arc4random.c revision 1.46
1 /* $NetBSD: arc4random.c,v 1.46 2025/03/09 18:11:55 riastradh Exp $ */ 2 3 /*- 4 * Copyright (c) 2014 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Taylor R. Campbell. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 * POSSIBILITY OF SUCH DAMAGE. 30 */ 31 32 /* 33 * Legacy arc4random(3) API from OpenBSD reimplemented using the 34 * ChaCha20 PRF, with per-thread state. 35 * 36 * Security model: 37 * - An attacker who sees some outputs cannot predict past or future 38 * outputs. 39 * - An attacker who sees the PRNG state cannot predict past outputs. 40 * - An attacker who sees a child's PRNG state cannot predict past or 41 * future outputs in the parent, or in other children. 42 * 43 * The arc4random(3) API may abort the process if: 44 * 45 * (a) the crypto self-test fails, or 46 * (b) sysctl(KERN_ARND) fails when reseeding the PRNG. 47 * 48 * The crypto self-test occurs only once, on the first use of any of 49 * the arc4random(3) API. KERN_ARND is unlikely to fail later unless 50 * the kernel is seriously broken. 51 */ 52 53 #include <sys/cdefs.h> 54 __RCSID("$NetBSD: arc4random.c,v 1.46 2025/03/09 18:11:55 riastradh Exp $"); 55 56 #include "namespace.h" 57 #include "reentrant.h" 58 59 #include <sys/bitops.h> 60 #include <sys/endian.h> 61 #include <sys/errno.h> 62 #include <sys/mman.h> 63 #include <sys/sysctl.h> 64 65 #include <assert.h> 66 #include <sha2.h> 67 #include <stdatomic.h> 68 #include <stdbool.h> 69 #include <stdint.h> 70 #include <stdlib.h> 71 #include <string.h> 72 #include <unistd.h> 73 74 #include "arc4random.h" 75 #include "reentrant.h" 76 77 #ifdef __weak_alias 78 __weak_alias(arc4random,_arc4random) 79 __weak_alias(arc4random_addrandom,_arc4random_addrandom) 80 __weak_alias(arc4random_buf,_arc4random_buf) 81 __weak_alias(arc4random_stir,_arc4random_stir) 82 __weak_alias(arc4random_uniform,_arc4random_uniform) 83 #endif 84 85 /* 86 * For standard ChaCha, use le32dec/le32enc. We don't need that for 87 * the purposes of a nondeterministic random number generator -- we 88 * don't need to be bit-for-bit compatible over any wire. 89 */ 90 91 static inline uint32_t 92 crypto_le32dec(const void *p) 93 { 94 uint32_t v; 95 96 (void)memcpy(&v, p, sizeof v); 97 98 return v; 99 } 100 101 static inline void 102 crypto_le32enc(void *p, uint32_t v) 103 { 104 105 (void)memcpy(p, &v, sizeof v); 106 } 107 108 /* ChaCha core */ 109 110 #define crypto_core_OUTPUTBYTES 64 111 #define crypto_core_INPUTBYTES 16 112 #define crypto_core_KEYBYTES 32 113 #define crypto_core_CONSTBYTES 16 114 115 #define crypto_core_ROUNDS 20 116 117 static uint32_t 118 rotate(uint32_t u, unsigned c) 119 { 120 121 return (u << c) | (u >> (32 - c)); 122 } 123 124 #define QUARTERROUND(a, b, c, d) do { \ 125 (a) += (b); (d) ^= (a); (d) = rotate((d), 16); \ 126 (c) += (d); (b) ^= (c); (b) = rotate((b), 12); \ 127 (a) += (b); (d) ^= (a); (d) = rotate((d), 8); \ 128 (c) += (d); (b) ^= (c); (b) = rotate((b), 7); \ 129 } while (0) 130 131 static const uint8_t crypto_core_constant32[16] = "expand 32-byte k"; 132 133 static void 134 crypto_core(uint8_t *out, const uint8_t *in, const uint8_t *k, 135 const uint8_t *c) 