crypto_rand.c

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/* Copyright (c) 2001, Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2021, The Tor Project, Inc. */
/* See LICENSE for licensing information */
 
/**
 * \file crypto_rand.c
 *
 * \brief Functions for initialising and seeding (pseudo-)random
 * number generators, and working with randomness.
 **/
 
#define CRYPTO_RAND_PRIVATE
 
#include "lib/crypt_ops/crypto_rand.h"
 
#ifdef _WIN32
#include <windows.h>
#include <wincrypt.h>
#endif /* defined(_WIN32) */
 
#include "lib/container/smartlist.h"
#include "lib/crypt_ops/compat_openssl.h"
#include "lib/crypt_ops/crypto_util.h"
#include "lib/encoding/binascii.h"
#include "lib/intmath/weakrng.h"
#include "lib/log/log.h"
#include "lib/log/util_bug.h"
#include "lib/malloc/malloc.h"
#include "lib/sandbox/sandbox.h"
#include "lib/string/compat_string.h"
#include "lib/string/util_string.h"
#include "lib/testsupport/testsupport.h"
#include "lib/fs/files.h"
 
#include "lib/defs/digest_sizes.h"
#include "lib/crypt_ops/crypto_digest.h"
#include "lib/ctime/di_ops.h"
 
#ifdef ENABLE_NSS
#include "lib/crypt_ops/crypto_nss_mgt.h"
#endif
 
#ifdef ENABLE_OPENSSL
DISABLE_GCC_WARNING("-Wredundant-decls")
#include <openssl/rand.h>
#include <openssl/sha.h>
ENABLE_GCC_WARNING("-Wredundant-decls")
#endif /* defined(ENABLE_OPENSSL) */
 
#ifdef ENABLE_NSS
DISABLE_GCC_WARNING("-Wstrict-prototypes")
#include <pk11pub.h>
#include <secerr.h>
#include <prerror.h>
ENABLE_GCC_WARNING("-Wstrict-prototypes")
#endif
 
#if __GNUC__ && GCC_VERSION >= 402
#if GCC_VERSION >= 406
#pragma GCC diagnostic pop
#else
#pragma GCC diagnostic warning "-Wredundant-decls"
#endif
#endif /* __GNUC__ && GCC_VERSION >= 402 */
 
#ifdef HAVE_FCNTL_H
#include <fcntl.h>
#endif
#ifdef HAVE_SYS_FCNTL_H
#include <sys/fcntl.h>
#endif
#ifdef HAVE_SYS_STAT_H
#include <sys/stat.h>
#endif
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
#ifdef HAVE_SYS_SYSCALL_H
#include <sys/syscall.h>
#endif
#ifdef HAVE_SYS_RANDOM_H
#include <sys/random.h>
#endif
 
#include <string.h>
#include <errno.h>
 
/**
 * How many bytes of entropy we add at once.
 *
 * This is how much entropy OpenSSL likes to add right now, so maybe it will
 * work for us too.
 **/
#define ADD_ENTROPY 32
 
/**
 * Longest recognized DNS query.
 **/
#define MAX_DNS_LABEL_SIZE 63
 
/**
 * Largest strong entropy request permitted.
 **/
#define MAX_STRONGEST_RAND_SIZE 256
 
/**
 * Set the seed of the weak RNG to a random value.
 **/
void
crypto_seed_weak_rng(tor_weak_rng_t *rng)
{
  unsigned seed;
  crypto_rand((void*)&seed, sizeof(seed));
  tor_init_weak_random(rng, seed);
}
 
#ifdef TOR_UNIT_TESTS
int break_strongest_rng_syscall = 0;
int break_strongest_rng_fallback = 0;
#endif
 
/**
 * Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * via system calls, storing it into <b>out</b>. Return 0 on success, -1 on
 * failure.  A maximum request size of 256 bytes is imposed.
 **/
static int
crypto_strongest_rand_syscall(uint8_t *out, size_t out_len)
{
  tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE);
 
  /* We only log at notice-level here because in the case that this function
   * fails the crypto_strongest_rand_raw() caller will log with a warning-level
   * message and let crypto_strongest_rand() error out and finally terminating
   * Tor with an assertion error.
   */
 
