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cpython/Python/random.c

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#include "Python.h"
#ifdef MS_WINDOWS
# include <windows.h>
/* All sample MSDN wincrypt programs include the header below. It is at least
* required with Min GW. */
# include <wincrypt.h>
#else
# include <fcntl.h>
# ifdef HAVE_SYS_STAT_H
# include <sys/stat.h>
# endif
# ifdef HAVE_LINUX_RANDOM_H
# include <linux/random.h>
# endif
# if defined(HAVE_SYS_RANDOM_H) && (defined(HAVE_GETRANDOM) || defined(HAVE_GETENTROPY))
# include <sys/random.h>
# endif
# if !defined(HAVE_GETRANDOM) && defined(HAVE_GETRANDOM_SYSCALL)
# include <sys/syscall.h>
# endif
#endif
#ifdef Py_DEBUG
int _Py_HashSecret_Initialized = 0;
#else
static int _Py_HashSecret_Initialized = 0;
#endif
#ifdef MS_WINDOWS
static HCRYPTPROV hCryptProv = 0;
static int
win32_urandom_init(int raise)
{
/* Acquire context */
if (!CryptAcquireContext(&hCryptProv, NULL, NULL,
PROV_RSA_FULL, CRYPT_VERIFYCONTEXT))
goto error;
return 0;
error:
if (raise) {
PyErr_SetFromWindowsErr(0);
}
return -1;
}
/* Fill buffer with size pseudo-random bytes generated by the Windows CryptoGen
API. Return 0 on success, or raise an exception and return -1 on error. */
static int
win32_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
{
Py_ssize_t chunk;
if (hCryptProv == 0)
{
if (win32_urandom_init(raise) == -1) {
return -1;
}
}
while (size > 0)
{
chunk = size > INT_MAX ? INT_MAX : size;
if (!CryptGenRandom(hCryptProv, (DWORD)chunk, buffer))
{
/* CryptGenRandom() failed */
if (raise) {
PyErr_SetFromWindowsErr(0);
}
return -1;
}
buffer += chunk;
size -= chunk;
}
return 0;
}
#else /* !MS_WINDOWS */
#if defined(HAVE_GETRANDOM) || defined(HAVE_GETRANDOM_SYSCALL)
#define PY_GETRANDOM 1
/* Call getrandom() to get random bytes:
- Return 1 on success
- Return 0 if getrandom() is not available (failed with ENOSYS or EPERM),
or if getrandom(GRND_NONBLOCK) failed with EAGAIN (system urandom not
initialized yet).
- Raise an exception (if raise is non-zero) and return -1 on error:
if getrandom() failed with EINTR, raise is non-zero and the Python signal
handler raised an exception, or if getrandom() failed with a different
error.
getrandom() is retried if it failed with EINTR: interrupted by a signal. */
static int
py_getrandom(void *buffer, Py_ssize_t size, int raise)
{
/* Is getrandom() supported by the running kernel? Set to 0 if getrandom()
failed with ENOSYS or EPERM. 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;
char *dest;
long n;
if (!getrandom_works) {
return 0;
}
dest = buffer;
while (0 < size) {
#ifdef sun
/* Issue #26735: On Solaris, getrandom() is limited to returning up
to 1024 bytes. Call it multiple times if more bytes are
requested. */
n = Py_MIN(size, 1024);
#else
n = Py_MIN(size, LONG_MAX);
#endif
errno = 0;
#ifdef HAVE_GETRANDOM
if (raise) {
Py_BEGIN_ALLOW_THREADS
n = getrandom(dest, n, flags);
Py_END_ALLOW_THREADS
}
else {
n = getrandom(dest, n, flags);
}
#else
/* On Linux, use the syscall() function because the GNU libc doesn't
expose the Linux getrandom() syscall yet. See:
https://sourceware.org/bugzilla/show_bug.cgi?id=17252 */
if (raise) {
Py_BEGIN_ALLOW_THREADS
n = syscall(SYS_getrandom, dest, n, flags);
Py_END_ALLOW_THREADS
}
else {
n = syscall(SYS_getrandom, dest, n, flags);
}
#endif
if (n < 0) {
/* ENOSYS: the syscall is not supported by the kernel.
EPERM: the syscall is blocked by a security policy (ex: SECCOMP)
or something else. */
if (errno == ENOSYS || errno == EPERM) {
getrandom_works = 0;
return 0;
}
if (errno == EAGAIN) {
/* getrandom(GRND_NONBLOCK) fails with EAGAIN if the system
urandom is not initialiazed yet. In this case, fall back on
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;
return 0;
}
if (errno == EINTR) {
if (raise) {
if (PyErr_CheckSignals()) {
return -1;
}
}
/* retry getrandom() if it was interrupted by a signal */
continue;
}
if (raise) {
PyErr_SetFromErrno(PyExc_OSError);
}
return -1;
}
dest += n;
size -= n;
}
return 1;
}
#elif defined(HAVE_GETENTROPY)
#define PY_GETENTROPY 1
/* Fill buffer with size pseudo-random bytes generated by getentropy():
- Return 1 on success
- Return 0 if getentropy() syscall is not available (failed with ENOSYS or
EPERM).
