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os.urandom() now blocks on Linux 

Issue #27776: The os.urandom() function does now block on Linux 3.17 and newer
until the system urandom entropy pool is initialized to increase the security.

This change is part of the PEP 524.
This commit is contained in:
Victor Stinner 2016-09-06 16:33:52 -07:00
parent e256accd46
commit e66987e626
8 changed files with 130 additions and 59 deletions
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27
Doc/library/os.rst
View File

@ -3968,14 +3968,27 @@ Random numbers
returned data should be unpredictable enough for cryptographic applications,
though its exact quality depends on the OS implementation.
On Linux, the ``getrandom()`` syscall is used if available and the urandom
entropy pool is initialized (``getrandom()`` does not block).
On a Unix-like system this will query ``/dev/urandom``. On Windows, it
will use ``CryptGenRandom()``. If a randomness source is not found,
:exc:`NotImplementedError` will be raised.
On Linux, if the ``getrandom()`` syscall is available, it is used in
blocking mode: block until the system urandom entropy pool is initialized
(128 bits of entropy are collected by the kernel). See the :pep:`524` for
the rationale. On Linux, the :func:`getrandom` function can be used to get
random bytes in non-blocking mode (using the :data:`GRND_NONBLOCK` flag) or
to poll until the system urandom entropy pool is initialized.
For an easy-to-use interface to the random number generator
provided by your platform, please see :class:`random.SystemRandom`.
On a Unix-like system, random bytes are read from the ``/dev/urandom``
device. If the ``/dev/urandom`` device is not available or not readable, the
:exc:`NotImplementedError` exception is raised.
On Windows, it will use ``CryptGenRandom()``.
.. seealso::
The :mod:`secrets` module provides higher level functions. For an
easy-to-use interface to the random number generator provided by your
platform, please see :class:`random.SystemRandom`.
.. versionchanged:: 3.6.0
On Linux, ``getrandom()`` is now used in blocking mode to increase the
security.
.. versionchanged:: 3.5.2
On Linux, if the ``getrandom()`` syscall blocks (the urandom entropy pool

12
Doc/whatsnew/3.6.rst
View File

@ -70,6 +70,12 @@ Standard library improvements:
* PEP 519: :ref:`Adding a file system path protocol <pep-519>`
Security improvements:
* On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
is initialized to increase the security. See the :pep:`524` for the
rationale.
Windows improvements:
* The ``py.exe`` launcher, when used interactively, no longer prefers
@ -345,6 +351,9 @@ New Modules
Improved Modules
================
On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
is initialized to increase the security. See the :pep:`524` for the rationale.
asyncio
-------
@ -913,6 +922,9 @@ Changes in 'python' Command Behavior
Changes in the Python API
-------------------------
* On Linux, :func:`os.urandom` now blocks until the system urandom entropy pool
is initialized to increase the security.
* When :meth:`importlib.abc.Loader.exec_module` is defined,
:meth:`importlib.abc.Loader.create_module` must also be defined.

3
Include/pylifecycle.h
View File

@ -117,7 +117,8 @@ PyAPI_FUNC(PyOS_sighandler_t) PyOS_getsig(int);
PyAPI_FUNC(PyOS_sighandler_t) PyOS_setsig(int, PyOS_sighandler_t);
/* Random */
PyAPI_FUNC(int) _PyOS_URandom (void *buffer, Py_ssize_t size);
PyAPI_FUNC(int) _PyOS_URandom(void *buffer, Py_ssize_t size);
PyAPI_FUNC(int) _PyOS_URandomNonblock(void *buffer, Py_ssize_t size);
#ifdef __cplusplus
}

9
Lib/random.py
View File

@ -105,15 +105,6 @@ class Random(_random.Random):
"""
if a is None:
try:
# Seed with enough bytes to span the 19937 bit
# state space for the Mersenne Twister
a = int.from_bytes(_urandom(2500), 'big')
except NotImplementedError:
import time
a = int(time.time() * 256) # use fractional seconds
if version == 1 and isinstance(a, (str, bytes)):
x = ord(a[0]) << 7 if a else 0
for c in a:

4
Misc/NEWS
View File

@ -89,6 +89,10 @@ Core and Builtins
Library
-------
- Issue #27776: The :func:`os.urandom` function does now block on Linux 3.17
and newer until the system urandom entropy pool is initialized to increase
the security. This change is part of the :pep:`524`.
- Issue #27778: Expose the Linux ``getrandom()`` syscall as a new
:func:`os.getrandom` function. This change is part of the :pep:`524`.

