[Python-checkins] gh-95778: Correctly pre-check for int-to-str conversion (GH-96537)
miss-islington
webhook-mailer at python.org
Sun Sep 4 12:45:07 EDT 2022
https://github.com/python/cpython/commit/8a776d1d51c57e2ab390fe8dee10763bc478f256
commit: 8a776d1d51c57e2ab390fe8dee10763bc478f256
branch: 3.11
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: miss-islington <31488909+miss-islington at users.noreply.github.com>
date: 2022-09-04T09:45:02-07:00
summary:
gh-95778: Correctly pre-check for int-to-str conversion (GH-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 at krypto.org>
(cherry picked from commit b126196838bbaf5f4d35120e0e6bcde435b0b480)
Co-authored-by: Mark Dickinson <dickinsm at gmail.com>
files:
M Include/internal/pycore_long.h
M Lib/test/test_int.py
M Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
M Objects/longobject.c
diff --git a/Include/internal/pycore_long.h b/Include/internal/pycore_long.h
index ef57a60bd530..0f466eb60fea 100644
--- a/Include/internal/pycore_long.h
+++ b/Include/internal/pycore_long.h
@@ -18,9 +18,9 @@ extern "C" {
* everyone's existing deployed numpy test suite passes before
* https://github.com/numpy/numpy/issues/22098 is widely available.
*
- * $ python -m timeit -s 's = * "1"*4300' 'int(s)'
+ * $ python -m timeit -s 's = "1"*4300' 'int(s)'
* 2000 loops, best of 5: 125 usec per loop
- * $ python -m timeit -s 's = * "1"*4300; v = int(s)' 'str(v)'
+ * $ python -m timeit -s 's = "1"*4300; v = int(s)' 'str(v)'
* 1000 loops, best of 5: 311 usec per loop
* (zen2 cloud VM)
*
diff --git a/Lib/test/test_int.py b/Lib/test/test_int.py
index e9561b02fcac..800c0b006cdc 100644
--- a/Lib/test/test_int.py
+++ b/Lib/test/test_int.py
@@ -1,4 +1,5 @@
import sys
+import time
import unittest
from test import support
@@ -632,6 +633,87 @@ def test_max_str_digits(self):
with self.assertRaises(ValueError):
str(i)
+ def test_denial_of_service_prevented_int_to_str(self):
+ """Regression test: ensure we fail before performing O(N**2) work."""
+ maxdigits = sys.get_int_max_str_digits()
+ assert maxdigits < 50_000, maxdigits # A test prerequisite.
+ get_time = time.process_time
+ if get_time() <= 0: # some platforms like WASM lack process_time()
+ get_time = time.monotonic
+
+ huge_int = int(f'0x{"c"*65_000}', base=16) # 78268 decimal digits.
+ digits = 78_268
+ with support.adjust_int_max_str_digits(digits):
+ start = get_time()
+ huge_decimal = str(huge_int)
+ seconds_to_convert = get_time() - start
+ self.assertEqual(len(huge_decimal), digits)
+ # Ensuring that we chose a slow enough conversion to measure.
+ # It takes 0.1 seconds on a Zen based cloud VM in an opt build.
+ if seconds_to_convert < 0.005:
+ raise unittest.SkipTest('"slow" conversion took only '
+ f'{seconds_to_convert} seconds.')
+
+ # We test with the limit almost at the size needed to check performance.
+ # The performant limit check is slightly fuzzy, give it a some room.
+ with support.adjust_int_max_str_digits(int(.995 * digits)):
+ with self.assertRaises(ValueError) as err:
+ start = get_time()
+ str(huge_int)
+ seconds_to_fail_huge = get_time() - start
+ self.assertIn('conversion', str(err.exception))
+ self.assertLess(seconds_to_fail_huge, seconds_to_convert/8)
+
+ # Now we test that a conversion that would take 30x as long also fails
+ # in a similarly fast fashion.
+ extra_huge_int = int(f'0x{"c"*500_000}', base=16) # 602060 digits.
+ with self.assertRaises(ValueError) as err:
+ start = get_time()
+ # If not limited, 8 seconds said Zen based cloud VM.
+ str(extra_huge_int)
+ seconds_to_fail_extra_huge = get_time() - start
+ self.assertIn('conversion', str(err.exception))
+ self.assertLess(seconds_to_fail_extra_huge, seconds_to_convert/8)
+
+ def test_denial_of_service_prevented_str_to_int(self):
+ """Regression test: ensure we fail before performing O(N**2) work."""
+ maxdigits = sys.get_int_max_str_digits()
+ assert maxdigits < 100_000, maxdigits # A test prerequisite.
+ get_time = time.process_time
+ if get_time() <= 0: # some platforms like WASM lack process_time()
+ get_time = time.monotonic
+
+ digits = 133700
+ huge = '8'*digits
+ with support.adjust_int_max_str_digits(digits):
+ start = get_time()
+ int(huge)
+ seconds_to_convert = get_time() - start
+ # Ensuring that we chose a slow enough conversion to measure.
+ # It takes 0.1 seconds on a Zen based cloud VM in an opt build.
+ if seconds_to_convert < 0.005:
+ raise unittest.SkipTest('"slow" conversion took only '
+ f'{seconds_to_convert} seconds.')
+
+ with support.adjust_int_max_str_digits(digits - 1):
+ with self.assertRaises(ValueError) as err:
+ start = get_time()
+ int(huge)
+ seconds_to_fail_huge = get_time() - start
+ self.assertIn('conversion', str(err.exception))
+ self.assertLess(seconds_to_fail_huge, seconds_to_convert/8)
+
+ # Now we test that a conversion that would take 30x as long also fails
+ # in a similarly fast fashion.
+ extra_huge = '7'*1_200_000
+ with self.assertRaises(ValueError) as err:
+ start = get_time()
+ # If not limited, 8 seconds in the Zen based cloud VM.
+ int(extra_huge)
+ seconds_to_fail_extra_huge = get_time() - start
+ self.assertIn('conversion', str(err.exception))
+ self.assertLess(seconds_to_fail_extra_huge, seconds_to_convert/8)
+
def test_power_of_two_bases_unlimited(self):
"""The limit does not apply to power of 2 bases."""
maxdigits = sys.get_int_max_str_digits()
diff --git a/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst b/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
index ea3b85d632e0..8eb8a34884dc 100644
--- a/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
+++ b/Misc/NEWS.d/next/Security/2022-08-07-16-53-38.gh-issue-95778.ch010gps.rst
@@ -11,4 +11,4 @@ limitation <int_max_str_digits>` documentation. The default limit is 4300
digits in string form.
Patch by Gregory P. Smith [Google] and Christian Heimes [Red Hat] with feedback
-from Victor Stinner, Thomas Wouters, Steve Dower, and Ned Deily.
+from Victor Stinner, Thomas Wouters, Steve Dower, Ned Deily, and Mark Dickinson.
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 0701c4941337..17274d01cec4 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -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)
@@ -1726,6 +1727,23 @@ 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:
@@ -1791,8 +1809,8 @@ 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;
}
}
@@ -2466,7 +2484,7 @@ 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;
}
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