[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, 18 insertions, 0 deletions

diff --git a/Parser/pegen/pegen.c b/Parser/pegen/pegen.c
index cdfbc12..15b06ce 100644
--- a/Parser/pegen/pegen.c
+++ b/Parser/pegen/pegen.c
@@ -967,6 +967,24 @@ _PyPegen_number_token(Parser *p)
 
     if (c == NULL) {
         p->error_indicator = 1;
+        PyObject *exc_type, *exc_value, *exc_tb;
+        PyErr_Fetch(&exc_type, &exc_value, &exc_tb);
+        // The only way a ValueError should happen in _this_ code is via
+        // PyLong_FromString hitting a length limit.
+        if (exc_type == PyExc_ValueError && exc_value != NULL) {
+            // The Fetch acted as PyErr_Clear(), we're replacing the exception.
+            Py_XDECREF(exc_tb);
+            Py_DECREF(exc_type);
+            RAISE_ERROR_KNOWN_LOCATION(
+                p, PyExc_SyntaxError,
+                t->lineno, 0 /* col_offset */,
+                "%S - Consider hexadecimal for huge integer literals "
+                "to avoid decimal conversion limits.",
+                exc_value);
+            Py_DECREF(exc_value);
+        } else {
+            PyErr_Restore(exc_type, exc_value, exc_tb);
+        }
         return NULL;
     }