56
Modules/_randommodule.c
View File

@ -165,7 +165,7 @@ init_genrand(RandomObject *self, uint32_t s)
/* initialize by an array with array-length */
/* init_key is the array for initializing keys */
/* key_length is its length */
static PyObject *
static void
init_by_array(RandomObject *self, uint32_t init_key[], size_t key_length)
{
size_t i, j, k; /* was signed in the original code. RDH 12/16/2002 */
@ -190,8 +190,6 @@ init_by_array(RandomObject *self, uint32_t init_key[], size_t key_length)
}
mt[0] = 0x80000000U; /* MSB is 1; assuring non-zero initial array */
Py_INCREF(Py_None);
return Py_None;
}
/*
@ -199,6 +197,37 @@ init_by_array(RandomObject *self, uint32_t init_key[], size_t key_length)
* Twister download.
*/
static int
random_seed_urandom(RandomObject *self)
{
PY_UINT32_T key[N];
if (_PyOS_URandomNonblock(key, sizeof(key)) < 0) {
return -1;
}
init_by_array(self, key, Py_ARRAY_LENGTH(key));
return 0;
}
static void
random_seed_time_pid(RandomObject *self)
{
_PyTime_t now;
uint32_t key[5];
now = _PyTime_GetSystemClock();
key[0] = (PY_UINT32_T)(now & 0xffffffffU);
key[1] = (PY_UINT32_T)(now >> 32);
key[2] = (PY_UINT32_T)getpid();
now = _PyTime_GetMonotonicClock();
key[3] = (PY_UINT32_T)(now & 0xffffffffU);
key[4] = (PY_UINT32_T)(now >> 32);
init_by_array(self, key, Py_ARRAY_LENGTH(key));
}
static PyObject *
random_seed(RandomObject *self, PyObject *args)
{
@ -212,14 +241,17 @@ random_seed(RandomObject *self, PyObject *args)
if (!PyArg_UnpackTuple(args, "seed", 0, 1, &arg))
return NULL;
if (arg == NULL || arg == Py_None) {
time_t now;
if (arg == NULL || arg == Py_None) {
if (random_seed_urandom(self) >= 0) {
PyErr_Clear();
time(&now);
init_genrand(self, (uint32_t)now);
Py_INCREF(Py_None);
return Py_None;
/* Reading system entropy failed, fall back on the worst entropy:
use the current time and process identifier. */
random_seed_time_pid(self);
}
Py_RETURN_NONE;
}
/* This algorithm relies on the number being unsigned.
* So: if the arg is a PyLong, use its absolute value.
* Otherwise use its hash value, cast to unsigned.
@ -269,7 +301,11 @@ random_seed(RandomObject *self, PyObject *args)
}
}
#endif
result = init_by_array(self, key, keyused);
init_by_array(self, key, keyused);
Py_INCREF(Py_None);
result = Py_None;
Done:
Py_XDECREF(n);
PyMem_Free(key);

3
Modules/posixmodule.c
View File

@ -11168,8 +11168,7 @@ os_urandom_impl(PyObject *module, Py_ssize_t size)
if (bytes == NULL)
return NULL;
result = _PyOS_URandom(PyBytes_AS_STRING(bytes),
PyBytes_GET_SIZE(bytes));
result = _PyOS_URandom(PyBytes_AS_STRING(bytes), PyBytes_GET_SIZE(bytes));
if (result == -1) {
Py_DECREF(bytes);
return NULL;

