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.

1 files changed, 77 insertions, 1 deletions

diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
index ddf01a7..6864b8b 100644
--- a/Python/clinic/sysmodule.c.h
+++ b/Python/clinic/sysmodule.c.h
@@ -745,6 +745,82 @@ exit:
 
 #endif /* defined(USE_MALLOPT) */
 
+PyDoc_STRVAR(sys_get_int_max_str_digits__doc__,
+"get_int_max_str_digits($module, /)\n"
+"--\n"
+"\n"
+"Set the maximum string digits limit for non-binary int<->str conversions.");
+
+#define SYS_GET_INT_MAX_STR_DIGITS_METHODDEF    \
+    {"get_int_max_str_digits", (PyCFunction)sys_get_int_max_str_digits, METH_NOARGS, sys_get_int_max_str_digits__doc__},
+
+static PyObject *
+sys_get_int_max_str_digits_impl(PyObject *module);
+
+static PyObject *
+sys_get_int_max_str_digits(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return sys_get_int_max_str_digits_impl(module);
+}
+
+PyDoc_STRVAR(sys_set_int_max_str_digits__doc__,
+"set_int_max_str_digits($module, /, maxdigits)\n"
+"--\n"
+"\n"
+"Set the maximum string digits limit for non-binary int<->str conversions.");
+
+#define SYS_SET_INT_MAX_STR_DIGITS_METHODDEF    \
+    {"set_int_max_str_digits", _PyCFunction_CAST(sys_set_int_max_str_digits), METH_FASTCALL|METH_KEYWORDS, sys_set_int_max_str_digits__doc__},
+
+static PyObject *
+sys_set_int_max_str_digits_impl(PyObject *module, int maxdigits);
+
+static PyObject *
+sys_set_int_max_str_digits(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
+
+    #define NUM_KEYWORDS 1
+    static struct {
+        PyGC_Head _this_is_not_used;
+        PyObject_VAR_HEAD
+        PyObject *ob_item[NUM_KEYWORDS];
+    } _kwtuple = {
+        .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
+        .ob_item = { &_Py_ID(maxdigits), },
+    };
+    #undef NUM_KEYWORDS
+    #define KWTUPLE (&_kwtuple.ob_base.ob_base)
+
+    #else  // !Py_BUILD_CORE
+    #  define KWTUPLE NULL
+    #endif  // !Py_BUILD_CORE
+
+    static const char * const _keywords[] = {"maxdigits", NULL};
+    static _PyArg_Parser _parser = {
+        .keywords = _keywords,
+        .fname = "set_int_max_str_digits",
+        .kwtuple = KWTUPLE,
+    };
+    #undef KWTUPLE
+    PyObject *argsbuf[1];
+    int maxdigits;
+
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 1, 1, 0, argsbuf);
+    if (!args) {
+        goto exit;
+    }
+    maxdigits = _PyLong_AsInt(args[0]);
+    if (maxdigits == -1 && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = sys_set_int_max_str_digits_impl(module, maxdigits);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(sys_getrefcount__doc__,
 "getrefcount($module, object, /)\n"
 "--\n"
@@ -1267,4 +1343,4 @@ sys_is_stack_trampoline_active(PyObject *module, PyObject *Py_UNUSED(ignored))
 #ifndef SYS_GETANDROIDAPILEVEL_METHODDEF
     #define SYS_GETANDROIDAPILEVEL_METHODDEF
 #endif /* !defined(SYS_GETANDROIDAPILEVEL_METHODDEF) */
-/*[clinic end generated code: output=43b44240211afe95 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=15318cdd96b62b06 input=a9049054013a1b77]*/