136 { 137 uint32_t x0,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x11,x12,x13,x14,x15; 138 uint32_t j0,j1,j2,j3,j4,j5,j6,j7,j8,j9,j10,j11,j12,j13,j14,j15; 139 int i; 140 141 j0 = x0 = crypto_le32dec(c + 0); 142 j1 = x1 = crypto_le32dec(c + 4); 143 j2 = x2 = crypto_le32dec(c + 8); 144 j3 = x3 = crypto_le32dec(c + 12); 145 j4 = x4 = crypto_le32dec(k + 0); 146 j5 = x5 = crypto_le32dec(k + 4); 147 j6 = x6 = crypto_le32dec(k + 8); 148 j7 = x7 = crypto_le32dec(k + 12); 149 j8 = x8 = crypto_le32dec(k + 16); 150 j9 = x9 = crypto_le32dec(k + 20); 151 j10 = x10 = crypto_le32dec(k + 24); 152 j11 = x11 = crypto_le32dec(k + 28); 153 j12 = x12 = crypto_le32dec(in + 0); 154 j13 = x13 = crypto_le32dec(in + 4); 155 j14 = x14 = crypto_le32dec(in + 8); 156 j15 = x15 = crypto_le32dec(in + 12); 157 158 for (i = crypto_core_ROUNDS; i > 0; i -= 2) { 159 QUARTERROUND( x0, x4, x8,x12); 160 QUARTERROUND( x1, x5, x9,x13); 161 QUARTERROUND( x2, x6,x10,x14); 162 QUARTERROUND( x3, x7,x11,x15); 163 QUARTERROUND( x0, x5,x10,x15); 164 QUARTERROUND( x1, x6,x11,x12); 165 QUARTERROUND( x2, x7, x8,x13); 166 QUARTERROUND( x3, x4, x9,x14); 167 } 168 169 crypto_le32enc(out + 0, x0 + j0); 170 crypto_le32enc(out + 4, x1 + j1); 171 crypto_le32enc(out + 8, x2 + j2); 172 crypto_le32enc(out + 12, x3 + j3); 173 crypto_le32enc(out + 16, x4 + j4); 174 crypto_le32enc(out + 20, x5 + j5); 175 crypto_le32enc(out + 24, x6 + j6); 176 crypto_le32enc(out + 28, x7 + j7); 177 crypto_le32enc(out + 32, x8 + j8); 178 crypto_le32enc(out + 36, x9 + j9); 179 crypto_le32enc(out + 40, x10 + j10); 180 crypto_le32enc(out + 44, x11 + j11); 181 crypto_le32enc(out + 48, x12 + j12); 182 crypto_le32enc(out + 52, x13 + j13); 183 crypto_le32enc(out + 56, x14 + j14); 184 crypto_le32enc(out + 60, x15 + j15); 185 } 186 187 /* ChaCha self-test */ 188 189 /* 190 * Test vector for ChaCha20 from 191 * <http://tools.ietf.org/html/draft-strombergson-chacha-test-vectors-00>, 192 * test vectors for ChaCha12 and ChaCha8 and for big-endian machines 193 * generated by the same crypto_core code with crypto_core_ROUNDS and 194 * crypto_le32enc/dec varied. 195 */ 196 197 static const uint8_t crypto_core_selftest_vector[64] = { 198 #if _BYTE_ORDER == _LITTLE_ENDIAN 199 # if crypto_core_ROUNDS == 8 200 0x3e,0x00,0xef,0x2f,0x89,0x5f,0x40,0xd6, 201 0x7f,0x5b,0xb8,0xe8,0x1f,0x09,0xa5,0xa1, 202 0x2c,0x84,0x0e,0xc3,0xce,0x9a,0x7f,0x3b, 203 0x18,0x1b,0xe1,0x88,0xef,0x71,0x1a,0x1e, 204 0x98,0x4c,0xe1,0x72,0xb9,0x21,0x6f,0x41, 205 0x9f,0x44,0x53,0x67,0x45,0x6d,0x56,0x19, 206 0x31,0x4a,0x42,0xa3,0xda,0x86,0xb0,0x01, 207 0x38,0x7b,0xfd,0xb8,0x0e,0x0c,0xfe,0x42, 208 # elif crypto_core_ROUNDS == 12 209 0x9b,0xf4,0x9a,0x6a,0x07,0x55,0xf9,0x53, 210 0x81,0x1f,0xce,0x12,0x5f,0x26,0x83,0xd5, 211 0x04,0x29,0xc3,0xbb,0x49,0xe0,0x74,0x14, 212 0x7e,0x00,0x89,0xa5,0x2e,0xae,0x15,0x5f, 213 0x05,0x64,0xf8,0x79,0xd2,0x7a,0xe3,0xc0, 214 0x2c,0xe8,0x28,0x34,0xac,0xfa,0x8c,0x79, 215 0x3a,0x62,0x9f,0x2c,0xa0,0xde,0x69,0x19, 216 0x61,0x0b,0xe8,0x2f,0x41,0x13,0x26,0xbe, 217 # elif crypto_core_ROUNDS == 20 218 0x76,0xb8,0xe0,0xad,0xa0,0xf1,0x3d,0x90, 219 0x40,0x5d,0x6a,0xe5,0x53,0x86,0xbd,0x28, 220 0xbd,0xd2,0x19,0xb8,0xa0,0x8d,0xed,0x1a, 221 0xa8,0x36,0xef,0xcc,0x8b,0x77,0x0d,0xc7, 222 0xda,0x41,0x59,0x7c,0x51,0x57,0x48,0x8d, 223 0x77,0x24,0xe0,0x3f,0xb8,0xd8,0x4a,0x37, 224 0x6a,0x43,0xb8,0xf4,0x15,0x18,0xa1,0x1c, 225 0xc3,0x87,0xb6,0x69,0xb2,0xee,0x65,0x86, 226 # else 227 # error crypto_core_ROUNDS must be 8, 12, or 20. 