Address CVE-2020-10735 for long objects

Doc/library/functions.rst

@@ -788,6 +788,14 @@ section.
 
    The long type is described in :ref:`typesnumeric`.
 
+   .. versionchanged:: 2.7.18.6
+      :class:`long` string inputs and string representations can be limited to
+      help avoid denial of service attacks. A :exc:`ValueError` is raised when
+      the limit is exceeded while converting a string *x* to an :class:`long` or
+      when converting an :class:`long` into a string would exceed the limit.
+      See the :ref:`integer string conversion length limitation
+      <int_max_str_digits>` documentation.
+
 
 .. function:: map(function, iterable, ...)
 

Doc/library/json.rst

@@ -14,6 +14,11 @@ is a lightweight data interchange format inspired by
 `JavaScript <https://en.wikipedia.org/wiki/JavaScript>`_ object literal syntax
 (although it is not a strict subset of JavaScript [#rfc-errata]_ ).
 
+.. warning::
+   Be cautious when parsing JSON data from untrusted sources. A malicious
+   JSON string may cause the decoder to consume considerable CPU and memory
+   resources. Limiting the size of data to be parsed is recommended.
+
 :mod:`json` exposes an API familiar to users of the standard library
 :mod:`marshal` and :mod:`pickle` modules.
 
@@ -249,6 +254,12 @@ Basic Usage
    be used to use another datatype or parser for JSON integers
    (e.g. :class:`float`).
 
+   .. versionchanged:: 2.7.18.6
+      The default *parse_int* of :func:`int` now limits the maximum length of
+      the integer string via the interpreter's :ref:`integer string
+      conversion length limitation <int_max_str_digits>` to help avoid denial
+      of service attacks.
+
    *parse_constant*, if specified, will be called with one of the following
    strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.
    This can be used to raise an exception if invalid JSON numbers

Doc/library/stdtypes.rst

@@ -521,6 +521,13 @@ class`. float also has the following additional methods.
 
    .. versionadded:: 2.6
 
+   .. note::
+
+      The values returned by ``as_integer_ratio()`` can be huge. Attempts
+      to render such integers into decimal strings may bump into the
+      :ref:`integer string conversion length limitation
+      <int_max_str_digits>`.
+
 .. method:: float.is_integer()
 
    Return ``True`` if the float instance is finite with integral
@@ -3190,6 +3197,167 @@ The following attributes are only supported by :term:`new-style class`\ es.
       [<type 'bool'>]
 
