[3.10] gh-95778: CVE-2020-10735: Prevent DoS by very large int() (#96501)

Integer to and from text conversions via CPython's bignum `int` type is not safe against denial of service attacks due to malicious input. Very large input strings with hundred thousands of digits can consume several CPU seconds.

This PR comes fresh from a pile of work done in our private PSRT security response team repo.

This backports https://github.com/python/cpython/pull/96499 aka 511ca9452033ef95bc7d7fc404b8161068226002

Signed-off-by: Christian Heimes [Red Hat] <christian@python.org>
Tons-of-polishing-up-by: Gregory P. Smith [Google] <greg@krypto.org>
Reviews via the private PSRT repo via many others (see the NEWS entry in the PR).

<!-- gh-issue-number: gh-95778 -->
* Issue: gh-95778
<!-- /gh-issue-number -->

I wrote up [a one pager for the release managers](https://docs.google.com/document/d/1KjuF_aXlzPUxTK4BMgezGJ2Pn7uevfX7g0_mvgHlL7Y/edit#).

1 files changed, 45 insertions, 1 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index ac49f78..1d5a06a 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -17,6 +17,7 @@ Data members:
 #include "Python.h"
 #include "pycore_ceval.h"         // _Py_RecursionLimitLowerWaterMark()
 #include "pycore_initconfig.h"    // _PyStatus_EXCEPTION()
+#include "pycore_long.h"          // _PY_LONG_MAX_STR_DIGITS_THRESHOLD
 #include "pycore_object.h"        // _PyObject_IS_GC()
 #include "pycore_pathconfig.h"    // _PyPathConfig_ComputeSysPath0()
 #include "pycore_pyerrors.h"      // _PyErr_Fetch()
@@ -1652,6 +1653,45 @@ sys_mdebug_impl(PyObject *module, int flag)
 }
 #endif /* USE_MALLOPT */
 
+
+/*[clinic input]
+sys.get_int_max_str_digits
+
+Set the maximum string digits limit for non-binary int<->str conversions.
+[clinic start generated code]*/
+
+static PyObject *
+sys_get_int_max_str_digits_impl(PyObject *module)
+/*[clinic end generated code: output=0042f5e8ae0e8631 input=8dab13e2023e60d5]*/
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return PyLong_FromSsize_t(interp->int_max_str_digits);
+}
+
+/*[clinic input]
+sys.set_int_max_str_digits
+
+    maxdigits: int
+
+Set the maximum string digits limit for non-binary int<->str conversions.
+[clinic start generated code]*/
+
+static PyObject *
+sys_set_int_max_str_digits_impl(PyObject *module, int maxdigits)
+/*[clinic end generated code: output=734d4c2511f2a56d input=d7e3f325db6910c5]*/
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    if ((!maxdigits) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD)) {
+        tstate->interp->int_max_str_digits = maxdigits;
+        Py_RETURN_NONE;
+    } else {
+        PyErr_Format(
+            PyExc_ValueError, "maxdigits must be 0 or larger than %d",
+            _PY_LONG_MAX_STR_DIGITS_THRESHOLD);
+        return NULL;
+    }
+}
+
 size_t
 _PySys_GetSizeOf(PyObject *o)
 {
@@ -2027,6 +2067,8 @@ static PyMethodDef sys_methods[] = {
     SYS_GETANDROIDAPILEVEL_METHODDEF
     SYS_UNRAISABLEHOOK_METHODDEF
     SYS__DEACTIVATE_OPCACHE_METHODDEF
+    SYS_GET_INT_MAX_STR_DIGITS_METHODDEF
+    SYS_SET_INT_MAX_STR_DIGITS_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -2516,6 +2558,7 @@ static PyStructSequence_Field flags_fields[] = {
     {"dev_mode",                "-X dev"},
     {"utf8_mode",               "-X utf8"},
     {"warn_default_encoding",   "-X warn_default_encoding"},
+    {"int_max_str_digits",      "-X int_max_str_digits"},
     {0}
 };
 
@@ -2523,7 +2566,7 @@ static PyStructSequence_Desc flags_desc = {
     "sys.flags",        /* name */
     flags__doc__,       /* doc */
     flags_fields,       /* fields */
-    16
+    17
 };
 
 static int
@@ -2563,6 +2606,7 @@ set_flags_from_config(PyInterpreterState *interp, PyObject *flags)
     SetFlagObj(PyBool_FromLong(config->dev_mode));
     SetFlag(preconfig->utf8_mode);
     SetFlag(config->warn_default_encoding);
+    SetFlag(_Py_global_config_int_max_str_digits);
 #undef SetFlagObj
 #undef SetFlag
     return 0;