228 # endif 229 #elif _BYTE_ORDER == _BIG_ENDIAN 230 # if crypto_core_ROUNDS == 8 231 0x9a,0x13,0x07,0xe3,0x38,0x18,0x9e,0x99, 232 0x15,0x37,0x16,0x4d,0x04,0xe6,0x48,0x9a, 233 0x07,0xd6,0xe8,0x7a,0x02,0xf9,0xf5,0xc7, 234 0x3f,0xa9,0xc2,0x0a,0xe1,0xc6,0x62,0xea, 235 0x80,0xaf,0xb6,0x51,0xca,0x52,0x43,0x87, 236 0xe3,0xa6,0xa6,0x61,0x11,0xf5,0xe6,0xcf, 237 0x09,0x0f,0xdc,0x9d,0xc3,0xc3,0xbb,0x43, 238 0xd7,0xfa,0x70,0x42,0xbf,0xa5,0xee,0xa2, 239 # elif crypto_core_ROUNDS == 12 240 0xcf,0x6c,0x16,0x48,0xbf,0xf4,0xba,0x85, 241 0x32,0x69,0xd3,0x98,0xc8,0x7d,0xcd,0x3f, 242 0xdc,0x76,0x6b,0xa2,0x7b,0xcb,0x17,0x4d, 243 0x05,0xda,0xdd,0xd8,0x62,0x54,0xbf,0xe0, 244 0x65,0xed,0x0e,0xf4,0x01,0x7e,0x3c,0x05, 245 0x35,0xb2,0x7a,0x60,0xf3,0x8f,0x12,0x33, 246 0x24,0x60,0xcd,0x85,0xfe,0x4c,0xf3,0x39, 247 0xb1,0x0e,0x3e,0xe0,0xba,0xa6,0x2f,0xa9, 248 # elif crypto_core_ROUNDS == 20 249 0x83,0x8b,0xf8,0x75,0xf7,0xde,0x9d,0x8c, 250 0x33,0x14,0x72,0x28,0xd1,0xbe,0x88,0xe5, 251 0x94,0xb5,0xed,0xb8,0x56,0xb5,0x9e,0x0c, 252 0x64,0x6a,0xaf,0xd9,0xa7,0x49,0x10,0x59, 253 0xba,0x3a,0x82,0xf8,0x4a,0x70,0x9c,0x00, 254 0x82,0x2c,0xae,0xc6,0xd7,0x1c,0x2e,0xda, 255 0x2a,0xfb,0x61,0x70,0x2b,0xd1,0xbf,0x8b, 256 0x95,0xbc,0x23,0xb6,0x4b,0x60,0x02,0xec, 257 # else 258 # error crypto_core_ROUNDS must be 8, 12, or 20. 259 # endif 260 #else 261 # error Byte order must be little-endian or big-endian. 262 #endif 263 }; 264 265 static int 266 crypto_core_selftest(void) 267 { 268 const uint8_t nonce[crypto_core_INPUTBYTES] = {0}; 269 const uint8_t key[crypto_core_KEYBYTES] = {0}; 270 uint8_t block[64]; 271 unsigned i; 272 273 crypto_core(block, nonce, key, crypto_core_constant32); 274 for (i = 0; i < 64; i++) { 275 if (block[i] != crypto_core_selftest_vector[i]) 276 return EIO; 277 } 278 279 return 0; 280 } 281 282 /* PRNG */ 283 284 /* 285 * For a state s, rather than use ChaCha20 as a stream cipher to 286 * generate the concatenation ChaCha20_s(0) || ChaCha20_s(1) || ..., we 287 * split ChaCha20_s(0) into s' || x and yield x for the first request, 288 * split ChaCha20_s'(0) into s'' || y and yield y for the second 289 * request, &c. This provides backtracking resistance: an attacker who 290 * finds s'' can't recover s' or x. 291 */ 292 293 #define crypto_prng_SEEDBYTES crypto_core_KEYBYTES 294 #define crypto_prng_MAXOUTPUTBYTES \ 295 (crypto_core_OUTPUTBYTES - crypto_prng_SEEDBYTES) 296 297 __CTASSERT(sizeof(struct crypto_prng) == crypto_prng_SEEDBYTES); 298 299 static void 300 crypto_prng_seed(struct crypto_prng *prng, const void *seed) 301 { 302 303 (void)memcpy(prng->state, seed, crypto_prng_SEEDBYTES); 304 } 305 306 static void 307 crypto_prng_buf(struct crypto_prng *prng, void *buf, size_t n) 308 { 309 const uint8_t nonce[crypto_core_INPUTBYTES] = {0}; 310 uint8_t output[crypto_core_OUTPUTBYTES]; 311 312 _DIAGASSERT(n <= crypto_prng_MAXOUTPUTBYTES); 313 __CTASSERT(sizeof