fix #881, check overlap before calling strcpy in cJSON_SetValuestring

cJSON.c

@@ -403,6 +403,8 @@ CJSON_PUBLIC(double) cJSON_SetNumberHelper(cJSON *object, double number)
 CJSON_PUBLIC(char*) cJSON_SetValuestring(cJSON *object, const char *valuestring)
 {
     char *copy = NULL;
+    size_t v1_len;
+    size_t v2_len;
     /* if object's type is not cJSON_String or is cJSON_IsReference, it should not set valuestring */
     if ((object == NULL) || !(object->type & cJSON_String) || (object->type & cJSON_IsReference))
     {
@@ -413,8 +415,17 @@ CJSON_PUBLIC(char*) cJSON_SetValuestring(cJSON *object, const char *valuestring)
     {
         return NULL;
     }
-    if (strlen(valuestring) <= strlen(object->valuestring))
+
+    v1_len = strlen(valuestring);
+    v2_len = strlen(object->valuestring);
+
+    if (v1_len <= v2_len)
     {
+        /* strcpy does not handle overlapping string: [X1, X2] [Y1, Y2] => X2 < Y1 or Y2 < X1 */
+        if (!( valuestring + v1_len < object->valuestring || object->valuestring + v2_len < valuestring ))
+        {
+            return NULL;
+        }
         strcpy(object->valuestring, valuestring);
         return object->valuestring;
     }