#ifdef TOR_UNIT_TESTS
  if (break_strongest_rng_syscall)
    return -1;
#endif
 
#if defined(_WIN32)
  static int provider_set = 0;
  static HCRYPTPROV provider;
 
  if (!provider_set) {
    if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
                             CRYPT_VERIFYCONTEXT)) {
      log_notice(LD_CRYPTO, "Unable to set Windows CryptoAPI provider [1].");
      return -1;
    }
    provider_set = 1;
  }
  if (!CryptGenRandom(provider, out_len, out)) {
    log_notice(LD_CRYPTO, "Unable get entropy from the Windows CryptoAPI.");
    return -1;
  }
 
  return 0;
#elif defined(__linux__) && defined(SYS_getrandom)
  static int getrandom_works = 1; /* Be optimistic about our chances... */
 
  /* getrandom() isn't as straightforward as getentropy(), and has
   * no glibc wrapper.
   *
   * As far as I can tell from getrandom(2) and the source code, the
   * requests we issue will always succeed (though it will block on the
   * call if /dev/urandom isn't seeded yet), since we are NOT specifying
   * GRND_NONBLOCK and the request is <= 256 bytes.
   *
   * The manpage is unclear on what happens if a signal interrupts the call
   * while the request is blocked due to lack of entropy....
   *
   * We optimistically assume that getrandom() is available and functional
   * because it is the way of the future, and 2 branch mispredicts pale in
   * comparison to the overheads involved with failing to open
   * /dev/srandom followed by opening and reading from /dev/urandom.
   */
  if (PREDICT_LIKELY(getrandom_works)) {
    long ret;
    /* A flag of '0' here means to read from '/dev/urandom', and to
     * block if insufficient entropy is available to service the
     * request.
     */
    const unsigned int flags = 0;
    do {
      ret = syscall(SYS_getrandom, out, out_len, flags);
    } while (ret == -1 && ((errno == EINTR) ||(errno == EAGAIN)));
 
    if (PREDICT_UNLIKELY(ret == -1)) {
      /* LCOV_EXCL_START we can't actually make the syscall fail in testing. */
      tor_assert(errno != EAGAIN);
      tor_assert(errno != EINTR);
 
      /* Useful log message for errno. */
      if (errno == ENOSYS) {
        log_notice(LD_CRYPTO, "Can't get entropy from getrandom()."
                   " You are running a version of Tor built to support"
                   " getrandom(), but the kernel doesn't implement this"
                   " function--probably because it is too old?"
                   " Trying fallback method instead.");
      } else {
        log_notice(LD_CRYPTO, "Can't get entropy from getrandom(): %s."
                              " Trying fallback method instead.",
                   strerror(errno));
      }
 
      getrandom_works = 0; /* Don't bother trying again. */
      return -1;
      /* LCOV_EXCL_STOP */
    }
 
    tor_assert(ret == (long)out_len);
    return 0;
  }
 
  return -1; /* getrandom() previously failed unexpectedly. */
#elif defined(HAVE_GETENTROPY)
  /* getentropy() is what Linux's getrandom() wants to be when it grows up.
   * the only gotcha is that requests are limited to 256 bytes.
   */
  return getentropy(out, out_len);
#else
  (void) out;
#endif /* defined(_WIN32) || ... */
 
  /* This platform doesn't have a supported syscall based random. */
  return -1;
}
 
/**
 * Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * via the per-platform fallback mechanism, storing it into <b>out</b>.
 * Return 0 on success, -1 on failure.  A maximum request size of 256 bytes
 * is imposed.
 **/
static int
crypto_strongest_rand_fallback(uint8_t *out, size_t out_len)
{
#ifdef TOR_UNIT_TESTS
  if (break_strongest_rng_fallback)
    return -1;
#endif
 
