Python: Handle view functions with decorators

python/change-notes/2021-12-21-django-view-function-with-decorator.md

@@ -0,0 +1,2 @@
+lgtm,codescanning
+* Improved modeling of `django` to recognize request handlers functions that are decorated (for example with `django.views.decorators.http.require_GET`). This leads to more sources of remote user input (`RemoteFlowSource`), since we correctly identify the first parameter as being passed a django request.

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

@@ -1975,6 +1975,14 @@ private module Django {
     }
   }
 
+  /**
+   * Gets the last decorator call for the function `func`, if `func` has decorators.
+   */
+  private Expr lastDecoratorCall(Function func) {
+    result = func.getDefinition().(FunctionExpr).getADecoratorCall() and
+    not exists(Call other_decorator | other_decorator.getArg(0) = result)
+  }
+
   // ---------------------------------------------------------------------------
   // routing modeling
   // ---------------------------------------------------------------------------
@@ -1987,7 +1995,18 @@ private module Django {
    */
   private DataFlow::Node djangoRouteHandlerFunctionTracker(DataFlow::TypeTracker t, Function func) {
     t.start() and
-    result = DataFlow::exprNode(func.getDefinition())
+    (
+      not exists(func.getADecorator()) and
+      result.asExpr() = func.getDefinition()
+      or
+      // If the function has decorators, we still want to model the function as being
+      // the request handler for a route setup. In such situations, we must track the
+      // last decorator call instead of the function itself.
+      //
+      // Note that this means that we blindly ignore what the decorator actually does to
+      // the function, which seems like an OK tradeoff.
+      result.asExpr() = lastDecoratorCall(func)
+    )
     or
     exists(DataFlow::TypeTracker t2 |
       result = djangoRouteHandlerFunctionTracker(t2, func).track(t2, t)

python/ql/test/experimental/library-tests/frameworks/django-v2-v3/routing_test.py

@@ -134,7 +134,7 @@ class PossiblyNotRouted(View):
 
 
 @require_GET
-def with_decorator(request, foo):  # $ MISSING: requestHandler routedParameter=foo
+def with_decorator(request, foo):  # $ requestHandler routedParameter=foo
     pass
 
 urlpatterns = [