref := C.CFStringRef(C.CFDictionaryGetValue(C.CFDictionaryRef(v.Value), unsafe.Pointer(C.kSecAttrTokenID)))
-	if cstr := C.CFStringGetCStringPtr(ref, C.kCFStringEncodingUTF8); cstr != nil {
-		tokenID = C.GoString(cstr)
-	}
-	return tokenID
+	return goString(ref)
 }
 
 func GetSecAttrAccessControl(v *cf.DictionaryRef) *SecAccessControlRef {
@@ -376,10 +364,7 @@ func GetSecAttrAccessControl(v *cf.DictionaryRef) *SecAccessControlRef {
 
 func GetSecValueData(v *cf.DictionaryRef) []byte {
 	data := C.CFDataRef(C.CFDictionaryGetValue(C.CFDictionaryRef(v.Value), unsafe.Pointer(C.kSecValueData)))
-	return C.GoBytes(
-		unsafe.Pointer(C.CFDataGetBytePtr(data)),
-		C.int(C.CFDataGetLength(data)),
-	)
+	return goBytes(data)
 }
 
 type osStatusError struct {
@@ -406,9 +391,7 @@ func goOSStatus(status C.OSStatus) error {
 
 	var message string
 	if ref := SecCopyErrorMessageString(status); ref.Value != 0 {
-		if cstr := C.CFStringGetCStringPtr(C.CFStringRef(ref.Value), C.kCFStringEncodingUTF8); cstr != nil {
-			message = C.GoString(cstr)
-		}
+		message = goString(C.CFStringRef(ref.Value))
 		defer ref.Release()
 	}
 	return osStatusError{
@@ -417,6 +400,41 @@ func goOSStatus(status C.OSStatus) error {
 	}
 }
 
+func goBytes(data C.CFDataRef) []byte {
+	if data == 0 {
+		return nil
+	}
+	return C.GoBytes(
+		unsafe.Pointer(C.CFDataGetBytePtr(data)),
+		C.int(C.CFDataGetLength(data)),
+	)
+}
+
+func goString(ref C.CFStringRef) string {
+	if ref == 0 {
+		return ""
+	}
+
+	// CFStringGetCStringPtr either returns the requested pointer immediately,
+	// with no memory allocations and no copying, in constant time, or returns
+	// NULL.
+	if cstr := C.CFStringGetCStringPtr(ref, C.kCFStringEncodingUTF8); cstr != nil {
+		return C.GoString(cstr)
+	}
+
+	// The documentation recommends using CFStringGetCString if the previous one
+	// fails.
+	length := C.CFStringGetLength(ref)
+	buf := (*C.char)(C.malloc(C.size_t(length) + 1))
+	defer C.free(unsafe.Pointer(buf))
+
+	if C.CFStringGetCString(ref, buf, length+1, C.kCFStringEncodingUTF8) == 0 {
+		return ""
+	}
+
+	return C.GoString(buf)
+}
+
 type cfError struct {
 	code    int
 	message string