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gh-95778: Correctly pre-check for int-to-str conversion (#96537)

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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>
This commit is contained in:
Mark Dickinson 2022-09-04 17:21:18 +01:00 • committed by GitHub
parent 6adb89f50a
commit b126196838
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4 changed files with 107 additions and 7 deletions
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26
Objects/longobject.c
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@ -36,7 +36,8 @@ medium_value(PyLongObject *x)
#define IS_SMALL_INT(ival) (-_PY_NSMALLNEGINTS <= (ival) && (ival) < _PY_NSMALLPOSINTS)
#define IS_SMALL_UINT(ival) ((ival) < _PY_NSMALLPOSINTS)
#define _MAX_STR_DIGITS_ERROR_FMT "Exceeds the limit (%d) for integer string conversion: value has %zd digits"
#define _MAX_STR_DIGITS_ERROR_FMT_TO_INT "Exceeds the limit (%d) for integer string conversion: value has %zd digits"
#define _MAX_STR_DIGITS_ERROR_FMT_TO_STR "Exceeds the limit (%d) for integer string conversion"
static inline void
_Py_DECREF_INT(PyLongObject *op)
@ -1758,6 +1759,23 @@ long_to_decimal_string_internal(PyObject *aa,
size_a = Py_ABS(Py_SIZE(a));
negative = Py_SIZE(a) < 0;
/* quick and dirty pre-check for overflowing the decimal digit limit,
based on the inequality 10/3 >= log2(10)
explanation in https://github.com/python/cpython/pull/96537
*/
if (size_a >= 10 * _PY_LONG_MAX_STR_DIGITS_THRESHOLD
/ (3 * PyLong_SHIFT) + 2) {
PyInterpreterState *interp = _PyInterpreterState_GET();
int max_str_digits = interp->int_max_str_digits;
if ((max_str_digits > 0) &&
(max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10)) {
PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_STR,
max_str_digits);
return -1;
}
}
/* quick and dirty upper bound for the number of digits
required to express a in base _PyLong_DECIMAL_BASE:
@ -1823,8 +1841,8 @@ long_to_decimal_string_internal(PyObject *aa,
Py_ssize_t strlen_nosign = strlen - negative;
if ((max_str_digits > 0) && (strlen_nosign > max_str_digits)) {
Py_DECREF(scratch);
PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT,
max_str_digits, strlen_nosign);
PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_STR,
max_str_digits);
return -1;
}
}
@ -2498,7 +2516,7 @@ digit beyond the first.
PyInterpreterState *interp = _PyInterpreterState_GET();
int max_str_digits = interp->int_max_str_digits;
if ((max_str_digits > 0) && (digits > max_str_digits)) {
PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT,
PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_INT,
max_str_digits, digits);
return NULL;
}