arc4random.c revision 1.50
1 /* $NetBSD: arc4random.c,v 1.50 2025/03/11 14:30:27 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 1.50 2025/03/11 14:30:27 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 <stdbool.h> 68 #include <stdint.h> 69 #include <stdlib.h> 70 #include <string.h> 71 #include <unistd.h> 72 73 #include "arc4random.h" 74 #include "reentrant.h" 75 76 #ifdef __weak_alias 77 __weak_alias(arc4random,_arc4random) 78 __weak_alias(arc4random_addrandom,_arc4random_addrandom) 79 __weak_alias(arc4random_buf,_arc4random_buf) 80 __weak_alias(arc4random_stir,_arc4random_stir) 81 __weak_alias(arc4random_uniform,_arc4random_uniform) 82 #endif 83 84 /* 85 * For standard ChaCha, use le32dec/le32enc. We don't need that for 86 * the purposes of a nondeterministic random number generator -- we 87 * don't need to be bit-for-bit compatible over any wire. 88 */ 89 90 static inline uint32_t 91 crypto_le32dec(const void *p) 92 { 93 uint32_t v; 94 95 (void)memcpy(&v, p, sizeof v); 96 97 return v; 98 } 99 100 static inline void 101 crypto_le32enc(void *p, uint32_t v) 102 { 103 104 (void)memcpy(p, &v, sizeof v); 105 } 106 107 /* ChaCha core */ 108 109 #define crypto_core_OUTPUTBYTES 64 110 #define crypto_core_INPUTBYTES 16 111 #define crypto_core_KEYBYTES 32 112 #define crypto_core_CONSTBYTES 16 113 114 #define crypto_core_ROUNDS 20 115 116 static uint32_t 117 rotate(uint32_t u, unsigned c) 118 { 119 120 return (u << c) | (u >> (32 - c)); 121 } 122 123 #define QUARTERROUND(a, b, c, d) do { \ 124 (a) += (b); (d) ^= (a); (d) = rotate((d), 16); \ 125 (c) += (d); (b) ^= (c); (b) = rotate((b), 12); \ 126 (a) += (b); (d) ^= (a); (d) = rotate((d), 8); \ 127 (c) += (d); (b) ^= (c); (b) = rotate((b), 7); \ 128 } while (0) 129 130 static const uint8_t crypto_core_constant32[16] = "expand 32-byte k"; 131 132 static void 133 crypto_core(uint8_t *out, const uint8_t *in, const uint8_t *k, 134 const uint8_t *c) 135 { 136 uint32_t x0,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x11,x12,x13,x14,x15; 137 uint32_t j0,j1,j2,j3,j4,j5,j6,j7,j8,j9,j10,j11,j12,j13,j14,j15; 138 int i; 139 140 j0 = x0 = crypto_le32dec(c + 0); 141 j1 = x1 = crypto_le32dec(c + 4); 142 j2 = x2 = crypto_le32dec(c + 8); 143 j3 = x3 = crypto_le32dec(c + 12); 144 j4 = x4 = crypto_le32dec(k + 0); 145 j5 = x5 = crypto_le32dec(k + 4); 146 j6 = x6 = crypto_le32dec(k + 8); 147 j7 = x7 = crypto_le32dec(k + 12); 148 j8 = x8 = crypto_le32dec(k + 16); 149 j9 = x9 = crypto_le32dec(k + 20); 150 j10 = x10 = crypto_le32dec(k + 24); 151 j11 = x11 = crypto_le32dec(k + 28); 152 j12 = x12 = crypto_le32dec(in + 0); 153 j13 = x13 = crypto_le32dec(in + 4); 154 j14 = x14 = crypto_le32dec(in + 8); 155 j15 = x15 = crypto_le32dec(in + 12); 156 157 for (i = crypto_core_ROUNDS; i > 0; i -= 2) { 158 QUARTERROUND( x0, x4, x8,x12); 159 QUARTERROUND( x1, x5, x9,x13); 160 QUARTERROUND( x2, x6,x10,x14); 161 QUARTERROUND( x3, x7,x11,x15); 162 QUARTERROUND( x0, x5,x10,x15); 163 QUARTERROUND( x1, x6,x11,x12); 164 QUARTERROUND( x2, x7, x8,x13); 165 QUARTERROUND( x3, x4, x9,x14); 166 } 167 168 crypto_le32enc(out + 0, x0 + j0); 169 crypto_le32enc(out + 4, x1 + j1); 170 crypto_le32enc(out + 8, x2 + j2); 171 crypto_le32enc(out + 12, x3 + j3); 172 crypto_le32enc(out + 16, x4 + j4); 173 crypto_le32enc(out + 20, x5 + j5); 174 crypto_le32enc(out + 24, x6 + j6); 175 crypto_le32enc(out + 28, x7 + j7); 176 crypto_le32enc(out + 32, x8 + j8); 177 crypto_le32enc(out + 36, x9 + j9); 178 crypto_le32enc(out + 40, x10 + j10); 179 crypto_le32enc(out + 44, x11 + j11); 180 crypto_le32enc(out + 48, x12 + j12); 181 crypto_le32enc(out + 52, x13 + j13); 182 crypto_le32enc(out + 56, x14 + j14); 183 crypto_le32enc(out + 60, x15 + j15); 184 } 185 186 /* ChaCha self-test */ 187 188 /* 189 * Test vector for ChaCha20 from 190 * <http://tools.ietf.org/html/draft-strombergson-chacha-test-vectors-00>, 191 * test vectors for ChaCha12 and ChaCha8 and for big-endian machines 192 * generated by the same crypto_core code with crypto_core_ROUNDS and 193 * crypto_le32enc/dec varied. 194 */ 195 196 static const uint8_t crypto_core_selftest_vector[64] = { 197 #if _BYTE_ORDER == _LITTLE_ENDIAN 198 # if crypto_core_ROUNDS == 8 199 0x3e,0x00,0xef,0x2f,0x89,0x5f,0x40,0xd6, 200 0x7f,0x5b,0xb8,0xe8,0x1f,0x09,0xa5,0xa1, 201 0x2c,0x84,0x0e,0xc3,0xce,0x9a,0x7f,0x3b, 202 0x18,0x1b,0xe1,0x88,0xef,0x71,0x1a,0x1e, 203 0x98,0x4c,0xe1,0x72,0xb9,0x21,0x6f,0x41, 204 0x9f,0x44,0x53,0x67,0x45,0x6d,0x56,0x19, 205 0x31,0x4a,0x42,0xa3,0xda,0x86,0xb0,0x01, 206 0x38,0x7b,0xfd,0xb8,0x0e,0x0c,0xfe,0x42, 207 # elif crypto_core_ROUNDS == 12 208 0x9b,0xf4,0x9a,0x6a,0x07,0x55,0xf9,0x53, 209 0x81,0x1f,0xce,0x12,0x5f,0x26,0x83,0xd5, 210 0x04,0x29,0xc3,0xbb,0x49,0xe0,0x74,0x14, 211 0x7e,0x00,0x89,0xa5,0x2e,0xae,0x15,0x5f, 212 0x05,0x64,0xf8,0x79,0xd2,0x7a,0xe3,0xc0, 213 0x2c,0xe8,0x28,0x34,0xac,0xfa,0x8c,0x79, 214 0x3a,0x62,0x9f,0x2c,0xa0,0xde,0x69,0x19, 215 0x61,0x0b,0xe8,0x2f,0x41,0x13,0x26,0xbe, 216 # elif crypto_core_ROUNDS == 20 217 0x76,0xb8,0xe0,0xad,0xa0,0xf1,0x3d,0x90, 218 0x40,0x5d,0x6a,0xe5,0x53,0x86,0xbd,0x28, 219 0xbd,0xd2,0x19,0xb8,0xa0,0x8d,0xed,0x1a, 220 0xa8,0x36,0xef,0xcc,0x8b,0x77,0x0d,0xc7, 221 0xda,0x41,0x59,0x7c,0x51,0x57,0x48,0x8d, 222 0x77,0x24,0xe0,0x3f,0xb8,0xd8,0x4a,0x37, 223 0x6a,0x43,0xb8,0xf4,0x15,0x18,0xa1,0x1c, 224 0xc3,0x87,0xb6,0x69,0xb2,0xee,0x65,0x86, 225 # else 226 # error crypto_core_ROUNDS must be 8, 12, or 20. 