JavaScript: Add basic support for reasoning about reflective parameter accesses.

Currently, only `arguments[c]` for a constant value `c` is supported.

This allows us to detect the prototype-pollution vulnerabilities in (old versions of) `extend`, `jquery`, and `node.extend`.

javascript/ql/src/semmle/javascript/dataflow/internal/FlowSteps.qll

@@ -151,11 +151,14 @@ private module CachedSteps {
   ) {
     calls(invk, f) and
     (
-      exists(int i, Parameter p |
-        f.getParameter(i) = p and
-        not p.isRestParameter() and
-        arg = invk.getArgument(i) and
-        parm = DataFlow::parameterNode(p)
+      exists(int i | arg = invk.getArgument(i) |
+        exists(Parameter p |
+          f.getParameter(i) = p and
+          not p.isRestParameter() and
+          parm = DataFlow::parameterNode(p)
+        )
+        or
+        parm = reflectiveParameterAccess(f, i)
       )
       or
       arg = invk.(DataFlow::CallNode).getReceiver() and
@@ -185,6 +188,22 @@ private module CachedSteps {
     )
   }
 
+  /**
+   * Gets a data-flow node inside `f` that refers to the `arguments` object of `f`.
+   */
+  private DataFlow::Node argumentsAccess(Function f) {
+    result.getContainer().getEnclosingContainer*() = f and
+    result.analyze().getAValue().(AbstractArguments).getFunction() = f
+  }
+
+  /**
+   * Gets a data-flow node that refers to the `i`th parameter of `f` through its `arguments`
+   * object.
+   */
+  private DataFlow::SourceNode reflectiveParameterAccess(Function f, int i) {
+    result.(DataFlow::PropRead).accesses(argumentsAccess(f), any(string p | i = p.toInt()))
+  }
+
   /**
    * Holds if there is a flow step from `pred` to `succ` through parameter passing
    * to a function call.

javascript/ql/test/library-tests/DataFlow/argumentPassing.expected

@@ -1,3 +1,7 @@
+| arguments.js:11:5:11:14 | f(1, 2, 3) | arguments.js:11:7:11:7 | 1 | arguments.js:2:5:10:5 | functio ... ;\\n    } | arguments.js:2:16:2:16 | x |
+| arguments.js:11:5:11:14 | f(1, 2, 3) | arguments.js:11:7:11:7 | 1 | arguments.js:2:5:10:5 | functio ... ;\\n    } | arguments.js:4:28:4:39 | arguments[0] |
+| arguments.js:11:5:11:14 | f(1, 2, 3) | arguments.js:11:10:11:10 | 2 | arguments.js:2:5:10:5 | functio ... ;\\n    } | arguments.js:5:25:5:36 | arguments[1] |
+| arguments.js:11:5:11:14 | f(1, 2, 3) | arguments.js:11:13:11:13 | 3 | arguments.js:2:5:10:5 | functio ... ;\\n    } | arguments.js:7:24:7:30 | args[2] |
 | sources.js:3:1:5:6 | (functi ... \\n})(23) | sources.js:5:4:5:5 | 23 | sources.js:3:2:5:1 | functio ... x+19;\\n} | sources.js:3:11:3:11 | x |
 | tst.js:16:1:20:9 | (functi ... ("arg") | tst.js:20:4:20:8 | "arg" | tst.js:16:2:20:1 | functio ... n "";\\n} | tst.js:16:13:16:13 | a |
 | tst.js:35:1:35:7 | g(true) | tst.js:35:3:35:6 | true | tst.js:32:1:34:1 | functio ... ables\\n} | tst.js:32:12:32:12 | b |

javascript/ql/test/library-tests/DataFlow/arguments.js

@@ -0,0 +1,12 @@
+(function() {
+    function f(x) {
+        let firstArg = x;
+        let alsoFirstArg = arguments[0];
+        let secondArg = arguments[1];
+        let args = arguments;
+        let thirdArg = args[2];
+        arguments = {};
+        let notFirstArg = arguments[0];
+    }
+    f(1, 2, 3);
+})();