#ifdef _WIN32
  /* Windows exclusively uses crypto_strongest_rand_syscall(). */
  (void)out;
  (void)out_len;
  return -1;
#else /* !defined(_WIN32) */
  static const char *filenames[] = {
    "/dev/srandom", "/dev/urandom", "/dev/random", NULL
  };
  int fd, i;
  size_t n;
 
  for (i = 0; filenames[i]; ++i) {
    log_debug(LD_FS, "Considering %s as entropy source", filenames[i]);
    fd = open(sandbox_intern_string(filenames[i]), O_RDONLY, 0);
    if (fd<0) continue;
    log_info(LD_CRYPTO, "Reading entropy from \"%s\"", filenames[i]);
    n = read_all_from_fd(fd, (char*)out, out_len);
    close(fd);
    if (n != out_len) {
      /* LCOV_EXCL_START
       * We can't make /dev/foorandom actually fail. */
      log_notice(LD_CRYPTO,
                 "Error reading from entropy source %s (read only %lu bytes).",
                 filenames[i],
                 (unsigned long)n);
      return -1;
      /* LCOV_EXCL_STOP */
    }
 
    return 0;
  }
 
  return -1;
#endif /* defined(_WIN32) */
}
 
/**
 * Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * storing it into <b>out</b>. Return 0 on success, -1 on failure.  A maximum
 * request size of 256 bytes is imposed.
 **/
STATIC int
crypto_strongest_rand_raw(uint8_t *out, size_t out_len)
{
  static const size_t sanity_min_size = 16;
  static const int max_attempts = 3;
  tor_assert(out_len <= MAX_STRONGEST_RAND_SIZE);
 
  /* For buffers >= 16 bytes (128 bits), we sanity check the output by
   * zero filling the buffer and ensuring that it actually was at least
   * partially modified.
   *
   * Checking that any individual byte is non-zero seems like it would
   * fail too often (p = out_len * 1/256) for comfort, but this is an
   * "adjust according to taste" sort of check.
   */
  memwipe(out, 0, out_len);
  for (int i = 0; i < max_attempts; i++) {
    /* Try to use the syscall/OS favored mechanism to get strong entropy. */
    if (crypto_strongest_rand_syscall(out, out_len) != 0) {
      /* Try to use the less-favored mechanism to get strong entropy. */
      if (crypto_strongest_rand_fallback(out, out_len) != 0) {
        /* Welp, we tried.  Hopefully the calling code terminates the process
         * since we're basically boned without good entropy.
         */
        log_warn(LD_CRYPTO,
                 "Cannot get strong entropy: no entropy source found.");
        return -1;
      }
    }
 
    if ((out_len < sanity_min_size) || !safe_mem_is_zero((char*)out, out_len))
      return 0;
  }
 
  /* LCOV_EXCL_START
   *
   * We tried max_attempts times to fill a buffer >= 128 bits long,
   * and each time it returned all '0's.  Either the system entropy
   * source is busted, or the user should go out and buy a ticket to
   * every lottery on the planet.
   */
  log_warn(LD_CRYPTO, "Strong OS entropy returned all zero buffer.");
 
  return -1;
  /* LCOV_EXCL_STOP */
}
 
/**
 * Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * storing it into <b>out</b>.
 **/
void
crypto_strongest_rand(uint8_t *out, size_t out_len)
{
  crypto_strongest_rand_(out, out_len);
}
 
/**
 * Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
 * storing it into <b>out</b>. (Mockable version.)
 **/
MOCK_IMPL(void,
crypto_strongest_rand_,(uint8_t *out, size_t out_len))
{
#define DLEN DIGEST512_LEN
 
  /* We're going to hash DLEN bytes from the system RNG together with some
   * bytes from the PRNGs from our crypto librar(y/ies), in order to yield
   * DLEN bytes.
   */
  uint8_t inp[DLEN*3];
  uint8_t tmp[DLEN];
  tor_assert(out);
  while (out_len) {
    memset(inp, 0, sizeof(inp));
#ifdef ENABLE_OPENSSL
    RAND_bytes(inp, DLEN);
#endif
#ifdef ENABLE_NSS
    PK11_GenerateRandom(inp+DLEN, DLEN);
#endif
    if (crypto_strongest_rand_raw(inp+DLEN*2, DLEN) < 0) {
      // LCOV_EXCL_START
      log_err(LD_CRYPTO, "Failed to load strong entropy when generating an "
              "important key. Exiting.");
      /* Die with an assertion so we get a stack trace. */
      tor_assert(0);
      // LCOV_EXCL_STOP
    }
    if (out_len >= DLEN) {
      crypto_digest512((char*)out, (char*)inp, sizeof(inp), DIGEST_SHA512);
      out += DLEN;
      out_len -= DLEN;
    } else {
      crypto_digest512((char*)tmp, (char*)inp, sizeof(inp), DIGEST_SHA512);
      memcpy(out, tmp, out_len);
      break;
    }
  }
  memwipe(tmp, 0, sizeof(tmp));
  memwipe(inp, 0, sizeof(inp));
#undef DLEN
}
 