prng->state + crypto_prng_MAXOUTPUTBYTES 314 <= sizeof output); 315 316 crypto_core(output, nonce, prng->state, crypto_core_constant32); 317 (void)memcpy(prng->state, output, sizeof prng->state); 318 (void)memcpy(buf, output + sizeof prng->state, n); 319 (void)explicit_memset(output, 0, sizeof output); 320 } 321 322 /* One-time stream: expand short single-use secret into long secret */ 323 324 #define crypto_onetimestream_SEEDBYTES crypto_core_KEYBYTES 325 326 static void 327 crypto_onetimestream(const void *seed, void *buf, size_t n) 328 { 329 uint32_t nonce[crypto_core_INPUTBYTES / sizeof(uint32_t)] = {0}; 330 uint8_t block[crypto_core_OUTPUTBYTES]; 331 uint8_t *p8, *p32; 332 const uint8_t *nonce8 = (const uint8_t *)(void *)nonce; 333 size_t ni, nb, nf; 334 335 /* 336 * Guarantee we can generate up to n bytes. We have 337 * 2^(8*INPUTBYTES) possible inputs yielding output of 338 * OUTPUTBYTES*2^(8*INPUTBYTES) bytes. It suffices to require 339 * that sizeof n > (1/CHAR_BIT) log_2 n be less than 340 * (1/CHAR_BIT) log_2 of the total output stream length. We 341 * have 342 * 343 * log_2 (o 2^(8 i)) = log_2 o + log_2 2^(8 i) 344 * = log_2 o + 8 i. 345 */ 346 #ifndef __lint__ 347 __CTASSERT(CHAR_BIT * sizeof n <= (ilog2(crypto_core_OUTPUTBYTES) + 348 8 * crypto_core_INPUTBYTES)); 349 #endif 350 351 p8 = buf; 352 p32 = (uint8_t *)roundup2((uintptr_t)p8, 4); 353 ni = p32 - p8; 354 if (n < ni) 355 ni = n; 356 nb = (n - ni) / sizeof block; 357 nf = (n - ni) % sizeof block; 358 359 _DIAGASSERT(((uintptr_t)p32 & 3) == 0); 360 _DIAGASSERT(ni <= n); 361 _DIAGASSERT(nb <= (n / sizeof block)); 362 _DIAGASSERT(nf <= n); 363 _DIAGASSERT(n == (ni + (nb * sizeof block) + nf)); 364 _DIAGASSERT(ni < 4); 365 _DIAGASSERT(nf < sizeof block); 366 367 if (ni) { 368 crypto_core(block, nonce8, seed, crypto_core_constant32); 369 nonce[0]++; 370 (void)memcpy(p8, block, ni); 371 } 372 while (nb--) { 373 crypto_core(p32, nonce8, seed, crypto_core_constant32); 374 if (++nonce[0] == 0) 375 nonce[1]++; 376 p32 += crypto_core_OUTPUTBYTES; 377 } 378 if (nf) { 379 crypto_core(block, nonce8, seed, crypto_core_constant32); 380 if (++nonce[0] == 0) 381 nonce[1]++; 382 (void)memcpy(p32, block, nf); 383 } 384 385 if (ni | nf) 386 (void)explicit_memset(block, 0, sizeof block); 387 } 388 389 /* 390 * entropy_epoch() 391 * 392 * Return the current entropy epoch, from the sysctl node 393 * kern.entropy.epoch. 394 * 395 * The entropy epoch is never zero. Initially, or on error, it is 396 * (unsigned)-1. It may wrap around but it skips (unsigned)-1 and 397 * 0 when it does. Changes happen less than once per second, so 398 * wraparound will only affect systems after 136 years of uptime. 399 * 400 * XXX This should get it from a page shared read-only by kernel 401 * with userland, but until we implement such a mechanism, this 402 * sysctl -- incurring the cost of a syscall -- will have to 403 * serve. 