fipsmodule/rand/rand.c

/* Copyright (c) 2014, Google Inc.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#include <openssl/rand.h>

#include <assert.h>
#include <limits.h>
#include <string.h>

#if defined(BORINGSSL_FIPS)
#include <unistd.h>
#endif

#include <openssl/chacha.h>
#include <openssl/cpu.h>
#include <openssl/mem.h>

#include "internal.h"
#include "../../internal.h"
#include "../delocate.h"


// It's assumed that the operating system always has an unfailing source of
// entropy which is accessed via |CRYPTO_sysrand[_for_seed]|. (If the operating
// system entropy source fails, it's up to |CRYPTO_sysrand| to abort the
// process—we don't try to handle it.)
//
// In addition, the hardware may provide a low-latency RNG. Intel's rdrand
// instruction is the canonical example of this. When a hardware RNG is
// available we don't need to worry about an RNG failure arising from fork()ing
// the process or moving a VM, so we can keep thread-local RNG state and use it
// as an additional-data input to CTR-DRBG.
//
// (We assume that the OS entropy is safe from fork()ing and VM duplication.
// This might be a bit of a leap of faith, esp on Windows, but there's nothing
// that we can do about it.)

// kReseedInterval is the number of generate calls made to CTR-DRBG before
// reseeding.
static const unsigned kReseedInterval = 4096;

// CRNGT_BLOCK_SIZE is the number of bytes in a “block” for the purposes of the
// continuous random number generator test in FIPS 140-2, section 4.9.2.
#define CRNGT_BLOCK_SIZE 16

// rand_thread_state contains the per-thread state for the RNG.
struct rand_thread_state {
  CTR_DRBG_STATE drbg;
  // calls is the number of generate calls made on |drbg| since it was last
  // (re)seeded. This is bound by |kReseedInterval|.
  unsigned calls;
  // last_block_valid is non-zero iff |last_block| contains data from
  // |CRYPTO_sysrand_for_seed|.
  int last_block_valid;

#if defined(BORINGSSL_FIPS)
  // last_block contains the previous block from |CRYPTO_sysrand_for_seed|.
  uint8_t last_block[CRNGT_BLOCK_SIZE];
  // next and prev form a NULL-terminated, double-linked list of all states in
  // a process.
  struct rand_thread_state *next, *prev;
#endif
};

#if defined(BORINGSSL_FIPS)
// thread_states_list is the head of a linked-list of all |rand_thread_state|
// objects in the process, one per thread. This is needed because FIPS requires
// that they be zeroed on process exit, but thread-local destructors aren't
// called when the whole process is exiting.
DEFINE_BSS_GET(struct rand_thread_state *, thread_states_list);
DEFINE_STATIC_MUTEX(thread_states_list_lock);

static void rand_thread_state_clear_all(void) __attribute__((destructor));
static void rand_thread_state_clear_all(void) {
  CRYPTO_STATIC_MUTEX_lock_write(thread_states_list_lock_bss_get());
  for (struct rand_thread_state *cur = *thread_states_list_bss_get();
       cur != NULL; cur = cur->next) {
    CTR_DRBG_clear(&cur->drbg);
  }
  // |thread_states_list_lock is deliberately left locked so that any threads
  // that are still running will hang if they try to call |RAND_bytes|.
}
#endif

// rand_thread_state_free frees a |rand_thread_state|. This is called when a
// thread exits.
static void rand_thread_state_free(void *state_in) {
  struct rand_thread_state *state = state_in;

  if (state_in == NULL) {
    return;
  }

#if defined(BORINGSSL_FIPS)
  CRYPTO_STATIC_MUTEX_lock_write(thread_states_list_lock_bss_get());

  if (state->prev != NULL) {
    state->prev->next = state->next;
  } else {
    *thread_states_list_bss_get() = state->next;
  }

  if (state->next != NULL) {
    state->next->prev = state->prev;
  }

  CRYPTO_STATIC_MUTEX_unlock_write(thread_states_list_lock_bss_get());