 
+.. _int_max_str_digits:
+
+Integer string conversion length limitation
+===========================================
+
+CPython has a global limit for converting between :class:`long` and :class:`str`
+or :class:`unicode` to mitigate denial of service attacks. This limit *only* applies
+to decimal or other non-power-of-two number bases. Hexadecimal, octal, and binary
+conversions are unlimited. The limit can be configured.
+
+The :class:`long` type in CPython is an arbitrary length number stored in binary
+form (commonly known as a "bignum"). There exists no algorithm that can convert
+a string to a binary integer or a binary integer to a string in linear time,
+*unless* the base is a power of 2. Even the best known algorithms for base 10
+have sub-quadratic complexity. Converting a large value such as ``long('1' *
+500_000)`` can take over a second on a fast CPU.
+
+Limiting conversion size offers a practical way to avoid `CVE-2020-10735
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10735>`_.
+
+The limit is applied to the number of digit characters in the input or output
+string when a non-linear conversion algorithm would be involved.  Trailing *L*s
+and the sign are not counted towards the limit.
+
+When an operation would exceed the limit, a :exc:`ValueError` is raised:
+
+.. doctest::
+
+   >>> import sys
+   >>> sys.set_int_max_str_digits(4300)  # Illustrative, this is the default.
+   >>> _ = long('2' * 5432)
+   Traceback (most recent call last):
+   ...
+   ValueError: Exceeds the limit (4300) for integer string conversion: value has 5432 digits.
+   >>> i = long('2' * 4300)
+   >>> len(str(i))
+   4300
+   >>> i_squared = i*i
+   >>> len(str(i_squared))
+   Traceback (most recent call last):
+   ...
+   ValueError: Exceeds the limit (4300) for integer string conversion: value has 8599 digits.
+   >>> len(hex(i_squared))
+   7144
+   >>> assert long(hex(i_squared), base=16) == i*i  # Hexadecimal is unlimited.
+
+The default limit is 4300 digits as provided in
+:data:`sys.long_info.default_max_str_digits <sys.long_info>`.
+The lowest limit that can be configured is 640 digits as provided in
+:data:`sys.long_info.str_digits_check_threshold <sys.long_info>`.
+
+Verification:
+
+.. doctest::
+
+   >>> import sys
+   >>> assert sys.int_info.default_max_str_digits == 4300, sys.int_info
+   >>> assert sys.int_info.str_digits_check_threshold == 640, sys.int_info
+   >>> msg = long('578966293710682886880994035146873798396722250538762761564'
+   ...           '9252925514383915483333812743580549779436104706260696366600'
+   ...           '571186405732').to_bytes(53, 'big')
+   ...
+
+.. versionadded:: 2.7.18.6
+
+
+Affected APIs
+-------------
+
+Because int automatically converts to long if the value is larger than
+:data:`sys.maxint` this limitation applies to potentially slow conversions
+between any of :class:`int` or :class:`long` and :class:`str` or :class:`unicode`:
+
+* ``int(string)`` with default base 10.
+* ``int(string, base)`` for all bases that are not a power of 2.
+* ``long(string)`` with default base 10.
+* ``long(string, base)`` for all bases that are not a power of 2.
+* ``int(unicode)`` with default base 10.
+* ``int(unicode, base)`` for all bases that are not a power of 2.
+* ``long(unicode)`` with default base 10.
+* ``long(unicode, base)`` for all bases that are not a power of 2.
+* ``str(long)``.
+* ``repr(long)``.
+* ``unicode(long)``.
+* any other string conversion to base 10, for example ``"{}".format(long)``.
+
+The limitations do not apply to functions with a linear algorithm:
+
+* ``long(string, base)`` with base 2, 4, 8, 16, or 32.
+* :func:`hex`, :func:`oct`, :func:`bin`.
+* :ref:`formatspec` for hex, octal, and binary numbers.
+* :class:`str` to :class:`float`.
+* :class:`str` to :class:`decimal.Decimal`.
+
+Configuring the limit
+---------------------
+
+Before Python starts up you can use an environment variable to configure the limit:
+
+* :envvar:`PYTHONINTMAXSTRDIGITS`, e.g.
+  ``PYTHONINTMAXSTRDIGITS=640 python`` to set the limit to 640 or
+  ``PYTHONINTMAXSTRDIGITS=0 python`` to disable the limitation.
+* :data:`sys.flags.long_max_str_digits` contains the value of
+  :envvar:`PYTHONINTMAXSTRDIGITS`. A value of *-1* indicates that none was set,
+  thus a value of :data:`sys.int_info.default_max_str_digits` was used during
+  initialization.
+
+From code, you can inspect the current limit and set a new one using these
+:mod:`sys` APIs:
+
+* :func:`sys.get_int_max_str_digits` and :func:`sys.set_int_max_str_digits` are
+  a getter and setter for the interpreter-wide limit.
+
+Information about the default and minimum can be found in :attr:`sys.long_info`:
+
+* :data:`sys.long_info.default_max_str_digits <sys.long_info>` is the compiled-in
+  default limit.
+* :data:`sys.long_info.str_digits_check_threshold <sys.long_info>` is the lowest
+  accepted value for the limit (other than 0 which disables it).
+
+.. versionadded:: 2.7.18.6
+
+.. caution::
+
+   Setting a low limit *can* lead to problems. While rare, code exists that
+   contains integer constants in decimal in their source that exceed the
+   minimum threshold. A consequence of setting the limit is that Python source
+   code containing decimal integer literals longer than the limit will
+   encounter an error during parsing, usually at startup time or import time or
+   even at installation time - anytime an up to date ``.pyc`` does not already
+   exist for the code. A workaround for source that contains such large
+   constants is to convert them to ``0x`` hexadecimal form as it has no limit.
+
+   Test your application thoroughly if you use a low limit. Ensure your tests
+   run with the limit set early via the environment so that it applies during
+   startup and even during any installation step that may invoke Python to
+   precompile ``.py`` sources to ``.pyc`` files.
+
+Recommended configuration
+-------------------------
+
+The default :data:`sys.long_info.default_max_str_digits` is expected to be
+reasonable for most applications. If your application requires a different
+limit, set it from your main entry point using Python version agnostic code as
+these APIs were ported from the original fix in version 3.12.
+
+Example::
+
+   >>> import sys
+   >>> if hasattr(sys, "set_int_max_str_digits"):
+   ...     upper_bound = 68000
+   ...     lower_bound = 4004
+   ...     current_limit = sys.get_int_max_str_digits()
+   ...     if current_limit == 0 or current_limit > upper_bound:
+   ...         sys.set_int_max_str_digits(upper_bound)
+   ...     elif current_limit < lower_bound:
+   ...         sys.set_int_max_str_digits(lower_bound)
+
+If you need to disable it entirely, set it to ``0``.
+
+
 .. rubric:: Footnotes
 