404 */ 405 static unsigned 406 entropy_epoch(void) 407 { 408 static atomic_int mib0[3]; 409 static atomic_bool initialized = false; 410 int mib[3]; 411 unsigned epoch = (unsigned)-1; 412 size_t epochlen = sizeof(epoch); 413 414 /* 415 * Resolve kern.entropy.epoch if we haven't already. Cache it 416 * for the next caller. Initialization is idempotent, so it's 417 * OK if two threads do it at once. 418 */ 419 if (atomic_load_explicit(&initialized, memory_order_acquire)) { 420 mib[0] = atomic_load_explicit(&mib0[0], memory_order_relaxed); 421 mib[1] = atomic_load_explicit(&mib0[1], memory_order_relaxed); 422 mib[2] = atomic_load_explicit(&mib0[2], memory_order_relaxed); 423 } else { 424 size_t nmib = __arraycount(mib); 425 426 if (sysctlnametomib("kern.entropy.epoch", mib, &nmib) == -1) 427 return (unsigned)-1; 428 if (nmib != __arraycount(mib)) 429 return (unsigned)-1; 430 atomic_store_explicit(&mib0[0], mib[0], memory_order_relaxed); 431 atomic_store_explicit(&mib0[1], mib[1], memory_order_relaxed); 432 atomic_store_explicit(&mib0[2], mib[2], memory_order_relaxed); 433 atomic_store_explicit(&initialized, true, 434 memory_order_release); 435 } 436 437 if (sysctl(mib, __arraycount(mib), &epoch, &epochlen, NULL, 0) == -1) 438 return (unsigned)-1; 439 if (epochlen != sizeof(epoch)) 440 return (unsigned)-1; 441 442 return epoch; 443 } 444 445 /* arc4random state: per-thread, per-process (zeroed in child on fork) */ 446 447 static void 448 arc4random_prng_addrandom(struct arc4random_prng *prng, const void *data, 449 size_t datalen) 450 { 451 const int mib[] = { CTL_KERN, KERN_ARND }; 452 SHA256_CTX ctx; 453 uint8_t buf[crypto_prng_SEEDBYTES]; 454 size_t buflen = sizeof buf; 455 unsigned epoch = entropy_epoch(); 456 457 __CTASSERT(sizeof buf == SHA256_DIGEST_LENGTH); 458 459 SHA256_Init(&ctx); 460 461 crypto_prng_buf(&prng->arc4_prng, buf, sizeof buf); 462 SHA256_Update(&ctx, buf, sizeof buf); 463 464 if (sysctl(mib, (u_int)__arraycount(mib), buf, &buflen, NULL, 0) == -1) 465 abort(); 466 if (buflen != sizeof buf) 467 abort(); 468 SHA256_Update(&ctx, buf, sizeof buf); 469 470 if (data != NULL) 471 SHA256_Update(&ctx, data, datalen); 472 473 SHA256_Final(buf, &ctx); 474 (void)explicit_memset(&ctx, 0, sizeof ctx); 475 476 /* reseed(SHA256(prng() || sysctl(KERN_ARND) || data)) */ 477 crypto_prng_seed(&prng->arc4_prng, buf); 478 (void)explicit_memset(buf, 0, sizeof buf); 479 prng->arc4_epoch = epoch; 480 } 481 482 #ifdef _REENTRANT 483 static struct arc4random_prng * 484 arc4random_prng_create(void) 485 { 486 struct arc4random_prng *prng; 487 const size_t size = roundup(sizeof(*prng), sysconf(_SC_PAGESIZE)); 488 489 prng = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 490 0); 491 if (prng == MAP_FAILED) 492 goto fail0; 493 if (minherit(prng, size, MAP_INHERIT_ZERO) == -1) 494 goto fail1; 495 496 return prng; 497 498 fail1: (void)munmap(prng, size); 499 fail0: return NULL; 500 } 501 #endif 502 503 #ifdef _REENTRANT 504 static void 505 arc4random_prng_destroy(struct arc4random_prng *prng) 506 { 507 const size_t size = roundup(sizeof(*prng), sysconf(_SC_PAGESIZE)); 508 509 (void)explicit_memset(prng, 0, sizeof(*prng)); 510 (void)munmap(prng, size); 511 } 512 #endif 513 514 /* Library state */ 515 516 struct arc4random_global_state arc4random_global = { 517 #ifdef _REENTRANT 518 .lock = MUTEX_INITIALIZER, 519 #endif 520 .once = ONCE_INITIALIZER, 521 }; 522 523 static void 524 