#ifdef ENABLE_OPENSSL
/**
 * Seed OpenSSL's random number generator with bytes from the operating
 * system.  Return 0 on success, -1 on failure.
 **/
static int
crypto_seed_openssl_rng(void)
{
  int rand_poll_ok = 0, load_entropy_ok = 0;
  uint8_t buf[ADD_ENTROPY];
 
  /* OpenSSL has a RAND_poll function that knows about more kinds of
   * entropy than we do.  We'll try calling that, *and* calling our own entropy
   * functions.  If one succeeds, we'll accept the RNG as seeded. */
  rand_poll_ok = RAND_poll();
  if (rand_poll_ok == 0)
    log_warn(LD_CRYPTO, "RAND_poll() failed."); // LCOV_EXCL_LINE
 
  load_entropy_ok = !crypto_strongest_rand_raw(buf, sizeof(buf));
  if (load_entropy_ok) {
    RAND_seed(buf, sizeof(buf));
  }
 
  memwipe(buf, 0, sizeof(buf));
 
  if ((rand_poll_ok || load_entropy_ok) && RAND_status() == 1)
    return 0;
  else
    return -1;
}
#endif /* defined(ENABLE_OPENSSL) */
 
#ifdef ENABLE_NSS
/**
 * Seed OpenSSL's random number generator with bytes from the operating
 * system.  Return 0 on success, -1 on failure.
 **/
static int
crypto_seed_nss_rng(void)
{
  uint8_t buf[ADD_ENTROPY];
 
  int load_entropy_ok = !crypto_strongest_rand_raw(buf, sizeof(buf));
  if (load_entropy_ok) {
    if (PK11_RandomUpdate(buf, sizeof(buf)) != SECSuccess) {
      load_entropy_ok = 0;
    }
  }
 
  memwipe(buf, 0, sizeof(buf));
 
  return load_entropy_ok ? 0 : -1;
}
#endif /* defined(ENABLE_NSS) */
 
/**
 * Seed the RNG for any and all crypto libraries that we're using with bytes
 * from the operating system.  Return 0 on success, -1 on failure.
 */
int
crypto_seed_rng(void)
{
  int seeded = 0;
#ifdef ENABLE_NSS
  if (crypto_seed_nss_rng() < 0)
    return -1;
  ++seeded;
#endif
#ifdef ENABLE_OPENSSL
  if (crypto_seed_openssl_rng() < 0)
    return -1;
  ++seeded;
#endif
  tor_assert(seeded);
  return 0;
}
 
/**
 * Write <b>n</b> bytes of strong random data to <b>to</b>. Supports mocking
 * for unit tests.
 *
 * This function is not allowed to fail; if it would fail to generate strong
 * entropy, it must terminate the process instead.
 **/
MOCK_IMPL(void,
crypto_rand, (char *to, size_t n))
{
  crypto_rand_unmocked(to, n);
}
 
/**
 * Write <b>n</b> bytes of strong random data to <b>to</b>.  Most callers
 * will want crypto_rand instead.
 *
 * This function is not allowed to fail; if it would fail to generate strong
 * entropy, it must terminate the process instead.
 **/
void
crypto_rand_unmocked(char *to, size_t n)
{
  if (n == 0)
    return;
 
  tor_assert(n < INT_MAX);
  tor_assert(to);
 
#ifdef ENABLE_NSS
  SECStatus s = PK11_GenerateRandom((unsigned char*)to, (int)n);
  if (s != SECSuccess) {
    /* NSS rather sensibly might refuse to generate huge amounts of random
     * data at once.  Unfortunately, our unit test do this in a couple of
     * places.  To solve this issue, we use our XOF to stretch a shorter
     * output when a longer one is needed.
     *
     * Yes, this is secure. */
 
