/* * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. * * Licensed under the Apache License, Version 2.0 (the "License"). * You may not use this file except in compliance with the License. * A copy of the License is located at * * http://aws.amazon.com/apache2.0 * * or in the "license" file accompanying this file. This file 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. */ /* * _XOPEN_SOURCE is needed for resolving the constant O_CLOEXEC in some * environments. We use _XOPEN_SOURCE instead of _GNU_SOURCE because * _GNU_SOURCE is not portable and breaks when attempting to build rust * bindings on MacOS. * * https://man7.org/linux/man-pages/man2/open.2.html * The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not * specified in POSIX.1-2001, but are specified in POSIX.1-2008. * Since glibc 2.12, one can obtain their definitions by defining * either _POSIX_C_SOURCE with a value greater than or equal to * 200809L or _XOPEN_SOURCE with a value greater than or equal to * 700. In glibc 2.11 and earlier, one obtains the definitions by * defining _GNU_SOURCE. * * We use two feature probes to detect the need to perform this workaround. * It is only applied if we can't get CLOEXEC without it and the build doesn't * fail with _XOPEN_SOURCE being defined. * * # Relevent Links * * - POSIX.1-2017: https://pubs.opengroup.org/onlinepubs/9699919799 * - https://stackoverflow.com/a/5724485 * - https://stackoverflow.com/a/5583764 */ #if !defined(S2N_CLOEXEC_SUPPORTED) && defined(S2N_CLOEXEC_XOPEN_SUPPORTED) && !defined(_XOPEN_SOURCE) #define _XOPEN_SOURCE 700 #include #undef _XOPEN_SOURCE #else #include #endif #include #include #include #include #include #include #include #include #include #include #include #include #include #if defined(S2N_CPUID_AVAILABLE) #include #endif #include "api/s2n.h" #include "crypto/s2n_drbg.h" #include "crypto/s2n_fips.h" #include "error/s2n_errno.h" #include "s2n_io.h" #include "stuffer/s2n_stuffer.h" #include "utils/s2n_fork_detection.h" #include "utils/s2n_init.h" #include "utils/s2n_mem.h" #include "utils/s2n_random.h" #include "utils/s2n_result.h" #include "utils/s2n_safety.h" #if defined(O_CLOEXEC) #define ENTROPY_FLAGS O_RDONLY | O_CLOEXEC #else #define ENTROPY_FLAGS O_RDONLY #endif /* See https://en.wikipedia.org/wiki/CPUID */ #define RDRAND_ECX_FLAG 0x40000000 /* One second in nanoseconds */ #define ONE_S INT64_C(1000000000) /* Placeholder value for an uninitialized entropy file descriptor */ #define UNINITIALIZED_ENTROPY_FD -1 static struct s2n_rand_device s2n_dev_urandom = { .source = "/dev/urandom", .fd = UNINITIALIZED_ENTROPY_FD, }; struct s2n_rand_state { uint64_t cached_fork_generation_number; struct s2n_drbg public_drbg; struct s2n_drbg private_drbg; bool drbgs_initialized; }; /* Key which will control per-thread freeing of drbg memory */ static pthread_key_t s2n_per_thread_rand_state_key; /* Needed to ensure key is initialized only once */ static pthread_once_t s2n_per_thread_rand_state_key_once = PTHREAD_ONCE_INIT; /* Tracks if call to pthread_key_create failed */ static int pthread_key_create_result; static __thread struct s2n_rand_state s2n_per_thread_rand_state = { .cached_fork_generation_number = 0, .public_drbg = { 0 }, .private_drbg = { 0 }, .drbgs_initialized = false }; static int s2n_rand_init_cb_impl(void); static int s2n_rand_cleanup_cb_impl(void); static int s2n_rand_get_entropy_from_urandom(void *ptr, uint32_t size); static int s2n_rand_get_entropy_from_rdrand(void *ptr, uint32_t size); static s2n_rand_init_callback s2n_rand_init_cb = s2n_rand_init_cb_impl; static s2n_rand_cleanup_callback s2n_rand_cleanup_cb = s2n_rand_cleanup_cb_impl; static s2n_rand_seed_callback s2n_rand_seed_cb = s2n_rand_get_entropy_from_urandom; static s2n_rand_mix_callback s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom; static int s2n_rand_entropy_fd_close_ptr(int *fd) { if (fd && *fd != UNINITIALIZED_ENTROPY_FD) { close(*fd); } return S2N_SUCCESS; } /* non-static for SAW proof */ bool s2n_cpu_supports_rdrand() { #if defined(S2N_CPUID_AVAILABLE) uint32_t eax, ebx, ecx, edx; if (!__get_cpuid(1, &eax, &ebx, &ecx, &edx)) { return false; } if (ecx & RDRAND_ECX_FLAG) { return true; } #endif return false; } int s2n_rand_set_callbacks(s2n_rand_init_callback rand_init_callback, s2n_rand_cleanup_callback rand_cleanup_callback, s2n_rand_seed_callback rand_seed_callback, s2n_rand_mix_callback rand_mix_callback) { POSIX_ENSURE_REF(rand_init_callback); POSIX_ENSURE_REF(rand_cleanup_callback); POSIX_ENSURE_REF(rand_seed_callback); POSIX_ENSURE_REF(rand_mix_callback); s2n_rand_init_cb = rand_init_callback; s2n_rand_cleanup_cb = rand_cleanup_callback; s2n_rand_seed_cb = rand_seed_callback; s2n_rand_mix_cb = rand_mix_callback; return S2N_SUCCESS; } S2N_RESULT s2n_get_seed_entropy(struct s2n_blob *blob) { RESULT_ENSURE_REF(blob); RESULT_ENSURE(s2n_rand_seed_cb(blob->data, blob->size) >= S2N_SUCCESS, S2N_ERR_CANCELLED); return S2N_RESULT_OK; } S2N_RESULT s2n_get_mix_entropy(struct s2n_blob *blob) { RESULT_ENSURE_REF(blob); RESULT_GUARD_POSIX(s2n_rand_mix_cb(blob->data, blob->size)); return S2N_RESULT_OK; } /* Deletes pthread key at process-exit */ static void __attribute__((destructor)) s2n_drbg_rand_state_key_cleanup(void) { if (s2n_is_initialized()) { pthread_key_delete(s2n_per_thread_rand_state_key); } } static void s2n_drbg_destructor(void *_unused_argument) { (void) _unused_argument; s2n_result_ignore(s2n_rand_cleanup_thread()); } static void s2n_drbg_make_rand_state_key(void) { /* We can't return the output of pthread_key_create due to the parameter constraints of pthread_once. * Here we store the result in a global variable that will be error checked later. */ pthread_key_create_result = pthread_key_create(&s2n_per_thread_rand_state_key, s2n_drbg_destructor); } static S2N_RESULT s2n_init_drbgs(void) { uint8_t s2n_public_drbg[] = "s2n public drbg"; uint8_t s2n_private_drbg[] = "s2n private drbg"; struct s2n_blob public = { 0 }; RESULT_GUARD_POSIX(s2n_blob_init(&public, s2n_public_drbg, sizeof(s2n_public_drbg))); struct s2n_blob private = { 0 }; RESULT_GUARD_POSIX(s2n_blob_init(&private, s2n_private_drbg, sizeof(s2n_private_drbg))); RESULT_ENSURE(pthread_once(&s2n_per_thread_rand_state_key_once, s2n_drbg_make_rand_state_key) == 0, S2N_ERR_DRBG); RESULT_ENSURE_EQ(pthread_key_create_result, 0); RESULT_GUARD(s2n_drbg_instantiate(&s2n_per_thread_rand_state.public_drbg, &public, S2N_AES_128_CTR_NO_DF_PR)); RESULT_GUARD(s2n_drbg_instantiate(&s2n_per_thread_rand_state.private_drbg, &private, S2N_AES_256_CTR_NO_DF_PR)); RESULT_ENSURE(pthread_setspecific(s2n_per_thread_rand_state_key, &s2n_per_thread_rand_state) == 