arc4random_atfork_prepare(void) 525 { 526 527 mutex_lock(&arc4random_global.lock); 528 (void)explicit_memset(&arc4random_global.prng, 0, 529 sizeof arc4random_global.prng); 530 } 531 532 static void 533 arc4random_atfork_parent(void) 534 { 535 536 mutex_unlock(&arc4random_global.lock); 537 } 538 539 static void 540 arc4random_atfork_child(void) 541 { 542 543 mutex_unlock(&arc4random_global.lock); 544 } 545 546 #ifdef _REENTRANT 547 static void 548 arc4random_tsd_destructor(void *p) 549 { 550 struct arc4random_prng *const prng = p; 551 552 arc4random_prng_destroy(prng); 553 } 554 #endif 555 556 static void 557 arc4random_initialize(void) 558 { 559 560 /* 561 * If the crypto software is broken, abort -- something is 562 * severely wrong with this process image. 563 */ 564 if (crypto_core_selftest() != 0) 565 abort(); 566 567 /* 568 * Set up a pthread_atfork handler to lock the global state 569 * around fork so that if forked children can't use the 570 * per-thread state, they can take the lock and use the global 571 * state without deadlock. If this fails, we will fall back to 572 * PRNG state on the stack reinitialized from the kernel 573 * entropy pool at every call. 574 */ 575 if (pthread_atfork(&arc4random_atfork_prepare, 576 &arc4random_atfork_parent, &arc4random_atfork_child) 577 == 0) 578 arc4random_global.forksafe = true; 579 580 /* 581 * For multithreaded builds, try to allocate a per-thread PRNG 582 * state to avoid contention due to arc4random. 583 */ 584 #ifdef _REENTRANT 585 if (thr_keycreate(&arc4random_global.thread_key, 586 &arc4random_tsd_destructor) == 0) 587 arc4random_global.per_thread = true; 588 #endif 589 590 /* 591 * Note that the arc4random library state has been initialized 592 * for the sake of automatic tests. 593 */ 594 arc4random_global.initialized = true; 595 } 596 597 static struct arc4random_prng * 598 arc4random_prng_get(struct arc4random_prng *fallback) 599 { 600 struct arc4random_prng *prng = NULL; 601 602 /* Make sure the library is initialized. */ 603 thr_once(&arc4random_global.once, &arc4random_initialize); 604 605 #ifdef _REENTRANT 606 /* Get or create the per-thread PRNG state. */ 607 prng = __predict_true(arc4random_global.per_thread) 608 ? thr_getspecific(arc4random_global.thread_key) 609 : NULL; 610 if (__predict_false(prng == NULL) && arc4random_global.per_thread) { 611 prng = arc4random_prng_create(); 612 thr_setspecific(arc4random_global.thread_key, prng); 613 } 614 #endif 615 616 /* 617 * If we can't create it, fall back to the global PRNG -- or an 618 * on-stack PRNG, in the unlikely event that pthread_atfork 619 * failed, which we have to seed from scratch each time 620 * (suboptimal, but unlikely, so not worth optimizing). 621 */ 622 if (__predict_false(prng == NULL)) { 623 if (__predict_true(arc4random_global.forksafe)) { 624 mutex_lock(&arc4random_global.lock); 625 prng = &arc4random_global.prng; 626 } else { 627 prng = fallback; 628 memset(prng, 0, sizeof(*prng)); 629 } 630 } 631 632 /* Guarantee the PRNG is seeded. */ 633 if (__predict_false(prng->arc4_epoch != entropy_epoch())) 634 arc4random_prng_addrandom(prng, NULL, 0); 635 636 return prng; 637 } 638 639 static void 640 arc4random_prng_put(struct arc4random_prng *prng, 641 struct arc4random_prng *fallback) 642 { 643 644 /* 645 * If we had to use a stack fallback, zero it before we return 646 * so that after we return we avoid leaving secrets on the 647 * stack that could recover the parent's future outputs in an 648 * unprivileged forked child (of course, we can't guarantee 649 * that the compiler hasn't spilled anything; this is 650 * best-effort, not a guarantee). 