 .. [1] Additional information on these special methods may be found in the Python

Doc/library/sys.rst

@@ -431,6 +431,14 @@ always available.
    an argument to :func:`getrefcount`.
 
 
+.. function:: get_int_max_str_digits()
+
+   Returns the current value for the :ref:`integer string conversion length
+   limitation <int_max_str_digits>`. See also :func:`set_int_max_str_digits`.
+
+   .. versionadded:: 2.7.18.6
+
+
 .. function:: getrecursionlimit()
 
    Return the current value of the recursion limit, the maximum depth of the Python
@@ -603,19 +611,30 @@ always available.
 
    .. tabularcolumns:: |l|L|
 
-   +-------------------------+----------------------------------------------+
-   | Attribute               | Explanation                                  |
-   +=========================+==============================================+
-   | :const:`bits_per_digit` | number of bits held in each digit.  Python   |
-   |                         | integers are stored internally in base       |
-   |                         | ``2**long_info.bits_per_digit``              |
-   +-------------------------+----------------------------------------------+
-   | :const:`sizeof_digit`   | size in bytes of the C type used to          |
-   |                         | represent a digit                            |
-   +-------------------------+----------------------------------------------+
+   +----------------------------------------+-----------------------------------------------+
+   | Attribute                              | Explanation                                   |
+   +========================================+===============================================+
+   | :const:`bits_per_digit`                | number of bits held in each digit.  Python    |
+   |                                        | integers are stored internally in base        |
+   |                                        | ``2**int_info.bits_per_digit``                |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`sizeof_digit`                  | size in bytes of the C type used to           |
+   |                                        | represent a digit                             |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`default_max_str_digits`        | default value for                             |
+   |                                        | :func:`sys.get_int_max_str_digits` when it    |
+   |                                        | is not otherwise explicitly configured.       |
+   +----------------------------------------+-----------------------------------------------+
+   | :const:`str_digits_check_threshold`    | minimum non-zero value for                    |
+   |                                        | :func:`sys.set_int_max_str_digits`,           |
+   |                                        | :envvar:`PYTHONINTMAXSTRDIGITS`.              |
+   +----------------------------------------+-----------------------------------------------+
 
    .. versionadded:: 2.7
 
+   .. versionchanged:: 2.7.18.6
+      Added ``default_max_str_digits`` and ``str_digits_check_threshold``.
+
 
 .. data:: last_type
           last_value
@@ -848,6 +867,15 @@ always available.
    .. versionadded:: 2.2
 
 
+.. function:: set_int_max_str_digits(n)
+
+   Set the :ref:`integer string conversion length limitation
+   <int_max_str_digits>` used by this interpreter. See also
+   :func:`get_int_max_str_digits`.
+
+   .. versionadded:: 2.7.18.6
+
+
 .. function:: setprofile(profilefunc)
 
    .. index::