0, S2N_ERR_DRBG); s2n_per_thread_rand_state.drbgs_initialized = true; return S2N_RESULT_OK; } static S2N_RESULT s2n_ensure_initialized_drbgs(void) { if (s2n_per_thread_rand_state.drbgs_initialized == false) { RESULT_GUARD(s2n_init_drbgs()); /* Then cache the fork generation number. We just initialized the drbg * states with new entropy and forking is not an external event. */ uint64_t returned_fork_generation_number = 0; RESULT_GUARD(s2n_get_fork_generation_number(&returned_fork_generation_number)); s2n_per_thread_rand_state.cached_fork_generation_number = returned_fork_generation_number; } return S2N_RESULT_OK; } /* s2n_ensure_uniqueness() implements defenses against uniqueness * breaking events that might cause duplicated drbg states. Currently, only * implements fork detection. */ static S2N_RESULT s2n_ensure_uniqueness(void) { uint64_t returned_fork_generation_number = 0; RESULT_GUARD(s2n_get_fork_generation_number(&returned_fork_generation_number)); if (returned_fork_generation_number != s2n_per_thread_rand_state.cached_fork_generation_number) { /* This assumes that s2n_rand_cleanup_thread() doesn't mutate any other * state than the drbg states and it resets the drbg initialization * boolean to false. s2n_ensure_initialized_drbgs() will cache the new * fork generation number in the per thread state. */ RESULT_GUARD(s2n_rand_cleanup_thread()); RESULT_GUARD(s2n_ensure_initialized_drbgs()); } return S2N_RESULT_OK; } static S2N_RESULT s2n_get_libcrypto_random_data(struct s2n_blob *out_blob) { RESULT_GUARD_PTR(out_blob); RESULT_GUARD_OSSL(RAND_bytes(out_blob->data, out_blob->size), S2N_ERR_DRBG); return S2N_RESULT_OK; } static S2N_RESULT s2n_get_custom_random_data(struct s2n_blob *out_blob, struct s2n_drbg *drbg_state) { RESULT_GUARD_PTR(out_blob); RESULT_GUARD_PTR(drbg_state); RESULT_ENSURE(!s2n_is_in_fips_mode(), S2N_ERR_DRBG); RESULT_GUARD(s2n_ensure_initialized_drbgs()); RESULT_GUARD(s2n_ensure_uniqueness()); uint32_t offset = 0; uint32_t remaining = out_blob->size; while (remaining) { struct s2n_blob slice = { 0 }; RESULT_GUARD_POSIX(s2n_blob_slice(out_blob, &slice, offset, MIN(remaining, S2N_DRBG_GENERATE_LIMIT))); RESULT_GUARD(s2n_drbg_generate(drbg_state, &slice)); remaining -= slice.size; offset += slice.size; } return S2N_RESULT_OK; } static S2N_RESULT s2n_get_random_data(struct s2n_blob *blob, struct s2n_drbg *drbg_state) { /* By default, s2n-tls uses a custom random implementation to generate random data for the TLS * handshake. When operating in FIPS mode, the FIPS-validated libcrypto implementation is used * instead. */ if (s2n_is_in_fips_mode()) { RESULT_GUARD(s2n_get_libcrypto_random_data(blob)); return S2N_RESULT_OK; } RESULT_GUARD(s2n_get_custom_random_data(blob, drbg_state)); return S2N_RESULT_OK; } S2N_RESULT s2n_get_public_random_data(struct s2n_blob *blob) { RESULT_GUARD(s2n_get_random_data(blob, &s2n_per_thread_rand_state.public_drbg)); return S2N_RESULT_OK; } S2N_RESULT s2n_get_private_random_data(struct s2n_blob *blob) { RESULT_GUARD(s2n_get_random_data(blob, &s2n_per_thread_rand_state.private_drbg)); return S2N_RESULT_OK; } S2N_RESULT