651 */ 652 if (__predict_false(prng == fallback)) 653 explicit_memset(fallback, 0, sizeof(*fallback)); 654 655 /* If we had fallen back to the global PRNG, unlock it. */ 656 if (__predict_false(prng == &arc4random_global.prng)) 657 mutex_unlock(&arc4random_global.lock); 658 } 659 660 /* Public API */ 661 662 uint32_t 663 arc4random(void) 664 { 665 struct arc4random_prng *prng, fallback; 666 uint32_t v; 667 668 prng = arc4random_prng_get(&fallback); 669 crypto_prng_buf(&prng->arc4_prng, &v, sizeof v); 670 arc4random_prng_put(prng, &fallback); 671 672 return v; 673 } 674 675 void 676 arc4random_buf(void *buf, size_t len) 677 { 678 struct arc4random_prng *prng, fallback; 679 680 if (len <= crypto_prng_MAXOUTPUTBYTES) { 681 prng = arc4random_prng_get(&fallback); 682 crypto_prng_buf(&prng->arc4_prng, buf, len); 683 arc4random_prng_put(prng, &fallback); 684 } else { 685 uint8_t seed[crypto_onetimestream_SEEDBYTES]; 686 687 prng = arc4random_prng_get(&fallback); 688 crypto_prng_buf(&prng->arc4_prng, seed, sizeof seed); 689 arc4random_prng_put(prng, &fallback); 690 691 crypto_onetimestream(seed, buf, len); 692 (void)explicit_memset(seed, 0, sizeof seed); 693 } 694 } 695 696 uint32_t 697 arc4random_uniform(uint32_t bound) 698 { 699 struct arc4random_prng *prng, fallback; 700 uint32_t minimum, r; 701 702 /* 703 * We want a uniform random choice in [0, n), and arc4random() 704 * makes a uniform random choice in [0, 2^32). If we reduce 705 * that modulo n, values in [0, 2^32 mod n) will be represented 706 * slightly more than values in [2^32 mod n, n). Instead we 707 * choose only from [2^32 mod n, 2^32) by rejecting samples in 708 * [0, 2^32 mod n), to avoid counting the extra representative 709 * of [0, 2^32 mod n). To compute 2^32 mod n, note that 710 * 711 * 2^32 mod n = 2^32 mod n - 0 712 * = 2^32 mod n - n mod n 713 * = (2^32 - n) mod n, 714 * 715 * the last of which is what we compute in 32-bit arithmetic. 716 */ 717 minimum = (-bound % bound); 718 719 prng = arc4random_prng_get(&fallback); 720 do crypto_prng_buf(&prng->arc4_prng, &r, sizeof r); 721 while (__predict_false(r < minimum)); 722 arc4random_prng_put(prng, &fallback); 723 724 return (r % bound); 725 } 726 727 void 728 arc4random_stir(void) 729 { 730 struct arc4random_prng *prng, fallback; 731 732 prng = arc4random_prng_get(&fallback); 733 arc4random_prng_addrandom(prng, NULL, 0); 734 arc4random_prng_put(prng, &fallback); 735 } 736 737 /* 738 * Silly signature here is for hysterical raisins. Should instead be 739 * const void *data and size_t datalen. 