crypto/rand/urandom.cc

// Copyright 2014 The BoringSSL Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#if !defined(_GNU_SOURCE)
#define _GNU_SOURCE  // needed for syscall() on Linux.
#endif

#include <openssl/rand.h>

#include "../bcm_support.h"
#include "internal.h"

#if defined(OPENSSL_RAND_URANDOM)

#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#if defined(OPENSSL_LINUX)
#if defined(BORINGSSL_FIPS)
#include <linux/random.h>
#include <sys/ioctl.h>
#endif
#include <sys/syscall.h>

#if defined(OPENSSL_ANDROID)
#include <sys/system_properties.h>
#endif

#if !defined(OPENSSL_ANDROID)
#define OPENSSL_HAS_GETAUXVAL
#endif
// glibc prior to 2.16 does not have getauxval and sys/auxv.h. Android has some
// host builds (i.e. not building for Android itself, so |OPENSSL_ANDROID| is
// unset) which are still using a 2.15 sysroot.
//
// TODO(davidben): Remove this once Android updates their sysroot.
#if defined(__GLIBC_PREREQ)
#if !__GLIBC_PREREQ(2, 16)
#undef OPENSSL_HAS_GETAUXVAL
#endif
#endif
#if defined(OPENSSL_HAS_GETAUXVAL)
#include <sys/auxv.h>
#endif
#endif  // OPENSSL_LINUX

#include <openssl/mem.h>

#include "../internal.h"
#include "getrandom_fillin.h"


#if defined(USE_NR_getrandom)

#if defined(OPENSSL_MSAN)
extern "C" {
void __msan_unpoison(void *, size_t);
}
#endif

static ssize_t boringssl_getrandom(void *buf, size_t buf_len, unsigned flags) {
  ssize_t ret;
  do {
    ret = syscall(__NR_getrandom, buf, buf_len, flags);
  } while (ret == -1 && errno == EINTR);

#if defined(OPENSSL_MSAN)
  if (ret > 0) {
    // MSAN doesn't recognise |syscall| and thus doesn't notice that we have
    // initialised the output buffer.
    __msan_unpoison(buf, ret);
  }
#endif  // OPENSSL_MSAN

  return ret;
}

#endif  // USE_NR_getrandom

// kHaveGetrandom in |urandom_fd| signals that |getrandom| or |getentropy| is
// available and should be used instead.
static const int kHaveGetrandom = -3;

// urandom_fd is a file descriptor to /dev/urandom. It's protected by |once|.
static int urandom_fd;

#if defined(USE_NR_getrandom)

// getrandom_ready is one if |getrandom| had been initialized by the time
// |init_once| was called and zero otherwise.
static int getrandom_ready;

// extra_getrandom_flags_for_seed contains a value that is ORed into the flags
// for getrandom() when reading entropy for a seed.
static int extra_getrandom_flags_for_seed;

// On Android, check a system property to decide whether to set
// |extra_getrandom_flags_for_seed| otherwise they will default to zero.  If
// ro.oem_boringcrypto_hwrand is true then |extra_getrandom_flags_for_seed| will
// be set to GRND_RANDOM, causing all random data to be drawn from the same
// source as /dev/random.
static void maybe_set_extra_getrandom_flags(void) {
#if defined(BORINGSSL_FIPS) && defined(OPENSSL_ANDROID)
  char value[PROP_VALUE_MAX + 1];
  int length = __system_property_get("ro.boringcrypto.hwrand", value);
  if (length < 0 || length > PROP_VALUE_MAX) {
    return;
  }

  value[length] = 0;
  if (OPENSSL_strcasecmp(value, "true") == 0) {
    extra_getrandom_flags_for_seed = GRND_RANDOM;
  }
#endif
}

#endif  // USE_NR_getrandom

static CRYPTO_once_t rand_once = CRYPTO_ONCE_INIT;

// init_once initializes the state of this module to values previously
// requested. This is the only function that modifies |urandom_fd|, which may be
// read safely after calling the once.
static void init_once(void) {
#if defined(USE_NR_getrandom)
  int have_getrandom;
  uint8_t dummy;
  ssize_t getrandom_ret =
      boringssl_getrandom(&dummy, sizeof(dummy), GRND_NONBLOCK);
  if (getrandom_ret == 1) {
    getrandom_ready = 1;
    have_getrandom = 1;
  } else if (getrandom_ret == -1 && errno == EAGAIN) {
    // We have getrandom, but the entropy pool has not been initialized yet.
    have_getrandom = 1;
  } else if (getrandom_ret == -1 && errno == ENOSYS) {
    // Fallthrough to using /dev/urandom, below.
    have_getrandom = 0;
  } else {
    // Other errors are fatal.
    perror("getrandom");
    abort();
  }

  if (have_getrandom) {
    urandom_fd = kHaveGetrandom;
    maybe_set_extra_getrandom_flags();
    return;
  }
#endif  // USE_NR_getrandom