s2n_get_public_random_bytes_used(uint64_t *bytes_used) { RESULT_GUARD(s2n_drbg_bytes_used(&s2n_per_thread_rand_state.public_drbg, bytes_used)); return S2N_RESULT_OK; } S2N_RESULT s2n_get_private_random_bytes_used(uint64_t *bytes_used) { RESULT_GUARD(s2n_drbg_bytes_used(&s2n_per_thread_rand_state.private_drbg, bytes_used)); return S2N_RESULT_OK; } S2N_RESULT s2n_rand_get_urandom_for_test(struct s2n_rand_device **device) { RESULT_ENSURE_REF(device); RESULT_ENSURE(s2n_in_unit_test(), S2N_ERR_NOT_IN_UNIT_TEST); *device = &s2n_dev_urandom; return S2N_RESULT_OK; } static S2N_RESULT s2n_rand_device_open(struct s2n_rand_device *device) { RESULT_ENSURE_REF(device); RESULT_ENSURE_REF(device->source); DEFER_CLEANUP(int fd = -1, s2n_rand_entropy_fd_close_ptr); S2N_IO_RETRY_EINTR(fd, open(device->source, ENTROPY_FLAGS)); RESULT_ENSURE(fd >= 0, S2N_ERR_OPEN_RANDOM); struct stat st = { 0 }; RESULT_ENSURE(fstat(fd, &st) == 0, S2N_ERR_OPEN_RANDOM); device->dev = st.st_dev; device->ino = st.st_ino; device->mode = st.st_mode; device->rdev = st.st_rdev; device->fd = fd; /* Disable closing the file descriptor with defer cleanup */ fd = UNINITIALIZED_ENTROPY_FD; return S2N_RESULT_OK; } S2N_RESULT s2n_rand_device_validate(struct s2n_rand_device *device) { RESULT_ENSURE_REF(device); RESULT_ENSURE_NE(device->fd, UNINITIALIZED_ENTROPY_FD); /* Ensure that the random device is still valid by comparing it to the current file descriptor * status. From: * https://github.com/openssl/openssl/blob/260d97229c467d17934ca3e2e0455b1b5c0994a6/providers/implementations/rands/seeding/rand_unix.c#L513 */ struct stat st = { 0 }; RESULT_ENSURE(fstat(device->fd, &st) == 0, S2N_ERR_OPEN_RANDOM); RESULT_ENSURE_EQ(device->dev, st.st_dev); RESULT_ENSURE_EQ(device->ino, st.st_ino); RESULT_ENSURE_EQ(device->rdev, st.st_rdev); /* Ensure that the mode is the same (equal to 0 when xor'd), but don't check the permission bits. */ mode_t permission_mask = ~(S_IRWXU | S_IRWXG | S_IRWXO); RESULT_ENSURE_EQ((device->mode ^ st.st_mode) & permission_mask, 0); return S2N_RESULT_OK; } static int s2n_rand_get_entropy_from_urandom(void *ptr, uint32_t size) { POSIX_ENSURE_REF(ptr); POSIX_ENSURE(s2n_dev_urandom.fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED); /* It's possible that the file descriptor pointing to /dev/urandom was closed or changed from * when it was last opened. Ensure that the file descriptor is still valid, and if it isn't, * re-open it before getting entropy. * * If the file descriptor is invalid and the process doesn't have access to /dev/urandom (as is * the case within a chroot tree), an error is raised here before attempting to indefinitely * read. */ if (s2n_result_is_error(s2n_rand_device_validate(&s2n_dev_urandom))) { POSIX_GUARD_RESULT(s2n_rand_device_open(&s2n_dev_urandom)); } uint8_t *data = ptr; uint32_t n = size; struct timespec sleep_time = { .tv_sec = 0, .tv_nsec = 0 }; long backoff = 1; while (n) { errno = 0; int r = read(s2n_dev_urandom.fd, data, n); if (r <= 0) { /* * A non-blocking read() on /dev/urandom should "never" fail, * except for EINTR. If it does, briefly pause and use * exponential backoff to avoid creating a tight spinning loop. * * iteration delay * --------- ----------------- * 1 10 nsec * 2 100 nsec * 3 1,000 nsec * 4 10,000 nsec * 5 