Python/random.c

#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
}