227 # endif 228 #elif _BYTE_ORDER == _BIG_ENDIAN 229 # if crypto_core_ROUNDS == 8 230 0x9a,0x13,0x07,0xe3,0x38,0x18,0x9e,0x99, 231 0x15,0x37,0x16,0x4d,0x04,0xe6,0x48,0x9a, 232 0x07,0xd6,0xe8,0x7a,0x02,0xf9,0xf5,0xc7, 233 0x3f,0xa9,0xc2,0x0a,0xe1,0xc6,0x62,0xea, 234 0x80,0xaf,0xb6,0x51,0xca,0x52,0x43,0x87, 235 0xe3,0xa6,0xa6,0x61,0x11,0xf5,0xe6,0xcf, 236 0x09,0x0f,0xdc,0x9d,0xc3,0xc3,0xbb,0x43, 237 0xd7,0xfa,0x70,0x42,0xbf,0xa5,0xee,0xa2, 238 # elif crypto_core_ROUNDS == 12 239 0xcf,0x6c,0x16,0x48,0xbf,0xf4,0xba,0x85, 240 0x32,0x69,0xd3,0x98,0xc8,0x7d,0xcd,0x3f, 241 0xdc,0x76,0x6b,0xa2,0x7b,0xcb,0x17,0x4d, 242 0x05,0xda,0xdd,0xd8,0x62,0x54,0xbf,0xe0, 243 0x65,0xed,0x0e,0xf4,0x01,0x7e,0x3c,0x05, 244 0x35,0xb2,0x7a,0x60,0xf3,0x8f,0x12,0x33, 245 0x24,0x60,0xcd,0x85,0xfe,0x4c,0xf3,0x39, 246 0xb1,0x0e,0x3e,0xe0,0xba,0xa6,0x2f,0xa9, 247 # elif crypto_core_ROUNDS == 20 248 0x83,0x8b,0xf8,0x75,0xf7,0xde,0x9d,0x8c, 249 0x33,0x14,0x72,0x28,0xd1,0xbe,0x88,0xe5, 250 0x94,0xb5,0xed,0xb8,0x56,0xb5,0x9e,0x0c, 251 0x64,0x6a,0xaf,0xd9,0xa7,0x49,0x10,0x59, 252 0xba,0x3a,0x82,0xf8,0x4a,0x70,0x9c,0x00, 253 0x82,0x2c,0xae,0xc6,0xd7,0x1c,0x2e,0xda, 254 0x2a,0xfb,0x61,0x70,0x2b,0xd1,0xbf,0x8b, 255 0x95,0xbc,0x23,0xb6,0x4b,0x60,0x02,0xec, 256 # else 257 # error crypto_core_ROUNDS must be 8, 12, or 20. 258 # endif 259 #else 260 # error Byte order must be little-endian or big-endian. 261 #endif 262 }; 263 264 static int 265 crypto_core_selftest(void) 266 { 267 const uint8_t nonce[crypto_core_INPUTBYTES] = {0}; 268 const uint8_t key[crypto_core_KEYBYTES] = {0}; 269 uint8_t block[64]; 270 unsigned i; 271 272 crypto_core(block, nonce, key, crypto_core_constant32); 273 for (i = 0; i < 64; i++) { 274 if (block[i] != crypto_core_selftest_vector[i]) 275 return EIO; 276 } 277 278 return 0; 279 } 280 281 /* PRNG */ 282 283 /* 284 * For a state s, rather than use ChaCha20 as a stream cipher to 285 * generate the concatenation ChaCha20_s(0) || ChaCha20_s(1) || ..., we 286 * split ChaCha20_s(0) into s' || x and yield x for the first request, 287 * split ChaCha20_s'(0) into s'' || y and yield y for the second 288 * request, &c. This provides backtracking resistance: an attacker who 289 * finds s'' can't recover s' or x. 290 */ 291 292 #define crypto_prng_SEEDBYTES crypto_core_KEYBYTES 293 #define crypto_prng_MAXOUTPUTBYTES \ 294 (crypto_core_OUTPUTBYTES - crypto_prng_SEEDBYTES) 295 296 __CTASSERT(sizeof(struct crypto_prng) == crypto_prng_SEEDBYTES); 297 