100,000 nsec * 6 1,000,000 nsec * 7 10,000,000 nsec * 8 99,999,999 nsec * 9 99,999,999 nsec * ... */ if (errno != EINTR) { backoff = MIN(backoff * 10, ONE_S - 1); sleep_time.tv_nsec = backoff; do { r = nanosleep(&sleep_time, &sleep_time); } while (r != 0); } continue; } data += r; n -= r; } return S2N_SUCCESS; } /* * Return a random number in the range [0, bound) */ S2N_RESULT s2n_public_random(int64_t bound, uint64_t *output) { uint64_t r = 0; RESULT_ENSURE_GT(bound, 0); while (1) { struct s2n_blob blob = { 0 }; RESULT_GUARD_POSIX(s2n_blob_init(&blob, (void *) &r, sizeof(r))); RESULT_GUARD(s2n_get_public_random_data(&blob)); /* Imagine an int was one byte and UINT_MAX was 256. If the * caller asked for s2n_random(129, ...) we'd end up in * trouble. Each number in the range 0...127 would be twice * as likely as 128. That's because r == 0 % 129 -> 0, and * r == 129 % 129 -> 0, but only r == 128 returns 128, * r == 257 is out of range. * * To de-bias the dice, we discard values of r that are higher * that the highest multiple of 'bound' an int can support. If * bound is a uint, then in the worst case we discard 50% - 1 r's. * But since 'bound' is an int and INT_MAX is <= UINT_MAX / 2, * in the worst case we discard 25% - 1 r's. */ if (r < (UINT64_MAX - (UINT64_MAX % bound))) { *output = r % bound; return S2N_RESULT_OK; } } } #if S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND #define S2N_RAND_ENGINE_ID "s2n_rand" int s2n_openssl_compat_rand(unsigned char *buf, int num) { struct s2n_blob out = { 0 }; POSIX_GUARD(s2n_blob_init(&out, buf, num)); if (s2n_result_is_error(s2n_get_private_random_data(&out))) { return 0; } return 1; } int s2n_openssl_compat_status(void) { return 1; } int s2n_openssl_compat_init(ENGINE *unused) { return 1; } RAND_METHOD s2n_openssl_rand_method = { .seed = NULL, .bytes = s2n_openssl_compat_rand, .cleanup = NULL, .add = NULL, .pseudorand = s2n_openssl_compat_rand, .status = s2n_openssl_compat_status }; #endif static int s2n_rand_init_cb_impl(void) { /* Currently, s2n-tls may mix in entropy from urandom into every generation of random data. The * file descriptor is opened on initialization for better performance reading from urandom, and * to ensure that urandom is accessible from within a chroot tree. */ POSIX_GUARD_RESULT(s2n_rand_device_open(&s2n_dev_urandom)); if (s2n_cpu_supports_rdrand()) { s2n_rand_mix_cb = s2n_rand_get_entropy_from_rdrand; } return S2N_SUCCESS; } S2N_RESULT s2n_rand_init(void) { RESULT_ENSURE(s2n_rand_init_cb() >= S2N_SUCCESS, S2N_ERR_CANCELLED); RESULT_GUARD(s2n_ensure_initialized_drbgs()); #if S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND if (s2n_is_in_fips_mode()) { return S2N_RESULT_OK; } /* Unset any existing random engine */ RESULT_GUARD_OSSL(RAND_set_rand_engine(NULL), S2N_ERR_OPEN_RANDOM); /* Create an engine */ ENGINE *e = ENGINE_new(); RESULT_ENSURE(e != NULL, S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_id(e, S2N_RAND_ENGINE_ID), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_name(e, "s2n entropy generator"), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_flags(e, ENGINE_FLAGS_NO_REGISTER_ALL), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_init_function(e, s2n_openssl_compat_init), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_RAND(e, &s2n_openssl_rand_method), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_add(e), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_free(e), S2N_ERR_OPEN_RANDOM); /* Use