Python: Split modelling of query operators

`$where` and `$function` behave quite differently.

Author: yoff

python/ql/lib/semmle/python/frameworks/NoSQL.qll

@@ -99,9 +99,6 @@ private module NoSql {
       ]
   }
 
-  /** Gets the name of a mongo query operator that will interpret JavaScript. */
-  private string mongoQueryOperator() { result in ["$where", "$function"] }
-
   /**
    * Gets a reference to a `Mongo` collection method call
    *
@@ -125,12 +122,34 @@ private module NoSql {
     override predicate vulnerableToStrings() { none() }
   }
 
-  private class MongoCollectionQueryOperator extends API::CallNode, NoSqlQuery::Range {
+  /** The `$where` query operator executes a string as JavaScript. */
+  private class WhereQueryOperator extends API::CallNode, NoSqlQuery::Range {
     DataFlow::Node query;
 
-    MongoCollectionQueryOperator() {
+    WhereQueryOperator() {
       this = mongoCollection().getMember(mongoCollectionMethodName()).getACall() and
-      query = this.getParameter(0).getSubscript(mongoQueryOperator()).asSink()
+      query = this.getParameter(0).getSubscript("$where").asSink()
+    }
+
+    override DataFlow::Node getQuery() { result = query }
+
+    override predicate interpretsDict() { none() }
+
+    override predicate vulnerableToStrings() { any() }
+  }
+
+  /** The `$function` query operator executes its `body` string as JavaScript. */
+  private class FunctionQueryOperator extends API::CallNode, NoSqlQuery::Range {
+    DataFlow::Node query;
+
+    FunctionQueryOperator() {
+      this = mongoCollection().getMember(mongoCollectionMethodName()).getACall() and
+      query =
+        this.getParameter(0)
+            .getASubscript*()
+            .getSubscript("$function")
+            .getSubscript("body")
+            .asSink()
     }
 
     override DataFlow::Node getQuery() { result = query }

python/ql/test/query-tests/Security/CWE-943-NoSqlInjection/PoC/server.py

@@ -46,30 +46,29 @@ def by_where():
     post = posts.find_one({'$where': 'this.author === "'+author+'"'}) # $ result=BAD
     return show_post(post, author)
 
-
 @app.route('/byFunction', methods=['GET'])
 def by_function():
     author = request.args['author']
     search = {
-        "body": 'function(author) { return(author === "'+author+'") }',
+        "body": 'function(author) { return(author === "'+author+'") }', # $ result=BAD
         "args": [ "$author" ],
         "lang": "js"
     }
     # Use `" | "a" === "a` as author
     # making the query `this.author === "" | "a" === "a"`
     # Found by http://127.0.0.1:5000/byFunction?author=%22%20|%20%22a%22%20===%20%22a
-    post = posts.find_one({'$expr': {'$function': search}}) # $ MISING: result=BAD
+    post = posts.find_one({'$expr': {'$function': search}}) # $ result=BAD
     return show_post(post, author)
 
 @app.route('/byFunctionArg', methods=['GET'])
 def by_function_arg():
     author = request.args['author']
     search = {
-        "body": 'function(author, target) { return(author === target) }',
+        "body": 'function(author, target) { return(author === target) }', # $ result=OK
         "args": [ "$author", author ],
         "lang": "js"
     }
-    post = posts.find_one({'$expr': {'$function': search}}) # $ result=OK
+    post = posts.find_one({'$expr': {'$function': search}}) # $ SPURIOUS: result=BAD
     return show_post(post, author)
 
 @app.route('/', methods=['GET'])