  CTR_DRBG_clear(&state->drbg);
#endif

  OPENSSL_free(state);
}

#if defined(OPENSSL_X86_64) && !defined(OPENSSL_NO_ASM) && \
    !defined(BORINGSSL_UNSAFE_DETERMINISTIC_MODE)
static int hwrand(uint8_t *buf, const size_t len) {
  if (!have_rdrand()) {
    return 0;
  }

  const size_t len_multiple8 = len & ~7;
  if (!CRYPTO_rdrand_multiple8_buf(buf, len_multiple8)) {
    return 0;
  }
  const size_t remainder = len - len_multiple8;

  if (remainder != 0) {
    assert(remainder < 8);

    uint8_t rand_buf[8];
    if (!CRYPTO_rdrand(rand_buf)) {
      return 0;
    }
    OPENSSL_memcpy(buf + len_multiple8, rand_buf, remainder);
  }

#if defined(BORINGSSL_FIPS_BREAK_CRNG)
  // This breaks the "continuous random number generator test" defined in FIPS
  // 140-2, section 4.9.2, and implemented in rand_get_seed().
  OPENSSL_memset(buf, 0, len);
#endif

  return 1;
}

#else

static int hwrand(uint8_t *buf, size_t len) {
  return 0;
}

#endif

#if defined(BORINGSSL_FIPS)

static void rand_get_seed(struct rand_thread_state *state,
                          uint8_t seed[CTR_DRBG_ENTROPY_LEN]) {
  if (!state->last_block_valid) {
    if (!hwrand(state->last_block, sizeof(state->last_block))) {
      CRYPTO_sysrand_for_seed(state->last_block, sizeof(state->last_block));
    }
    state->last_block_valid = 1;
  }

  // We overread from /dev/urandom or RDRAND by a factor of 10 and XOR to
  // whiten.
#define FIPS_OVERREAD 10
  uint8_t entropy[CTR_DRBG_ENTROPY_LEN * FIPS_OVERREAD];

  int used_hwrand = hwrand(entropy, sizeof(entropy));
  if (!used_hwrand) {
    CRYPTO_sysrand_for_seed(entropy, sizeof(entropy));
  }

  // See FIPS 140-2, section 4.9.2. This is the “continuous random number
  // generator test” which causes the program to randomly abort. Hopefully the
  // rate of failure is small enough not to be a problem in practice.
  if (CRYPTO_memcmp(state->last_block, entropy, CRNGT_BLOCK_SIZE) == 0) {
    fprintf(stderr, "CRNGT failed.\n");
    BORINGSSL_FIPS_abort();
  }

  for (size_t i = CRNGT_BLOCK_SIZE; i < sizeof(entropy);
       i += CRNGT_BLOCK_SIZE) {
    if (CRYPTO_memcmp(entropy + i - CRNGT_BLOCK_SIZE, entropy + i,
                      CRNGT_BLOCK_SIZE) == 0) {
      fprintf(stderr, "CRNGT failed.\n");
      BORINGSSL_FIPS_abort();
    }
  }
  OPENSSL_memcpy(state->last_block,
                 entropy + sizeof(entropy) - CRNGT_BLOCK_SIZE,
                 CRNGT_BLOCK_SIZE);

  OPENSSL_memcpy(seed, entropy, CTR_DRBG_ENTROPY_LEN);

  for (size_t i = 1; i < FIPS_OVERREAD; i++) {
    for (size_t j = 0; j < CTR_DRBG_ENTROPY_LEN; j++) {
      seed[j] ^= entropy[CTR_DRBG_ENTROPY_LEN * i + j];
    }
  }

#if defined(OPENSSL_URANDOM)
  // If we used RDRAND, also opportunistically read from the system. This avoids
  // solely relying on the hardware once the entropy pool has been initialized.
  if (used_hwrand) {
    CRYPTO_sysrand_if_available(entropy, CTR_DRBG_ENTROPY_LEN);
    for (size_t i = 0; i < CTR_DRBG_ENTROPY_LEN; i++) {
      seed[i] ^= entropy[i];
    }
  }
#endif
}

#else

static void rand_get_seed(struct rand_thread_state *state,
                          uint8_t seed[CTR_DRBG_ENTROPY_LEN]) {
  // If not in FIPS mode, we don't overread from the system entropy source and
  // we don't depend only on the hardware RDRAND.
  CRYPTO_sysrand(seed, CTR_DRBG_ENTROPY_LEN);
}

#endif

void RAND_bytes_with_additional_data(uint8_t *out, size_t out_len,
                                     const uint8_t user_additional_data[32]) {
  if (out_len == 0) {
    return;
  }