that engine for rand() */ e = ENGINE_by_id(S2N_RAND_ENGINE_ID); RESULT_ENSURE(e != NULL, S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_init(e), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_set_default(e, ENGINE_METHOD_RAND), S2N_ERR_OPEN_RANDOM); RESULT_GUARD_OSSL(ENGINE_free(e), S2N_ERR_OPEN_RANDOM); #endif return S2N_RESULT_OK; } static int s2n_rand_cleanup_cb_impl(void) { POSIX_ENSURE(s2n_dev_urandom.fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED); if (s2n_result_is_ok(s2n_rand_device_validate(&s2n_dev_urandom))) { POSIX_GUARD(close(s2n_dev_urandom.fd)); } s2n_dev_urandom.fd = UNINITIALIZED_ENTROPY_FD; return S2N_SUCCESS; } S2N_RESULT s2n_rand_cleanup(void) { RESULT_ENSURE(s2n_rand_cleanup_cb() >= S2N_SUCCESS, S2N_ERR_CANCELLED); #if S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND /* Cleanup our rand ENGINE in libcrypto */ ENGINE *rand_engine = ENGINE_by_id(S2N_RAND_ENGINE_ID); if (rand_engine) { ENGINE_remove(rand_engine); ENGINE_finish(rand_engine); ENGINE_unregister_RAND(rand_engine); ENGINE_free(rand_engine); ENGINE_cleanup(); RAND_set_rand_engine(NULL); RAND_set_rand_method(NULL); } #endif s2n_rand_init_cb = s2n_rand_init_cb_impl; s2n_rand_cleanup_cb = s2n_rand_cleanup_cb_impl; s2n_rand_seed_cb = s2n_rand_get_entropy_from_urandom; s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom; return S2N_RESULT_OK; } S2N_RESULT s2n_rand_cleanup_thread(void) { /* Currently, it is only safe for this function to mutate the drbg states * in the per thread rand state. See s2n_ensure_uniqueness(). */ RESULT_GUARD(s2n_drbg_wipe(&s2n_per_thread_rand_state.private_drbg)); RESULT_GUARD(s2n_drbg_wipe(&s2n_per_thread_rand_state.public_drbg)); s2n_per_thread_rand_state.drbgs_initialized = false; /* Unset the thread-local destructor */ if (s2n_is_initialized()) { pthread_setspecific(s2n_per_thread_rand_state_key, NULL); } return S2N_RESULT_OK; } /* This must only be used for unit tests. Any real use is dangerous and will be * overwritten in s2n_ensure_uniqueness() if it is forked. This was added to * support known answer tests that use OpenSSL and s2n_get_private_random_data * directly. */ S2N_RESULT s2n_set_private_drbg_for_test(struct s2n_drbg drbg) { RESULT_ENSURE(s2n_in_unit_test(), S2N_ERR_NOT_IN_UNIT_TEST); RESULT_GUARD(s2n_drbg_wipe(&s2n_per_thread_rand_state.private_drbg)); s2n_per_thread_rand_state.private_drbg = drbg; return S2N_RESULT_OK; } S2N_RESULT s2n_rand_set_urandom_for_test() { RESULT_ENSURE(s2n_in_unit_test(), S2N_ERR_NOT_IN_UNIT_TEST); s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom; return S2N_RESULT_OK; } /* * volatile is important to prevent the compiler from * re-ordering or optimizing the use of RDRAND. */ static int s2n_rand_get_entropy_from_rdrand(void *data, uint32_t size) { #if defined(__x86_64__) || defined(__i386__) struct s2n_blob out = { 0 }; POSIX_GUARD(s2n_blob_init(&out, data, size)); size_t space_remaining = 0; struct s2n_stuffer stuffer = { 0 }; union { uint64_t u64; #if defined(__i386__) struct { /* since we check first