298 static void 299 crypto_prng_seed(struct crypto_prng *prng, const void *seed) 300 { 301 302 (void)memcpy(prng->state, seed, crypto_prng_SEEDBYTES); 303 } 304 305 static void 306 crypto_prng_buf(struct crypto_prng *prng, void *buf, size_t n) 307 { 308 const uint8_t nonce[crypto_core_INPUTBYTES] = {0}; 309 uint8_t output[crypto_core_OUTPUTBYTES]; 310 311 _DIAGASSERT(n <= crypto_prng_MAXOUTPUTBYTES); 312 __CTASSERT(sizeof prng->state + crypto_prng_MAXOUTPUTBYTES 313 <= sizeof output); 314 315 crypto_core(output, nonce, prng->state, crypto_core_constant32); 316 (void)memcpy(prng->state, output, sizeof prng->state); 317 (void)memcpy(buf, output + sizeof prng->state, n); 318 (void)explicit_memset(output, 0, sizeof output); 319 } 320 321 static int 322 crypto_prng_selftest(void) 323 { 324 const uint8_t expected[32] = { 325 #if _BYTE_ORDER == _LITTLE_ENDIAN 326 # if crypto_core_ROUNDS == 20 327 0x2b, /* first call */ 328 0x2d,0x41,0xa5,0x9c,0x90,0xe4,0x1a,0x8e, /* second call */ 329 0x7a,0x4d,0xcc,0xaa,0x1c,0x46,0x06,0x99, 330 0x83,0xb1,0xa3,0x33,0xce,0x25,0x71,0x9e, 331 0xc3,0x43,0x77,0x68,0xab,0x57, 332 0x5f, /* third call */ 333 # else 334 # error crypto_core_ROUNDS other than 20 left as exercise for reader. 335 # endif 336 #elif _BYTE_ORDER == _BIG_ENDIAN 337 # if crypto_core_ROUNDS == 20 338 0xae, /* first call */ 339 0x97,0x14,0x5a,0x05,0xad,0xa8,0x48,0xf1, /* second call */ 340 0x3a,0x81,0x84,0xd7,0x05,0xda,0x20,0x5d, 341 0xc0,0xef,0x86,0x65,0x98,0xbd,0xb0,0x16, 342 0x1b,0xfc,0xff,0xc4,0xc2,0xfd, 343 0xa0, /* third call */ 344 # else 345 # error crypto_core_ROUNDS other than 20 left as exercise for reader. 346 # endif 347 #else 348 # error Byte order must be little-endian or big-endian. 349 #endif 350 }; 351 uint8_t seed[crypto_prng_SEEDBYTES]; 352 struct crypto_prng prng; 353 uint8_t output[32]; 354 unsigned i; 355 356 for (i = 0; i < __arraycount(seed); i++) 357 seed[i] = i; 358 crypto_prng_seed(&prng, seed); 359 crypto_prng_buf(&prng, output, 1); 360 crypto_prng_buf(&prng, output + 1, 30); 361 crypto_prng_buf(&prng, output + 31, 1); 362 if (memcmp(output, expected, 32) != 0) 363 return EIO; 364 return 0; 365 } 366 367 /* One-time stream: expand short single-use secret into long secret */ 368 369 #define crypto_onetimestream_SEEDBYTES crypto_core_KEYBYTES 370 371 static void 372 crypto_onetimestream(const void *seed, void *buf, size_t n) 373 { 374 uint32_t nonce[crypto_core_INPUTBYTES / sizeof(uint32_t)] = {0}; 375 uint8_t block[crypto_core_OUTPUTBYTES]; 376 uint8_t *p8, *p32; 377 const uint8_t *nonce8 = (const uint8_t *)(void *)nonce; 378 size_t ni, nb, nf; 379 380 /* 