- Raise an exception (if raise is non-zero) and return -1 on error:
if getentropy() failed with EINTR, raise is non-zero and the Python signal
handler raised an exception, or if getentropy() failed with a different
error.
getentropy() is retried if it failed with EINTR: interrupted by a signal. */
static int
py_getentropy(char *buffer, Py_ssize_t size, int raise)
{
/* Is getentropy() supported by the running kernel? Set to 0 if
getentropy() failed with ENOSYS or EPERM. */
static int getentropy_works = 1;
if (!getentropy_works) {
return 0;
}
while (size > 0) {
/* getentropy() is limited to returning up to 256 bytes. Call it
multiple times if more bytes are requested. */
Py_ssize_t len = Py_MIN(size, 256);
int res;
if (raise) {
Py_BEGIN_ALLOW_THREADS
res = getentropy(buffer, len);
Py_END_ALLOW_THREADS
}
else {
res = getentropy(buffer, len);
}
if (res < 0) {
/* ENOSYS: the syscall is not supported by the running kernel.
EPERM: the syscall is blocked by a security policy (ex: SECCOMP)
or something else. */
if (errno == ENOSYS || errno == EPERM) {
getentropy_works = 0;
return 0;
}
if (errno == EINTR) {
if (raise) {
if (PyErr_CheckSignals()) {
return -1;
}
}
/* retry getentropy() if it was interrupted by a signal */
continue;
}
if (raise) {
PyErr_SetFromErrno(PyExc_OSError);
}
return -1;
}
buffer += len;
size -= len;
}
return 1;
}
#endif /* defined(HAVE_GETENTROPY) && !defined(sun) */
static struct {
int fd;
dev_t st_dev;
ino_t st_ino;
} urandom_cache = { -1 };
/* Read random bytes from the /dev/urandom device:
- Return 0 on success
- Raise an exception (if raise is non-zero) and return -1 on error
Possible causes of errors:
- open() failed with ENOENT, ENXIO, ENODEV, EACCES: the /dev/urandom device
was not found. For example, it was removed manually or not exposed in a
chroot or container.
- open() failed with a different error
- fstat() failed
- read() failed or returned 0
read() is retried if it failed with EINTR: interrupted by a signal.
The file descriptor of the device is kept open between calls to avoid using
many file descriptors when run in parallel from multiple threads:
see the issue #18756.
st_dev and st_ino fields of the file descriptor (from fstat()) are cached to
check if the file descriptor was replaced by a different file (which is
likely a bug in the application): see the issue #21207.
If the file descriptor was closed or replaced, open a new file descriptor
but don't close the old file descriptor: it probably points to something
important for some third-party code. */
static int
dev_urandom(char *buffer, Py_ssize_t size, int raise)
{
int fd;
Py_ssize_t n;
if (raise) {
struct _Py_stat_struct st;
if (urandom_cache.fd >= 0) {
/* Does the fd point to the same thing as before? (issue #21207) */
if (_Py_fstat_noraise(urandom_cache.fd, &st)
|| st.st_dev != urandom_cache.st_dev
|| st.st_ino != urandom_cache.st_ino) {
/* Something changed: forget the cached fd (but don't close it,
since it probably points to something important for some
third-party code). */
urandom_cache.fd = -1;
}
}
if (urandom_cache.fd >= 0)
fd = urandom_cache.fd;
else {
fd = _Py_open("/dev/urandom", O_RDONLY);
if (fd < 0) {
if (errno == ENOENT || errno == ENXIO ||
errno == ENODEV || errno == EACCES) {
PyErr_SetString(PyExc_NotImplementedError,
"/dev/urandom (or equivalent) not found");
}
/* otherwise, keep the OSError exception raised by _Py_open() */
return -1;
}
if (urandom_cache.fd >= 0) {
/* urandom_fd was initialized by another thread while we were
not holding the GIL, keep it. */
close(fd);
fd = urandom_cache.fd;
}
else {
if (_Py_fstat(fd, &st)) {
close(fd);
return -1;
}
else {
urandom_cache.fd = fd;
urandom_cache.st_dev = st.st_dev;
urandom_cache.st_ino = st.st_ino;
}
}
}
do {
n = _Py_read(fd, buffer, (size_t)size);
if (n == -1)
return -1;
if (n == 0) {
PyErr_Format(PyExc_RuntimeError,
"Failed to read %zi bytes from /dev/urandom",
size);
return -1;
}
buffer += n;
size -= n;
} while (0 < size);
}
else {
fd = _Py_open_noraise("/dev/urandom", O_RDONLY);
if (fd < 0) {
return -1;
}
while (0 < size)
{
do {
n = read(fd, buffer, (size_t)size);
} while (n < 0 && errno == EINTR);
if (n <= 0) {
/* stop on error or if read(size) returned 0 */
close(fd);
return -1;
}
buffer += n;
size -= n;
}
close(fd);
}
return 0;
}
static void
dev_urandom_close(void)
{
if (urandom_cache.fd >= 0) {
close(urandom_cache.fd);
urandom_cache.fd = -1;
}
}
#endif /* !MS_WINDOWS */
/* Fill buffer with pseudo-random bytes generated by a linear congruent
generator (LCG):
x(n+1) = (x(n) * 214013 + 2531011) % 2^32
Use bits 23..16 of x(n) to generate a byte. */
static void
lcg_urandom(unsigned int x0, unsigned char *buffer, size_t size)
{
size_t index;
unsigned int x;
x = x0;
for (index=0; index < size; index++) {
x *= 214013;
x += 2531011;
/* modulo 2 ^ (8 * sizeof(int)) */
buffer[index] = (x >> 16) & 0xff;
}
}
/* Read random bytes:
- Return 0 on success
- Raise an exception (if raise is non-zero) and return -1 on error
Used sources of entropy ordered by preference, preferred source first:
- CryptGenRandom() on Windows
- getrandom() function (ex: Linux and Solaris): call py_getrandom()
- getentropy() function (ex: OpenBSD): call py_getentropy()
- /dev/urandom device
Read from the /dev/urandom device if getrandom() or getentropy() function
is not available or does not work.