  // FIPS builds must support getrandom.
  //
  // Historically, only Android FIPS builds required getrandom, while Linux FIPS
  // builds had a /dev/urandom fallback which used RNDGETENTCNT as a poor
  // approximation for getrandom's blocking behavior. This is now removed, but
  // avoid making assumptions on this removal until March 2023, in case it needs
  // to be restored. This comment can be deleted after March 2023.
#if defined(BORINGSSL_FIPS)
  perror("getrandom not found");
  abort();
#endif

  int fd;
  do {
    fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
  } while (fd == -1 && errno == EINTR);

  if (fd < 0) {
    perror("failed to open /dev/urandom");
    abort();
  }

  urandom_fd = fd;
}

static CRYPTO_once_t wait_for_entropy_once = CRYPTO_ONCE_INIT;

static void wait_for_entropy(void) {
  int fd = urandom_fd;
  if (fd == kHaveGetrandom) {
    // |getrandom| and |getentropy| support blocking in |fill_with_entropy|
    // directly. For |getrandom|, we first probe with a non-blocking call to aid
    // debugging.
#if defined(USE_NR_getrandom)
    if (getrandom_ready) {
      // The entropy pool was already initialized in |init_once|.
      return;
    }

    uint8_t dummy;
    ssize_t getrandom_ret =
        boringssl_getrandom(&dummy, sizeof(dummy), GRND_NONBLOCK);
    if (getrandom_ret == -1 && errno == EAGAIN) {
      // Attempt to get the path of the current process to aid in debugging when
      // something blocks.
      const char *current_process = "<unknown>";
#if defined(OPENSSL_HAS_GETAUXVAL)
      const unsigned long getauxval_ret = getauxval(AT_EXECFN);
      if (getauxval_ret != 0) {
        current_process = (const char *)getauxval_ret;
      }
#endif

      fprintf(
          stderr,
          "%s: getrandom indicates that the entropy pool has not been "
          "initialized. Rather than continue with poor entropy, this process "
          "will block until entropy is available.\n",
          current_process);

      getrandom_ret =
          boringssl_getrandom(&dummy, sizeof(dummy), 0 /* no flags */);
    }

    if (getrandom_ret != 1) {
      perror("getrandom");
      abort();
    }
#endif  // USE_NR_getrandom
    return;
  }
}

// fill_with_entropy writes |len| bytes of entropy into |out|. It returns one
// on success and zero on error. If |block| is one, this function will block
// until the entropy pool is initialized. Otherwise, this function may fail,
// setting |errno| to |EAGAIN| if the entropy pool has not yet been initialized.
// If |seed| is one, this function will OR in the value of
// |*extra_getrandom_flags_for_seed()| when using |getrandom|.
static int fill_with_entropy(uint8_t *out, size_t len, int block, int seed) {
  if (len == 0) {
    return 1;
  }

#if defined(USE_NR_getrandom) || defined(FREEBSD_GETRANDOM)
  int getrandom_flags = 0;
  if (!block) {
    getrandom_flags |= GRND_NONBLOCK;
  }
#endif

#if defined(USE_NR_getrandom)
  if (seed) {
    getrandom_flags |= extra_getrandom_flags_for_seed;
  }
#endif

  CRYPTO_init_sysrand();
  if (block) {
    CRYPTO_once(&wait_for_entropy_once, wait_for_entropy);
  }

  // Clear |errno| so it has defined value if |read| or |getrandom|
  // "successfully" returns zero.
  errno = 0;
  while (len > 0) {
    ssize_t r;

    if (urandom_fd == kHaveGetrandom) {
#if defined(USE_NR_getrandom)
      r = boringssl_getrandom(out, len, getrandom_flags);
#else  // USE_NR_getrandom
      fprintf(stderr, "urandom fd corrupt.\n");
      abort();
#endif
    } else {
      do {
        r = read(urandom_fd, out, len);
      } while (r == -1 && errno == EINTR);
    }

    if (r <= 0) {
      return 0;
    }
    out += r;
    len -= r;
  }

  return 1;
}

void CRYPTO_init_sysrand(void) { CRYPTO_once(&rand_once, init_once); }

// CRYPTO_sysrand puts |requested| random bytes into |out|.
void CRYPTO_sysrand(uint8_t *out, size_t requested) {
  if (!fill_with_entropy(out, requested, /*block=*/1, /*seed=*/0)) {
    perror("entropy fill failed");
    abort();
  }
}

void CRYPTO_sysrand_for_seed(uint8_t *out, size_t requested) {
  if (!fill_with_entropy(out, requested, /*block=*/1, /*seed=*/1)) {
    perror("entropy fill failed");
    abort();
  }
}

int CRYPTO_sysrand_if_available(uint8_t *out, size_t requested) {
  if (fill_with_entropy(out, requested, /*block=*/0, /*seed=*/0)) {
    return 1;
  } else if (errno == EAGAIN) {
    OPENSSL_memset(out, 0, requested);
    return 0;
  } else {
    perror("opportunistic entropy fill failed");
    abort();
  }
}

#endif  // OPENSSL_RAND_URANDOM