[3.9] gh-95778: CVE-2020-10735: Prevent DoS by very large int() (#96502)

* Correctly pre-check for int-to-str conversion (#96537)

Converting a large enough `int` to a decimal string raises `ValueError` as expected. However, the raise comes _after_ the quadratic-time base-conversion algorithm has run to completion. For effective DOS prevention, we need some kind of check before entering the quadratic-time loop. Oops! =)

The quick fix: essentially we catch _most_ values that exceed the threshold up front. Those that slip through will still be on the small side (read: sufficiently fast), and will get caught by the existing check so that the limit remains exact.

The justification for the current check. The C code check is:
```c
max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10
```

In GitHub markdown math-speak, writing $M$ for `max_str_digits`, $L$ for `PyLong_SHIFT` and $s$ for `size_a`, that check is:
$$\left\lfloor\frac{M}{3L}\right\rfloor \le \left\lfloor\frac{s - 11}{10}\right\rfloor$$

From this it follows that
$$\frac{M}{3L} < \frac{s-1}{10}$$
hence that
$$\frac{L(s-1)}{M} > \frac{10}{3} > \log_2(10).$$
So
$$2^{L(s-1)} > 10^M.$$
But our input integer $a$ satisfies $|a| \ge 2^{L(s-1)}$, so $|a|$ is larger than $10^M$. This shows that we don't accidentally capture anything _below_ the intended limit in the check.

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

Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org>
Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 6aed8d2..dab4746 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -1587,3 +1587,17 @@ URL by the parser in :mod:`urllib.parse` preventing such attacks. The removal
 characters are controlled by a new module level variable
 ``urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE``. (See :issue:`43882`)
 
+Notable security feature in 3.9.14
+==================================
+
+Converting between :class:`int` and :class:`str` in bases other than 2
+(binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal)
+now raises a :exc:`ValueError` if the number of digits in string form is
+above a limit to avoid potential denial of service attacks due to the
+algorithmic complexity. This is a mitigation for `CVE-2020-10735
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_.
+This limit can be configured or disabled by environment variable, command
+line flag, or :mod:`sys` APIs. See the :ref:`integer string conversion
+length limitation <int_max_str_digits>` documentation.  The default limit
+is 4300 digits in string form.
+