gh-95185: Check recursion depth in the AST constructor

Lib/test/test_ast.py

@@ -793,6 +793,27 @@ def next(self):
                     return self
         enum._test_simple_enum(_Precedence, ast._Precedence)
 
+    @support.cpython_only
+    def test_ast_recursion_limit(self):
+        fail_depth = sys.getrecursionlimit() * 3
+        crash_depth = sys.getrecursionlimit() * 300
+        success_depth = int(fail_depth * 0.75)
+
+        def check_limit(prefix, repeated):
+            expect_ok = prefix + repeated * success_depth
+            ast.parse(expect_ok)
+            for depth in (fail_depth, crash_depth):
+                broken = prefix + repeated * depth
+                details = "Compiling ({!r} + {!r} * {})".format(
+                            prefix, repeated, depth)
+                with self.assertRaises(RecursionError, msg=details):
+                    ast.parse(broken)
+
+        check_limit("a", "()")
+        check_limit("a", ".b")
+        check_limit("a", "[0]")
+        check_limit("a", "*a")
+
 
 class ASTHelpers_Test(unittest.TestCase):
     maxDiff = None

Misc/NEWS.d/next/Core and Builtins/2022-07-24-00-27-47.gh-issue-95185.ghYTZx.rst

@@ -0,0 +1,3 @@
+Prevented crashes in the AST constructor when compiling some absurdly long
+expressions like ``"+0"*1000000``. :exc:`RecursionError` is now raised
+instead. Patch by Pablo Galindo

Parser/asdl_c.py

@@ -1112,6 +1112,8 @@ def visitModule(self, mod):
         for dfn in mod.dfns:
             self.visit(dfn)
         self.file.write(textwrap.dedent('''
+                state->recursion_depth = 0;
+                state->recursion_limit = 0;
                 state->initialized = 1;
                 return 1;
             }
@@ -1259,8 +1261,14 @@ def func_begin(self, name):
         self.emit('if (!o) {', 1)
         self.emit("Py_RETURN_NONE;", 2)
         self.emit("}", 1)
+        self.emit("if (++state->recursion_depth > state->recursion_limit) {", 1)
+        self.emit("PyErr_SetString(PyExc_RecursionError,", 2)
+        self.emit('"maximum recursion depth exceeded during ast construction");', 3)
+        self.emit("return 0;", 2)
+        self.emit("}", 1)
 
     def func_end(self):
+        self.emit("state->recursion_depth--;", 1)
         self.emit("return result;", 1)
         self.emit("failed:", 0)
         self.emit("Py_XDECREF(value);", 1)
@@ -1371,7 +1379,32 @@ class PartingShots(StaticVisitor):
     if (state == NULL) {
         return NULL;
     }
-    return ast2obj_mod(state, t);
+
+    int recursion_limit = Py_GetRecursionLimit();
+    int starting_recursion_depth;
+    /* Be careful here to prevent overflow. */
+    int COMPILER_STACK_FRAME_SCALE = 3;
+    PyThreadState *tstate = _PyThreadState_GET();
+    if (!tstate) {
+        return 0;
+    }
+    state->recursion_limit = (recursion_limit < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        recursion_limit * COMPILER_STACK_FRAME_SCALE : recursion_limit;
+    int recursion_depth = tstate->recursion_limit - tstate->recursion_remaining;
+    starting_recursion_depth = (recursion_depth < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        recursion_depth * COMPILER_STACK_FRAME_SCALE : recursion_depth;
+    state->recursion_depth = starting_recursion_depth;
+
+    PyObject *result = ast2obj_mod(state, t);
+
+    /* Check that the recursion depth counting balanced correctly */
+    if (result && state->recursion_depth != starting_recursion_depth) {
+        PyErr_Format(PyExc_SystemError,
+            "AST constructor recursion depth mismatch (before=%d, after=%d)",
+            starting_recursion_depth, state->recursion_depth);
+        return 0;
+    }
+    return result;
 }
 
 /* mode is 0 for "exec", 1 for "eval" and 2 for "single" input */
@@ -1437,6 +1470,8 @@ def visit(self, object):
 def generate_ast_state(module_state, f):
     f.write('struct ast_state {\n')
     f.write('    int initialized;\n')
+    f.write('    int recursion_depth;\n')
+    f.write('    int recursion_limit;\n')
     for s in module_state:
         f.write('    PyObject *' + s + ';\n')
     f.write('};')