gh-95778: CVE-2020-10735: Prevent DoS by very large int() (#96499)

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.

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#). Much of that text wound up in the Issue. Backports PRs already exist. See the issue for links.

Python/initconfig.c

@@ -3,6 +3,7 @@
 #include "pycore_getopt.h"        // _PyOS_GetOpt()
 #include "pycore_initconfig.h"    // _PyStatus_OK()
 #include "pycore_interp.h"        // _PyInterpreterState.runtime
+#include "pycore_long.h"          // _PY_LONG_MAX_STR_DIGITS_THRESHOLD
 #include "pycore_pathconfig.h"    // _Py_path_config
 #include "pycore_pyerrors.h"      // _PyErr_Fetch()
 #include "pycore_pylifecycle.h"   // _Py_PreInitializeFromConfig()
@@ -124,7 +125,11 @@ The following implementation-specific options are available:\n\
     do nothing if is not supported on the current system. The default value is \"off\".\n\
 \n\
 -X frozen_modules=[on|off]: whether or not frozen modules should be used.\n\
-   The default is \"on\" (or \"off\" if you are running a local build).";
+   The default is \"on\" (or \"off\" if you are running a local build).\n\
+\n\
+-X int_max_str_digits=number: limit the size of int<->str conversions.\n\
+    This helps avoid denial of service attacks when parsing untrusted data.\n\
+    The default is sys.int_info.default_max_str_digits.  0 disables.";
 
 /* Envvars that don't have equivalent command-line options are listed first */
 static const char usage_envvars[] =
@@ -144,6 +149,10 @@ static const char usage_envvars[] =
 "   to seed the hashes of str and bytes objects.  It can also be set to an\n"
 "   integer in the range [0,4294967295] to get hash values with a\n"
 "   predictable seed.\n"
+"PYTHONINTMAXSTRDIGITS: limits the maximum digit characters in an int value\n"
+"   when converting from a string and when converting an int back to a str.\n"
+"   A value of 0 disables the limit.  Conversions to or from bases 2, 4, 8,\n"
+"   16, and 32 are never limited.\n"
 "PYTHONMALLOC: set the Python memory allocators and/or install debug hooks\n"
 "   on Python memory allocators. Use PYTHONMALLOC=debug to install debug\n"
 "   hooks.\n"
@@ -784,6 +793,10 @@ _PyConfig_InitCompatConfig(PyConfig *config)
     config->code_debug_ranges = 1;
 }
 
+/* Excluded from public struct PyConfig for backporting reasons. */
+/* default to unconfigured, _PyLong_InitTypes() does the rest */
+int _Py_global_config_int_max_str_digits = -1;
+
 
 static void
 config_init_defaults(PyConfig *config)
@@ -1762,6 +1775,48 @@ config_init_tracemalloc(PyConfig *config)
     return _PyStatus_OK();
 }
 
+static PyStatus
+config_init_int_max_str_digits(PyConfig *config)
+{
+    int maxdigits;
+    int valid = 0;
+
+    const char *env = config_get_env(config, "PYTHONINTMAXSTRDIGITS");
+    if (env) {
+        if (!_Py_str_to_int(env, &maxdigits)) {
+            valid = ((maxdigits == 0) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD));
+        }
+        if (!valid) {
+#define STRINGIFY(VAL) _STRINGIFY(VAL)
+#define _STRINGIFY(VAL) #VAL
+            return _PyStatus_ERR(
+                    "PYTHONINTMAXSTRDIGITS: invalid limit; must be >= "
+                    STRINGIFY(_PY_LONG_MAX_STR_DIGITS_THRESHOLD)
+                    " or 0 for unlimited.");
+        }
+        _Py_global_config_int_max_str_digits = maxdigits;
+    }
+
+    const wchar_t *xoption = config_get_xoption(config, L"int_max_str_digits");
+    if (xoption) {
+        const wchar_t *sep = wcschr(xoption, L'=');
+        if (sep) {
+            if (!config_wstr_to_int(sep + 1, &maxdigits)) {
+                valid = ((maxdigits == 0) || (maxdigits >= _PY_LONG_MAX_STR_DIGITS_THRESHOLD));
+            }
+        }
+        if (!valid) {
+            return _PyStatus_ERR(
+                    "-X int_max_str_digits: invalid limit; must be >= "
+                    STRINGIFY(_PY_LONG_MAX_STR_DIGITS_THRESHOLD)
+                    " or 0 for unlimited.");
+#undef _STRINGIFY
+#undef STRINGIFY
+        }
+        _Py_global_config_int_max_str_digits = maxdigits;
+    }
+    return _PyStatus_OK();
+}
 
 static PyStatus
 config_init_pycache_prefix(PyConfig *config)
@@ -1818,6 +1873,7 @@ config_read_complex_options(PyConfig *config)
             return status;
         }
     }
+
     if (config->perf_profiling < 0) {
         status = config_init_perf_profiling(config);
         if (_PyStatus_EXCEPTION(status)) {
@@ -1825,6 +1881,13 @@ config_read_complex_options(PyConfig *config)
         }
     }
 
+    if (_Py_global_config_int_max_str_digits < 0) {
+        status = config_init_int_max_str_digits(config);
+        if (_PyStatus_EXCEPTION(status)) {
+            return status;
+        }
+    }
+
     if (config->pycache_prefix == NULL) {
         status = config_init_pycache_prefix(config);
         if (_PyStatus_EXCEPTION(status)) {