refactor(iast): prevent out-of-bounds read in slice aspect with negative step

ddtrace/appsec/_iast/_taint_tracking/aspects/aspect_slice.cpp

@@ -65,38 +65,26 @@ build_index_range_map(PyObject* text, TaintRangeRefs& ranges, PyObject* start, P
         index_range_map.emplace_back(nullptr);
         index++;
     }
-    TaintRangeRefs index_range_map_result;
-    long start_int = 0;
-    if (start != nullptr and start != Py_None) {
-        start_int = PyLong_AsLong(start);
-        if (start_int < 0) {
-            start_int = length_text + start_int;
-            if (start_int < 0) {
-                start_int = 0;
-            }
-        }
-    }
-
-    long stop_int = length_text;
-    if (stop != nullptr and stop != Py_None) {
-        stop_int = PyLong_AsLong(stop);
-        if (stop_int > length_text) {
-            stop_int = length_text;
-        } else if (stop_int < 0) {
-            stop_int = length_text + stop_int;
-            if (stop_int < 0) {
-                stop_int = 0;
-            }
-        }
+    PyObject* slice = PySlice_New(
+      (start != nullptr) ? start : Py_None, (stop != nullptr) ? stop : Py_None, (step != nullptr) ? step : Py_None);
+    if (slice == nullptr) {
+        return {};
     }
 
-    long step_int = 1;
-    if (step != nullptr and step != Py_None) {
-        step_int = PyLong_AsLong(step);
+    Py_ssize_t norm_start, norm_stop, norm_step, slicelength;
+    if (PySlice_GetIndicesEx(slice, length_text, &norm_start, &norm_stop, &norm_step, &slicelength) < 0) {
+        Py_DECREF(slice);
+        return {};
     }
+    Py_DECREF(slice);
 
-    for (auto i = start_int; i < stop_int; i += step_int) {
-        index_range_map_result.emplace_back(index_range_map[i]);
+    TaintRangeRefs index_range_map_result;
+    Py_ssize_t map_size = static_cast<Py_ssize_t>(index_range_map.size());
+    for (Py_ssize_t i = 0; i < slicelength; i++) {
+        Py_ssize_t idx = norm_start + i * norm_step;
+        if (idx >= 0 && idx < map_size) {
+            index_range_map_result.emplace_back(index_range_map[idx]);
+        }
     }
 
     return index_range_map_result;

tests/appsec/iast/aspects/test_slice_aspect_fixtures.py

@@ -288,6 +288,37 @@ def test_string_slice_negative():
     assert result == expected_result
 
 
+@pytest.mark.parametrize(
+    "input_str, start_pos, end_pos, step, expected_result",
+    [
+        # Regression: negative step triggered OOB read in build_index_range_map
+        # because start_int=0 < stop_int=len satisfied i<stop for all negative i.
+        ("hello", None, None, -1, "olleh"),
+        ("hello", None, None, -2, "olh"),
+        ("hello", 4, 0, -1, "olle"),
+    ],
+)
+def test_string_slice_negative_step_oob_regression(
+    input_str: str, start_pos: object, end_pos: object, step: int, expected_result: str
+) -> None:
+    """Regression test for out-of-bounds read in slice aspect with negative step."""
+    result = mod.do_slice(input_str, start_pos, end_pos, step)  # pylint: disable=no-member
+    assert result == expected_result
+
+    tainted_input = taint_pyobject(
+        pyobject=input_str,
+        source_name="input_str",
+        source_value="foo",
+        source_origin=OriginType.PARAMETER,
+    )
+    result = mod.do_slice(tainted_input, start_pos, end_pos, step)  # pylint: disable=no-member
+    assert result == expected_result
+    tainted_ranges = get_tainted_ranges(result)
+    assert len(tainted_ranges) == 1
+    assert tainted_ranges[0].start == 0
+    assert tainted_ranges[0].length == len(expected_result)
+
+
 @pytest.mark.skip_iast_check_logs
 def test_propagate_ranges_with_no_context(caplog):
     tainted_input = taint_pyobject(