740 */ 741 void 742 arc4random_addrandom(u_char *data, int datalen) 743 { 744 struct arc4random_prng *prng, fallback; 745 746 _DIAGASSERT(0 <= datalen); 747 748 prng = arc4random_prng_get(&fallback); 749 arc4random_prng_addrandom(prng, data, datalen); 750 arc4random_prng_put(prng, &fallback); 751 } 752 753 #ifdef _ARC4RANDOM_TEST 754 755 #include <sys/wait.h> 756 757 #include <err.h> 758 #include <stdio.h> 759 760 int 761 main(int argc __unused, char **argv __unused) 762 { 763 unsigned char gubbish[] = "random gubbish"; 764 const uint8_t zero64[64] = {0}; 765 uint8_t buf[2048]; 766 unsigned i, a, n; 767 768 /* Test arc4random: should not be deterministic. */ 769 if (printf("arc4random: %08"PRIx32"\n", arc4random()) < 0) 770 err(1, "printf"); 771 772 /* Test stirring: should definitely not be deterministic. */ 773 arc4random_stir(); 774 775 /* Test small buffer. */ 776 arc4random_buf(buf, 8); 777 if (printf("arc4randombuf small:") < 0) 778 err(1, "printf"); 779 for (i = 0; i < 8; i++) 780 if (printf(" %02x", buf[i]) < 0) 781 err(1, "printf"); 782 if (printf("\n") < 0) 783 err(1, "printf"); 784 785 /* Test addrandom: should not make the rest deterministic. */ 786 arc4random_addrandom(gubbish, sizeof gubbish); 787 788 /* Test large buffer. */ 789 arc4random_buf(buf, sizeof buf); 790 if (printf("arc4randombuf_large:") < 0) 791 err(1, "printf"); 792 for (i = 0; i < sizeof buf; i++) 793 if (printf(" %02x", buf[i]) < 0) 794 err(1, "printf"); 795 if (printf("\n") < 0) 796 err(1, "printf"); 797 798 /* Test misaligned small and large. */ 799 for (a = 0; a < 64; a++) { 800 for (n = a; n < sizeof buf; n++) { 801 (void)memset(buf, 0, sizeof buf); 802 arc4random_buf(buf, n - a); 803 if (memcmp(buf + n - a, zero64, a) != 0) 804 errx(1, "arc4random buffer overflow 0"); 805 806 (void)memset(buf, 0, sizeof buf); 807 arc4random_buf(buf + a, n - a); 808 if (memcmp(buf, zero64, a) != 0) 809 errx(1, "arc4random buffer overflow 1"); 810 811 if ((2*a) <= n) { 812 (void)memset(buf, 0, sizeof buf); 813 arc4random_buf(buf + a, n - a - a); 814 if (memcmp(buf + n - a, zero64, a) != 0) 815 errx(1, 816 "arc4random buffer overflow 2"); 817 } 818 } 819 } 820 821 /* Test fork-safety. */ 822 { 823 pid_t pid, rpid; 824 int status; 825 826 pid = fork(); 827 switch (pid) { 828 case -1: 829 err(1, "fork"); 830 case 0: { 831 /* 832 * Verify the epoch has been set to zero by fork. 833 */ 834 struct arc4random_prng *prng = NULL; 835 #ifdef _REENTRANT 836 prng = arc4random_global.per_thread 837 ? thr_getspecific(arc4random_global.thread_key) 838 : NULL; 839 #endif 840 if (prng == NULL) 841 prng = &arc4random_global.prng; 842 _exit(prng->arc4_epoch != 0); 843 } 844 default: 845 rpid = waitpid(pid, &status, 0); 846 if (rpid == -1) 847 err(1, "waitpid"); 848 if (rpid != pid) 849 errx(1, "waitpid returned wrong pid" 850 ": %"PRIdMAX" != %"PRIdMAX, 851 (intmax_t)rpid, 852 (intmax_t)pid); 853 if (WIFEXITED(status)) { 854 if (WEXITSTATUS(status) != 0) 855 errx(1, "child exited with %d", 856 WEXITSTATUS(status)); 857 } else if (WIFSIGNALED(status)) { 858 errx(1, "child terminated on signal %d", 859 WTERMSIG(status)); 860 } else { 861 errx(1, "child died mysteriously: %d", status); 862 } 863 } 864 } 865 866 /* XXX Test multithreaded fork safety...? */ 867 868 return 0; 869 } 870 #endif 871
Indexes created Mon Sep 29 21:09:56 GMT 2025