[3.14] gh-161388: [tkinter] Fix crash on raising __str__ method passed to setvar (GH-161397) (#161551)

(cherry picked from commit 6026791)

Author: sobolevn

Lib/test/test_tcl.py

@@ -565,6 +565,29 @@ class List(list):
         self.assertEqual(self.passValue(values := [Value(), "pad"]), expect)
         self.assertEqual(self.passValue(values := List([Value(), "pad"])), expect)
 
+    def test_setvar_evil(self):
+        # gh-161388
+        class StrRaises:
+            def __str__(self):
+                raise RuntimeError("setvar evil")
+
+        value = StrRaises()
+        tk = self.interp.tk
+
+        with self.assertRaisesRegex(RuntimeError, "setvar evil"):
+            tk.setvar("local_name1", value)
+        self.assertRaises(TclError, tk.getvar, "local_name1")
+        with self.assertRaisesRegex(RuntimeError, "setvar evil"):
+            tk.setvar("local_name2", "local_name3", value)
+        self.assertRaises(TclError, tk.getvar, "local_name2", "local_name3")
+
+        with self.assertRaisesRegex(RuntimeError, "setvar evil"):
+            tk.globalsetvar("global_name1", value)
+        self.assertRaises(TclError, tk.globalgetvar, "global_name1")
+        with self.assertRaisesRegex(RuntimeError, "setvar evil"):
+            tk.globalsetvar("global_name2", "global_name3", value)
+        self.assertRaises(TclError, tk.globalgetvar, "global_name2", "global_name3")
+
     def test_user_command(self):
         result = None
         def testfunc(arg):

Modules/_tkinter.c

@@ -1220,8 +1220,9 @@ AsObj(PyObject *value)
 
     {
         PyObject *v = PyObject_Str(value);
-        if (!v)
-            return 0;
+        if (!v) {
+            return NULL;
+        }
         result = AsObj(v);
         Py_DECREF(v);
         return result;
@@ -1958,6 +1959,9 @@ SetVar(TkappObject *self, PyObject *args, int flags)
 
         /* XXX must hold tcl lock already??? */
         newval = AsObj(newValue);
+        if (newval == NULL) {
+            return NULL;
+        }
         if (self->trace) {
             if (flags & TCL_GLOBAL_ONLY) {
                 TRACE(self, ("((sssNO))", "uplevel", "#0", "set",