that we're on intel, we can safely assume little endian. */ uint32_t u_low; uint32_t u_high; } i386_fields; #endif /* defined(__i386__) */ uint8_t u8[8]; } output; POSIX_GUARD(s2n_stuffer_init(&stuffer, &out)); while ((space_remaining = s2n_stuffer_space_remaining(&stuffer))) { unsigned char success = 0; output.u64 = 0; for (int tries = 0; tries < 10; tries++) { #if defined(__i386__) /* execute the rdrand instruction, store the result in a general purpose register (it's assigned to * output.i386_fields.u_low). Check the carry bit, which will be set on success. Then clober the register and reset * the carry bit. Due to needing to support an ancient assembler we use the opcode syntax. * the %b1 is to force compilers to use c1 instead of ecx. * Here's a description of how the opcode is encoded: * 0x0fc7 (rdrand) * 0xf0 (store the result in eax). */ unsigned char success_high = 0, success_low = 0; __asm__ __volatile__( ".byte 0x0f, 0xc7, 0xf0;\n" "setc %b1;\n" : "=&a"(output.i386_fields.u_low), "=qm"(success_low) : : "cc"); __asm__ __volatile__( ".byte 0x0f, 0xc7, 0xf0;\n" "setc %b1;\n" : "=&a"(output.i386_fields.u_high), "=qm"(success_high) : : "cc"); /* cppcheck-suppress knownConditionTrueFalse */ success = success_high & success_low; /* Treat either all 1 or all 0 bits in either the high or low order * bits as failure */ if (output.i386_fields.u_low == 0 || output.i386_fields.u_low == UINT32_MAX || output.i386_fields.u_high == 0 || output.i386_fields.u_high == UINT32_MAX) { success = 0; } #else /* execute the rdrand instruction, store the result in a general purpose register (it's assigned to * output.u64). Check the carry bit, which will be set on success. Then clober the carry bit. * Due to needing to support an ancient assembler we use the opcode syntax. * the %b1 is to force compilers to use c1 instead of ecx. * Here's a description of how the opcode is encoded: * 0x48 (pick a 64-bit register it does more too, but that's all that matters there) * 0x0fc7 (rdrand) * 0xf0 (store the result in rax). */ __asm__ __volatile__( ".byte 0x48, 0x0f, 0xc7, 0xf0;\n" "setc %b1;\n" : "=&a"(output.u64), "=qm"(success) : : "cc"); #endif /* defined(__i386__) */ /* Some AMD CPUs will find that RDRAND "sticks" on all 1s but still reports success. * Some other very old CPUs use all 0s as an error condition while still reporting success. * If we encounter either of these suspicious values (a 1/2^63 chance) we'll treat them as * a failure and generate a new value. * * In the future we could add CPUID checks to detect processors with these known bugs, * however it does not appear worth it. The entropy loss is negligible and the * corresponding likelihood that a healthy CPU generates either of these values is also * negligible (1/2^63). Finally, adding processor specific logic would greatly * increase the complexity and would cause us to "miss" any unknown processors with * similar bugs. */ if (output.u64 == UINT64_MAX || output.u64 == 0) { success = 0; } if (success) { break; } } POSIX_ENSURE(success, S2N_ERR_RDRAND_FAILED); size_t data_to_fill = MIN(sizeof(output), space_remaining); POSIX_GUARD(s2n_stuffer_write_bytes(&stuffer, output.u8, data_to_fill)); } return S2N_SUCCESS; #else POSIX_BAIL(S2N_ERR_UNSUPPORTED_CPU); #endif }