Python: Model HTTP responses in tornado

This is quite a simpel model, but ends up matching what we were able to do with
points-to.

I think this modeling excercise really shows that we need a bit of a different
way to model HTTP responses... but I'm not going to try to fix that in this PR.

python/ql/src/semmle/python/frameworks/Tornado.qll

@@ -216,6 +216,17 @@ private module Tornado {
         /** Gets a reference to one of the methods `get_arguments`, `get_body_arguments`, `get_query_arguments`. */
         DataFlow::Node argumentsMethod() { result = argumentsMethod(DataFlow::TypeTracker::end()) }
 
+        /** Gets a reference to the `write` method. */
+        private DataFlow::Node writeMethod(DataFlow::TypeTracker t) {
+          t.startInAttr("write") and
+          result = instance()
+          or
+          exists(DataFlow::TypeTracker t2 | result = writeMethod(t2).track(t2, t))
+        }
+
+        /** Gets a reference to the `write` method. */
+        DataFlow::Node writeMethod() { result = writeMethod(DataFlow::TypeTracker::end()) }
+
         private class AdditionalTaintStep extends TaintTracking::AdditionalTaintStep {
           override predicate step(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
             // Method access
@@ -540,4 +551,29 @@ private module Tornado {
       not result = this.getArg(0)
     }
   }
+
+  // ---------------------------------------------------------------------------
+  // Response modeling
+  // ---------------------------------------------------------------------------
+  /**
+   * A call to `tornado.web.RequestHandler.write` method.
+   *
+   * See https://www.tornadoweb.org/en/stable/web.html?highlight=write#tornado.web.RequestHandler.write
+   */
+  private class TornadoRequestHandlerWriteCall extends HTTP::Server::HttpResponse::Range,
+    DataFlow::CfgNode {
+    override CallNode node;
+
+    TornadoRequestHandlerWriteCall() {
+      node.getFunction() = tornado::web::RequestHandler::writeMethod().asCfgNode()
+    }
+
+    override DataFlow::Node getBody() {
+      result.asCfgNode() in [node.getArg(0), node.getArgByName("chunk")]
+    }
+
+    override string getMimetypeDefault() { result = "text/html" }
+
+    override DataFlow::Node getMimetypeOrContentTypeArg() { none() }
+  }
 }