Python: Don't report mutable parameters that are in fact immutable.

Fixes #1832.

In the taint sink, we add an additional check that the given control-flow node
can indeed point to a value that is mutable. This takes care of the guard on the
type.

If and when we get around to adding configurations for all of the taint
analyses, we may want to implement this as a barrier instead, pruning any steps
that go through a type test where the type is not mutable.

python/ql/src/Functions/ModificationOfParameterWithDefault.ql

@@ -73,6 +73,11 @@ class MutableDefaultValue extends TaintSource {
     }
 }
 
+private ClassValue mutable_class() {
+    result = Value::named("list") or
+    result = Value::named("dict")
+}
+
 class Mutation extends TaintSink {
     Mutation() {
         exists(AugAssign a | a.getTarget().getAFlowNode() = this)
@@ -80,7 +85,8 @@ class Mutation extends TaintSink {
         exists(Call c, Attribute a |
             c.getFunc() = a |
             a.getObject().getAFlowNode() = this and
-            not safe_method(a.getName())
+            not safe_method(a.getName()) and
+            this.(ControlFlowNode).pointsTo().getClass() = mutable_class()
         )
     }
 

python/ql/test/query-tests/Functions/general/ModificationOfParameterWithDefault.expected

@@ -7,8 +7,18 @@ edges
 | functions_test.py:157:27:157:27 | empty mutable value | functions_test.py:159:21:159:21 | empty mutable value |
 | functions_test.py:158:25:158:25 | empty mutable value | functions_test.py:151:25:151:25 | empty mutable value |
 | functions_test.py:159:21:159:21 | empty mutable value | functions_test.py:154:21:154:21 | empty mutable value |
+| functions_test.py:175:28:175:28 | non-empty mutable value | functions_test.py:179:9:179:9 | non-empty mutable value |
+| functions_test.py:175:28:175:28 | non-empty mutable value | functions_test.py:181:9:181:9 | non-empty mutable value |
+| functions_test.py:188:18:188:18 | non-empty mutable value | functions_test.py:189:28:189:28 | non-empty mutable value |
+| functions_test.py:189:28:189:28 | non-empty mutable value | functions_test.py:175:28:175:28 | non-empty mutable value |
+| functions_test.py:191:18:191:18 | non-empty mutable value | functions_test.py:192:28:192:28 | non-empty mutable value |
+| functions_test.py:192:28:192:28 | non-empty mutable value | functions_test.py:175:28:175:28 | non-empty mutable value |
 #select
 | functions_test.py:40:5:40:5 | x | functions_test.py:39:9:39:9 | empty mutable value | functions_test.py:40:5:40:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:39:9:39:9 | x | Default value |
 | functions_test.py:134:5:134:5 | x | functions_test.py:133:15:133:15 | empty mutable value | functions_test.py:134:5:134:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:133:15:133:15 | x | Default value |
 | functions_test.py:152:5:152:5 | x | functions_test.py:157:27:157:27 | empty mutable value | functions_test.py:152:5:152:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:157:27:157:27 | y | Default value |
 | functions_test.py:155:5:155:5 | x | functions_test.py:157:27:157:27 | empty mutable value | functions_test.py:155:5:155:5 | empty mutable value | $@ flows to here and is mutated. | functions_test.py:157:27:157:27 | y | Default value |
+| functions_test.py:179:9:179:9 | x | functions_test.py:188:18:188:18 | non-empty mutable value | functions_test.py:179:9:179:9 | non-empty mutable value | $@ flows to here and is mutated. | functions_test.py:188:18:188:18 | x | Default value |
+| functions_test.py:179:9:179:9 | x | functions_test.py:191:18:191:18 | non-empty mutable value | functions_test.py:179:9:179:9 | non-empty mutable value | $@ flows to here and is mutated. | functions_test.py:191:18:191:18 | x | Default value |
+| functions_test.py:181:9:181:9 | x | functions_test.py:188:18:188:18 | non-empty mutable value | functions_test.py:181:9:181:9 | non-empty mutable value | $@ flows to here and is mutated. | functions_test.py:188:18:188:18 | x | Default value |
+| functions_test.py:181:9:181:9 | x | functions_test.py:191:18:191:18 | non-empty mutable value | functions_test.py:181:9:181:9 | non-empty mutable value | $@ flows to here and is mutated. | functions_test.py:191:18:191:18 | x | Default value |

python/ql/test/query-tests/Functions/general/functions_test.py

@@ -168,3 +168,28 @@ def issue1143(expr, param=[]):
         return result
     for i in param:
         param.remove(i) # Mutation here
+
+
+# Type guarding of modification of parameter with default:
+
+def do_stuff_based_on_type(x):
+    if isinstance(x, str):
+        x = x.split()
+    elif isinstance(x, dict):
+        x.setdefault('foo', 'bar')
+    elif isinstance(x, list):
+        x.append(5)
+    elif isinstance(x, tuple):
+        x = x.unknown_method()
+
+def str_default(x="hello world"):
+    do_stuff_based_on_type(x)
+
+def dict_default(x={'baz':'quux'}):
+    do_stuff_based_on_type(x)
+
+def list_default(x=[1,2,3,4]):
+    do_stuff_based_on_type(x)
+
+def tuple_default(x=(1,2)):
+    do_stuff_based_on_type(x)