    /* This is longer than it needs to be; 1600 bits == 200 bytes is the
     * state-size of SHA3. */
#define BUFLEN 512
    tor_assert(PR_GetError() == SEC_ERROR_INVALID_ARGS && n > BUFLEN);
    unsigned char buf[BUFLEN];
    s = PK11_GenerateRandom(buf, BUFLEN);
    tor_assert(s == SECSuccess);
    crypto_xof_t *xof = crypto_xof_new();
    crypto_xof_add_bytes(xof, buf, BUFLEN);
    crypto_xof_squeeze_bytes(xof, (unsigned char *)to, n);
    crypto_xof_free(xof);
    memwipe(buf, 0, BUFLEN);
 
#undef BUFLEN
  }
#else /* !defined(ENABLE_NSS) */
  int r = RAND_bytes((unsigned char*)to, (int)n);
  /* We consider a PRNG failure non-survivable. Let's assert so that we get a
   * stack trace about where it happened.
   */
  tor_assert(r == 1);
#endif
}
 
/**
 * Draw an unsigned 32-bit integer uniformly at random.
 */
uint32_t
crypto_rand_u32(void)
{
  uint32_t rand;
  crypto_rand((void*)&rand, sizeof(rand));
  return rand;
}
 
/**
 * Generate and return a new random hostname starting with <b>prefix</b>,
 * ending with <b>suffix</b>, and containing no fewer than
 * <b>min_rand_len</b> and no more than <b>max_rand_len</b> random base32
 * characters. Does not check for failure.
 *
 * Clip <b>max_rand_len</b> to MAX_DNS_LABEL_SIZE.
 **/
char *
crypto_random_hostname(int min_rand_len, int max_rand_len, const char *prefix,
                       const char *suffix)
{
  char *result, *rand_bytes;
  int randlen, rand_bytes_len;
  size_t resultlen, prefixlen;
 
  if (max_rand_len > MAX_DNS_LABEL_SIZE)
    max_rand_len = MAX_DNS_LABEL_SIZE;
  if (min_rand_len > max_rand_len)
    min_rand_len = max_rand_len;
 
  randlen = crypto_rand_int_range(min_rand_len, max_rand_len+1);
 
  prefixlen = strlen(prefix);
  resultlen = prefixlen + strlen(suffix) + randlen + 16;
 
  /* (x+(n-1))/n is an idiom for dividing x by n, rounding up to the nearest
   * integer and thus why this construction. */
  rand_bytes_len = ((randlen*5)+7)/8;
  if (rand_bytes_len % 5)
    rand_bytes_len += 5 - (rand_bytes_len%5);
  rand_bytes = tor_malloc(rand_bytes_len);
  crypto_rand(rand_bytes, rand_bytes_len);
 
  result = tor_malloc(resultlen);
  memcpy(result, prefix, prefixlen);
  base32_encode(result+prefixlen, resultlen-prefixlen,
                rand_bytes, rand_bytes_len);
  tor_free(rand_bytes);
  strlcpy(result+prefixlen+randlen, suffix, resultlen-(prefixlen+randlen));
 
  return result;
}
 
/**
 * Return a randomly chosen element of <b>sl</b>; or NULL if <b>sl</b>
 * is empty.
 **/
void *
smartlist_choose(const smartlist_t *sl)
{
  int len = smartlist_len(sl);
  if (len)
    return smartlist_get(sl,crypto_rand_int(len));
  return NULL; /* no elements to choose from */
}
 
/**
 * Scramble the elements of <b>sl</b> into a random order.
 **/
void
smartlist_shuffle(smartlist_t *sl)
{
  int i;
  /* From the end of the list to the front, choose at random from the
     positions we haven't looked at yet, and swap that position into the
     current position.  Remember to give "no swap" the same probability as
     any other swap. */
  for (i = smartlist_len(sl)-1; i > 0; --i) {
    int j = crypto_rand_int(i+1);
    smartlist_swap(sl, i, j);
  }
}
 
/** Make sure that openssl is using its default PRNG. Return 1 if we had to
 * adjust it; 0 otherwise. */
int
crypto_force_rand_ssleay(void)
{
#ifdef ENABLE_OPENSSL
  RAND_METHOD *default_method;
  default_method = RAND_OpenSSL();
  if (RAND_get_rand_method() != default_method) {
    log_notice(LD_CRYPTO, "It appears that one of our engines has provided "
               "a replacement the OpenSSL RNG. Resetting it to the default "
               "implementation.");
    RAND_set_rand_method(default_method);
    return 1;
  }
#endif /* defined(ENABLE_OPENSSL) */
  return 0;
}