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Issue #29157: Prefer getrandom() over getentropy() 

* dev_urandom() now calls py_getentropy(). Prepare the fallback to support
  getentropy() failure and falls back on reading from /dev/urandom.
* Simplify dev_urandom(). pyurandom() is now responsible to call getentropy()
  or getrandom(). Enhance also dev_urandom() and pyurandom() documentation.
* getrandom() is now preferred over getentropy(). The glibc 2.24 now implements
  getentropy() on Linux using the getrandom() syscall.  But getentropy()
  doesn't support non-blocking mode. Since getrandom() is tried first, it's not
  more needed to explicitly exclude getentropy() on Solaris. Replace:
  "if defined(HAVE_GETENTROPY) && !defined(sun)"
  with "if defined(HAVE_GETENTROPY)"
* Enhance py_getrandom() documentation. py_getentropy() now supports ENOSYS,
  EPERM & EINTR
pull/22/head
Victor Stinner 10 years ago
parent
84b6fb0eea
commit
ff558f5aba
1 changed files with 187 additions and 87 deletions
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  1. 274
      Python/random.c

274
Python/random.c
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@ -77,57 +77,23 @@ win32_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
return 0;
}
/* Issue #25003: Don't use getentropy() on Solaris (available since
* Solaris 11.3), it is blocking whereas os.urandom() should not block. */
#elif defined(HAVE_GETENTROPY) && !defined(sun)
#define PY_GETENTROPY 1
/* Fill buffer with size pseudo-random bytes generated by getentropy().
Return 0 on success, or raise an exception and return -1 on error.
If raise is zero, don't raise an exception on error. */
static int
py_getentropy(char *buffer, Py_ssize_t size, int raise)
{
while (size > 0) {
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) {
if (raise) {
PyErr_SetFromErrno(PyExc_OSError);
}
return -1;
}
buffer += len;
size -= len;
}
return 0;
}
#else
#else /* !MS_WINDOWS */
#if defined(HAVE_GETRANDOM) || defined(HAVE_GETRANDOM_SYSCALL)
#define PY_GETRANDOM 1
/* Call getrandom()
/* Call getrandom() to get random bytes:
- Return 1 on success
- Return 0 if getrandom() syscall is not available (failed with ENOSYS or
EPERM) or if getrandom(GRND_NONBLOCK) failed with EAGAIN (system urandom
not initialized yet) and raise=0.
- 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) 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. */
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 blocking, int raise)
{
@ -148,7 +114,8 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
while (0 < size) {
#ifdef sun
/* Issue #26735: On Solaris, getrandom() is limited to returning up
to 1024 bytes */
to 1024 bytes. Call it multiple times if more bytes are
requested. */
n = Py_MIN(size, 1024);
#else
n = Py_MIN(size, LONG_MAX);
@ -179,18 +146,19 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
#endif
if (n < 0) {
/* ENOSYS: getrandom() syscall not supported by the kernel (but
* maybe supported by the host which built Python). EPERM:
* getrandom() syscall blocked by SECCOMP or something else. */
/* 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;
}
/* getrandom(GRND_NONBLOCK) fails with EAGAIN if the system urandom
is not initialiazed yet. For _PyRandom_Init(), we ignore their
is not initialiazed yet. For _PyRandom_Init(), we ignore the
error and fall back on reading /dev/urandom which never blocks,
even if the system urandom is not initialized yet. */
even if the system urandom is not initialized yet:
see the PEP 524. */
if (errno == EAGAIN && !raise && !blocking) {
return 0;
}
@ -217,7 +185,80 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
}
return 1;
}
#endif
#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;
@ -225,35 +266,38 @@ static struct {
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 'size' random bytes from py_getrandom(). Fall back on reading from
/dev/urandom if getrandom() is not available.
read() is retried if it failed with EINTR: interrupted by a signal.
Return 0 on success. Raise an exception (if raise is non-zero) and return -1
on error. */
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 blocking, int raise)
dev_urandom(char *buffer, Py_ssize_t size, int raise)
{
int fd;
Py_ssize_t n;
#ifdef PY_GETRANDOM
int res;
#endif
assert(size > 0);
#ifdef PY_GETRANDOM
res = py_getrandom(buffer, size, blocking, raise);
if (res < 0) {
return -1;
}
if (res == 1) {
return 0;
}
/* getrandom() failed with ENOSYS or EPERM,
fall back on reading /dev/urandom */
#endif
if (raise) {
struct _Py_stat_struct st;
@ -275,9 +319,10 @@ dev_urandom(char *buffer, Py_ssize_t size, int blocking, int raise)
fd = _Py_open("/dev/urandom", O_RDONLY);
if (fd < 0) {
if (errno == ENOENT || errno == ENXIO ||
errno == ENODEV || errno == EACCES)
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;
}
@ -349,8 +394,8 @@ dev_urandom_close(void)
urandom_cache.fd = -1;
}
}
#endif /* !MS_WINDOWS */
#endif
/* Fill buffer with pseudo-random bytes generated by a linear congruent
generator (LCG):
@ -373,14 +418,56 @@ lcg_urandom(unsigned int x0, unsigned char *buffer, size_t size)
}
}
/* If raise is zero:
- Don't raise exceptions on error
- Don't call PyErr_CheckSignals() on EINTR (retry directly the interrupted
syscall)
- Don't release the GIL to call syscalls. */
/* 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: see the PEP 524. Python requires non-blocking RNG at
startup to initialize its hash secret, but os.urandom() must block until the
system urandom is initialized (at least on Linux 3.17 and newer).
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 the getrandom() function supports non-blocking mode.
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 blocking, int raise)
{
#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
int res;
#endif
if (size < 0) {
if (raise) {
PyErr_Format(PyExc_ValueError,
@ -395,10 +482,25 @@ pyurandom(void *buffer, Py_ssize_t size, int blocking, int raise)
#ifdef MS_WINDOWS
return win32_urandom((unsigned char *)buffer, size, raise);
#elif defined(PY_GETENTROPY)
return py_getentropy(buffer, size, raise);
#else
return dev_urandom(buffer, size, blocking, raise);
#if defined(PY_GETRANDOM) || defined(PY_GETENTROPY)
#ifdef PY_GETRANDOM
res = py_getrandom(buffer, size, blocking, 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 or EPERM. Fall back on reading from /dev/urandom. */
#endif
return dev_urandom(buffer, size, raise);
#endif
}
@ -491,8 +593,6 @@ _PyRandom_Fini(void)
CryptReleaseContext(hCryptProv, 0);
hCryptProv = 0;
}
#elif defined(PY_GETENTROPY)
/* nothing to clean */
#else
dev_urandom_close();
#endif

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