Prefer getrandom() over getentropy() because getrandom() supports blocking
and non-blocking mode and Python requires non-blocking RNG at startup to
initialize its hash secret: see the PEP 524.
Prefer getrandom() and getentropy() over reading directly /dev/urandom
because these functions don't need file descriptors and so avoid ENFILE or
EMFILE errors (too many open files): see the issue #18756.
Only use RNG running in the kernel. They are more secure because it is
harder to get the internal state of a RNG running in the kernel land than a
RNG running in the user land. The kernel has a direct access to the hardware
and has access to hardware RNG, they are used as entropy sources.
Note: the OpenSSL RAND_pseudo_bytes() function does not automatically reseed
its RNG on fork(), two child processes (with the same pid) generate the same
random numbers: see issue #18747. Kernel RNGs don't have this issue,
they have access to good quality entropy sources.
If raise is zero:
- Don't raise an exception on error
- Don't call the Python signal handler (don't call PyErr_CheckSignals()) if
a function fails with EINTR: retry directly the interrupted function
- Don't release the GIL to call functions.
*/
static int
pyurandom(void *buffer, Py_ssize_t size, int raise)
{
#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
int res;
#endif
if (size < 0) {
if (raise) {
PyErr_Format(PyExc_ValueError,
"negative argument not allowed");
}
return -1;
}
if (size == 0) {
return 0;
}
#ifdef MS_WINDOWS
return win32_urandom((unsigned char *)buffer, size, raise);
#else
#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
#ifdef PY_GETRANDOM
res = py_getrandom(buffer, size, raise);
#else
res = py_getentropy(buffer, size, raise);
#endif
if (res < 0) {
return -1;
}
if (res == 1) {
return 0;
}
/* getrandom() or getentropy() function is not available: failed with
ENOSYS, EPERM or EAGAIN. Fall back on reading from /dev/urandom. */
#endif
return dev_urandom(buffer, size, raise);
#endif
}
/* Fill buffer with size pseudo-random bytes from the operating system random
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. */
int
_PyOS_URandom(void *buffer, Py_ssize_t size)
{
return pyurandom(buffer, size, 1);
}
void
_PyRandom_Init(void)
{
char *env;
unsigned char *secret = (unsigned char *)&_Py_HashSecret.uc;
Py_ssize_t secret_size = sizeof(_Py_HashSecret_t);
assert(secret_size == sizeof(_Py_HashSecret.uc));
if (_Py_HashSecret_Initialized)
return;
_Py_HashSecret_Initialized = 1;
/*
Hash randomization is enabled. Generate a per-process secret,
using PYTHONHASHSEED if provided.
*/
env = Py_GETENV("PYTHONHASHSEED");
if (env && *env != '\0' && strcmp(env, "random") != 0) {
char *endptr = env;
unsigned long seed;
seed = strtoul(env, &endptr, 10);
if (*endptr != '\0'
|| seed > 4294967295UL
|| (errno == ERANGE && seed == ULONG_MAX))
{
Py_FatalError("PYTHONHASHSEED must be \"random\" or an integer "
"in range [0; 4294967295]");
}
if (seed == 0) {
/* disable the randomized hash */
memset(secret, 0, secret_size);
}
else {
lcg_urandom(seed, secret, secret_size);
}
}
else {
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);
if (res < 0) {
Py_FatalError("failed to get random numbers to initialize Python");
}
}
}
void
_PyRandom_Fini(void)
{
#ifdef MS_WINDOWS
if (hCryptProv) {
CryptReleaseContext(hCryptProv, 0);
hCryptProv = 0;
}
#else
dev_urandom_close();
#endif
}
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