75
Python/random.c
View File

@ -77,7 +77,7 @@ win32_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
}
/* Issue #25003: Don't use getentropy() on Solaris (available since
Solaris 11.3), it is blocking whereas os.urandom() should not block. */
* Solaris 11.3), it is blocking whereas os.urandom() should not block. */
#elif defined(HAVE_GETENTROPY) && !defined(sun)
#define PY_GETENTROPY 1
@ -121,24 +121,20 @@ py_getentropy(char *buffer, Py_ssize_t size, int raise)
/* Call getrandom()
- Return 1 on success
- Return 0 if getrandom() syscall is not available (fails with ENOSYS).
- Return 0 if getrandom() syscall is not available (fails with ENOSYS)
or if getrandom(GRND_NONBLOCK) fails with EAGAIN (blocking=0 and system
urandom not initialized yet) and raise=0.
- Raise an exception (if raise is non-zero) and return -1 on error:
getrandom() failed with EINTR and the Python signal handler raised an
exception, or getrandom() failed with a different error. */
static int
py_getrandom(void *buffer, Py_ssize_t size, int raise)
py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
{
/* Is getrandom() supported by the running kernel?
Need Linux kernel 3.17 or newer, or Solaris 11.3 or newer */
/* Is getrandom() supported by the running kernel? Set to 0 if getrandom()
fails with ENOSYS. Need Linux kernel 3.17 or newer, or Solaris 11.3
or newer */
static int getrandom_works = 1;
/* getrandom() on Linux will block if called before the kernel has
initialized the urandom entropy pool. This will cause Python
to hang on startup if called very early in the boot process -
see https://bugs.python.org/issue26839. To avoid this, use the
GRND_NONBLOCK flag. */
const int flags = GRND_NONBLOCK;
int flags;
char *dest;
long n;
@ -146,6 +142,7 @@ py_getrandom(void *buffer, Py_ssize_t size, int raise)
return 0;
}
flags = blocking ? 0 : GRND_NONBLOCK;
dest = buffer;
while (0 < size) {
#ifdef sun
@ -185,15 +182,12 @@ py_getrandom(void *buffer, Py_ssize_t size, int raise)
getrandom_works = 0;
return 0;
}
if (errno == EAGAIN) {
/* If we failed with EAGAIN, the entropy pool was
uninitialized. In this case, we return failure to fall
back to reading from /dev/urandom.
Note: In this case the data read will not be random so
should not be used for cryptographic purposes. Retaining
the existing semantics for practical purposes. */
getrandom_works = 0;
/* getrandom(GRND_NONBLOCK) fails with EAGAIN if the system urandom
is not initialiazed yet. For _PyRandom_Init(), we ignore their
error and fall back on reading /dev/urandom which never blocks,
even if the system urandom is not initialized yet. */
if (errno == EAGAIN && !raise && !blocking) {
return 0;
}
@ -228,13 +222,13 @@ static struct {
} urandom_cache = { -1 };
/* Read 'size' random bytes from getrandom(). Fall back on reading from
/* Read 'size' random bytes from py_getrandom(). Fall back on reading from
/dev/urandom if getrandom() is not available.
Return 0 on success. Raise an exception (if raise is non-zero) and return -1
on error. */
static int
dev_urandom(char *buffer, Py_ssize_t size, int raise)
dev_urandom(char *buffer, Py_ssize_t size, int blocking, int raise)
{
int fd;
Py_ssize_t n;
@ -245,7 +239,7 @@ dev_urandom(char *buffer, Py_ssize_t size, int raise)
assert(size > 0);
#ifdef PY_GETRANDOM
res = py_getrandom(buffer, size, raise);
res = py_getrandom(buffer, size, blocking, raise);
if (res < 0) {
return -1;
}
@ -381,7 +375,7 @@ lcg_urandom(unsigned int x0, unsigned char *buffer, size_t size)
syscall)
- Don't release the GIL to call syscalls. */
static int
pyurandom(void *buffer, Py_ssize_t size, int raise)
pyurandom(void *buffer, Py_ssize_t size, int blocking, int raise)
{
if (size < 0) {
if (raise) {
@ -400,7 +394,7 @@ pyurandom(void *buffer, Py_ssize_t size, int raise)
#elif defined(PY_GETENTROPY)
return py_getentropy(buffer, size, raise);
#else
return dev_urandom(buffer, size, raise);
return dev_urandom(buffer, size, blocking, raise);
#endif
}
@ -408,11 +402,29 @@ pyurandom(void *buffer, Py_ssize_t size, int raise)
number generator (RNG). It is suitable for most cryptographic purposes
except long living private keys for asymmetric encryption.
Return 0 on success, raise an exception and return -1 on error. */
On Linux 3.17 and newer, the getrandom() syscall is used in blocking mode:
block until the system urandom entropy pool is initialized (128 bits are
collected by the kernel).
Return 0 on success. Raise an exception and return -1 on error. */
int
_PyOS_URandom(void *buffer, Py_ssize_t size)
{
return pyurandom(buffer, size, 1);
return pyurandom(buffer, size, 1, 1);
}
/* Fill buffer with size pseudo-random bytes from the operating system random
number generator (RNG). It is not suitable for cryptographic purpose.
On Linux 3.17 and newer (when getrandom() syscall is used), if the system
urandom is not initialized yet, the function returns "weak" entropy read
from /dev/urandom.
Return 0 on success. Raise an exception and return -1 on error. */
int
_PyOS_URandomNonblock(void *buffer, Py_ssize_t size)
{
return pyurandom(buffer, size, 0, 1);
}
void
@ -456,8 +468,11 @@ _PyRandom_Init(void)
int res;
/* _PyRandom_Init() is called very early in the Python initialization
and so exceptions cannot be used (use raise=0). */
res = pyurandom(secret, secret_size, 0);
and so exceptions cannot be used (use raise=0).
_PyRandom_Init() must not block Python initialization: call
pyurandom() is non-blocking mode (blocking=0): see the PEP 524. */
res = pyurandom(secret, secret_size, 0, 0);
if (res < 0) {
Py_FatalError("failed to get random numbers to initialize Python");
}