  // Additional data is mixed into every CTR-DRBG call to protect, as best we
  // can, against forks & VM clones. We do not over-read this information and
  // don't reseed with it so, from the point of view of FIPS, this doesn't
  // provide “prediction resistance”. But, in practice, it does.
  uint8_t additional_data[32];
  if (!hwrand(additional_data, sizeof(additional_data))) {
    // Without a hardware RNG to save us from address-space duplication, the OS
    // entropy is used. This can be expensive (one read per |RAND_bytes| call)
    // and so can be disabled by applications that we have ensured don't fork
    // and aren't at risk of VM cloning.
    if (!rand_fork_unsafe_buffering_enabled()) {
      CRYPTO_sysrand(additional_data, sizeof(additional_data));
    } else {
      OPENSSL_memset(additional_data, 0, sizeof(additional_data));
    }
  }

  for (size_t i = 0; i < sizeof(additional_data); i++) {
    additional_data[i] ^= user_additional_data[i];
  }

  struct rand_thread_state stack_state;
  struct rand_thread_state *state =
      CRYPTO_get_thread_local(OPENSSL_THREAD_LOCAL_RAND);

  if (state == NULL) {
    state = OPENSSL_malloc(sizeof(struct rand_thread_state));
    if (state == NULL ||
        !CRYPTO_set_thread_local(OPENSSL_THREAD_LOCAL_RAND, state,
                                 rand_thread_state_free)) {
      // If the system is out of memory, use an ephemeral state on the
      // stack.
      state = &stack_state;
    }

    state->last_block_valid = 0;
    uint8_t seed[CTR_DRBG_ENTROPY_LEN];
    rand_get_seed(state, seed);
    if (!CTR_DRBG_init(&state->drbg, seed, NULL, 0)) {
      abort();
    }
    state->calls = 0;

#if defined(BORINGSSL_FIPS)
    if (state != &stack_state) {
      CRYPTO_STATIC_MUTEX_lock_write(thread_states_list_lock_bss_get());
      struct rand_thread_state **states_list = thread_states_list_bss_get();
      state->next = *states_list;
      if (state->next != NULL) {
        state->next->prev = state;
      }
      state->prev = NULL;
      *states_list = state;
      CRYPTO_STATIC_MUTEX_unlock_write(thread_states_list_lock_bss_get());
    }
#endif
  }

  if (state->calls >= kReseedInterval) {
    uint8_t seed[CTR_DRBG_ENTROPY_LEN];
    rand_get_seed(state, seed);
#if defined(BORINGSSL_FIPS)
    // Take a read lock around accesses to |state->drbg|. This is needed to
    // avoid returning bad entropy if we race with
    // |rand_thread_state_clear_all|.
    //
    // This lock must be taken after any calls to |CRYPTO_sysrand| to avoid a
    // bug on ppc64le. glibc may implement pthread locks by wrapping user code
    // in a hardware transaction, but, on some older versions of glibc and the
    // kernel, syscalls made with |syscall| did not abort the transaction.
    CRYPTO_STATIC_MUTEX_lock_read(thread_states_list_lock_bss_get());
#endif
    if (!CTR_DRBG_reseed(&state->drbg, seed, NULL, 0)) {
      abort();
    }
    state->calls = 0;
  } else {
#if defined(BORINGSSL_FIPS)
    CRYPTO_STATIC_MUTEX_lock_read(thread_states_list_lock_bss_get());
#endif
  }

  int first_call = 1;
  while (out_len > 0) {
    size_t todo = out_len;
    if (todo > CTR_DRBG_MAX_GENERATE_LENGTH) {
      todo = CTR_DRBG_MAX_GENERATE_LENGTH;
    }

    if (!CTR_DRBG_generate(&state->drbg, out, todo, additional_data,
                           first_call ? sizeof(additional_data) : 0)) {
      abort();
    }

    out += todo;
    out_len -= todo;
    // Though we only check before entering the loop, this cannot add enough to
    // overflow a |size_t|.
    state->calls++;
    first_call = 0;
  }

  if (state == &stack_state) {
    CTR_DRBG_clear(&state->drbg);
  }

#if defined(BORINGSSL_FIPS)
  CRYPTO_STATIC_MUTEX_unlock_read(thread_states_list_lock_bss_get());
#endif
}

int RAND_bytes(uint8_t *out, size_t out_len) {
  static const uint8_t kZeroAdditionalData[32] = {0};
  RAND_bytes_with_additional_data(out, out_len, kZeroAdditionalData);
  return 1;
}

int RAND_pseudo_bytes(uint8_t *buf, size_t len) {
  return RAND_bytes(buf, len);
}