Doc/library/test.rst

@@ -443,6 +443,16 @@ The :mod:`test.support` module defines the following functions:
    .. versionadded:: 2.7
 
 
+.. function:: adjust_int_max_str_digits(max_digits)
+
+   This function returns a context manager that will change the global
+   :func:`sys.set_int_max_str_digits` setting for the duration of the
+   context to allow execution of test code that needs a different limit
+   on the number of digits when converting between an integer and string.
+
+   .. versionadded:: 2.7.18.6
+
+
 The :mod:`test.support` module defines the following classes:
 
 .. class:: TransientResource(exc[, **kwargs])

Doc/library/xmlrpclib.rst

@@ -557,6 +557,12 @@ Convenience Functions
    .. versionchanged:: 2.5
       The *use_datetime* flag was added.
 
+   .. versionchanged:: 2.7.18.6
+      The default *parse_int* of :func:`int` now limits the maximum length of
+      the integer string via the interpreter's :ref:`integer string
+      conversion length limitation <int_max_str_digits>` to help avoid denial
+      of service attacks.
+
 
 .. _xmlrpc-client-example:
 

Doc/using/cmdline.rst

@@ -638,6 +638,15 @@ conflict.
 
    .. versionadded:: 2.7.12
 
+
+.. envvar:: PYTHONINTMAXSTRDIGITS
+
+   If this variable is set to an integer, it is used to configure the
+   interpreter's global :ref:`integer string conversion length limitation
+   <int_max_str_digits>`.
+
+   .. versionadded:: 2.7.18.6
+
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~
 

Doc/whatsnew/2.7.rst

@@ -884,6 +884,18 @@ Some smaller changes made to the core Python language are:
   now only cleared if no one else is holding a reference to the
   dictionary (:issue:`7140`).
 
+* Converting between :class:`int` or :class:`long` and :class:`str` or
+  :class:`unicode` 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 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.
+
 .. ======================================================================
 
 .. _new-27-interpreter:

Include/longobject.h

@@ -129,6 +129,32 @@ PyAPI_FUNC(PyObject *) _PyLong_FormatAdvanced(PyObject *obj,
 					      char *format_spec,
 					      Py_ssize_t format_spec_len);
 
+#define _MAX_STR_DIGITS_ERROR_FMT "Exceeds the limit (%d) for integer string conversion: value has %zd digits"
+/*
+ * Default long base conversion size limitation: Denial of Service prevention.
+ *
+ * Chosen such that this isn't wildly slow on modern hardware
+ * 4300 decimal digits fits a ~14284 bit number.
+ */
+#define _PY_LONG_DEFAULT_MAX_STR_DIGITS 4300
+/*
+ * Threshold for max digits check.  For performance reasons long() and
+ * long.__str__() don't checks values that are smaller than this
+ * threshold.  Acts as a guaranteed minimum size limit for bignums that
+ * applications can expect from CPython.
+ *
+ * "640 digits should be enough for anyone." - gps
+ * fits a ~2126 bit decimal number.
+ */
+#define _PY_LONG_MAX_STR_DIGITS_THRESHOLD 640
+
+#if ((_PY_LONG_DEFAULT_MAX_STR_DIGITS != 0) && \
+   (_PY_LONG_DEFAULT_MAX_STR_DIGITS < _PY_LONG_MAX_STR_DIGITS_THRESHOLD))
+# error "_PY_LONG_DEFAULT_MAX_STR_DIGITS smaller than threshold."
+#endif
+
+int Py_LongMaxStrDigits;
+
 #ifdef __cplusplus
 }
 #endif

Include/pydebug.h

@@ -27,6 +27,7 @@ PyAPI_DATA(int) _Py_QnewFlag;
 /* Warn about 3.x issues */
 PyAPI_DATA(int) Py_Py3kWarningFlag;
 PyAPI_DATA(int) Py_HashRandomizationFlag;
+PyAPI_DATA(int) Py_LongMaxStrDigitsFlag;
 
 /* this is a wrapper around getenv() that pays attention to
    Py_IgnoreEnvironmentFlag.  It should be used for getting variables like