381 * Guarantee we can generate up to n bytes. We have 382 * 2^(8*INPUTBYTES) possible inputs yielding output of 383 * OUTPUTBYTES*2^(8*INPUTBYTES) bytes. It suffices to require 384 * that sizeof n > (1/CHAR_BIT) log_2 n be less than 385 * (1/CHAR_BIT) log_2 of the total output stream length. We 386 * have 387 * 388 * log_2 (o 2^(8 i)) = log_2 o + log_2 2^(8 i) 389 * = log_2 o + 8 i. 390 */ 391 #ifndef __lint__ 392 __CTASSERT(CHAR_BIT * sizeof n <= (ilog2(crypto_core_OUTPUTBYTES) + 393 8 * crypto_core_INPUTBYTES)); 394 #endif 395 396 p8 = buf; 397 p32 = (uint8_t *)roundup2((uintptr_t)p8, 4); 398 ni = p32 - p8; 399 if (n < ni) 400 ni = n; 401 nb = (n - ni) / sizeof block; 402 nf = (n - ni) % sizeof block; 403 404 _DIAGASSERT(((uintptr_t)p32 & 3) == 0); 405 _DIAGASSERT(ni <= n); 406 _DIAGASSERT(nb <= (n / sizeof block)); 407 _DIAGASSERT(nf <= n); 408 _DIAGASSERT(n == (ni + (nb * sizeof block) + nf)); 409 _DIAGASSERT(ni < 4); 410 _DIAGASSERT(nf < sizeof block); 411 412 if (ni) { 413 crypto_core(block, nonce8, seed, crypto_core_constant32); 414 crypto_le32enc(&nonce[0], 1 + crypto_le32dec(&nonce[0])); 415 (void)memcpy(p8, block, ni); 416 } 417 while (nb--) { 418 crypto_core(p32, nonce8, seed, crypto_core_constant32); 419 crypto_le32enc(&nonce[0], 1 + crypto_le32dec(&nonce[0])); 420 if (crypto_le32dec(&nonce[0]) == 0) { 421 crypto_le32enc(&nonce[1], 422 1 + crypto_le32dec(&nonce[1])); 423 } 424 p32 += crypto_core_OUTPUTBYTES; 425 } 426 if (nf) { 427 crypto_core(block, nonce8, seed, crypto_core_constant32); 428 crypto_le32enc(&nonce[0], 1 + crypto_le32dec(&nonce[0])); 429 if (crypto_le32dec(&nonce[0]) == 0) { 430 crypto_le32enc(&nonce[1], 431 1 + crypto_le32dec(&nonce[1])); 432 } 433 (void)memcpy(p32, block, nf); 434 } 435 436 if (ni | nf) 437 (void)explicit_memset(block, 0, sizeof block); 438 } 439 440 static int 441 crypto_onetimestream_selftest(void) 442 { 443 const uint8_t expected[70] = { 444 0x5a, /* guard byte */ 445 #if _BYTE_ORDER == _LITTLE_ENDIAN 446 # if crypto_core_ROUNDS == 20 447 0x39,0xfd,0x2b, /* initial block */ 448 0x18,0xb8,0x42,0x31,0xad,0xe6,0xa6,0xd1, 449 0x13,0x61,0x5c,0x61,0xaf,0x43,0x4e,0x27, 450 0xf8,0xb1,0xf3,0xf5,0xe1,0xad,0x5b,0x5c, 451 0xec,0xf8,0xfc,0x12,0x2a,0x35,0x75,0x5c, 452 0x72,0x08,0x08,0x6d,0xd1,0xee,0x3c,0x5d, 453 0x9d,0x81,0x58,0x24,0x64,0x0e,0x00,0x3c, 454 0x9b,0xa0,0xf6,0x5e,0xde,0x5d,0x59,0xce, 455 0x0d,0x2a,0x4a,0x7f,0x31,0x95,0x5a,0xcd, 456 0x42, /* final block */ 457 # else 458 # error crypto_core_ROUNDS other than 20 left as exercise for reader. 459 # endif 460 #elif _BYTE_ORDER == _BIG_ENDIAN 461 # if crypto_core_ROUNDS == 20 462 0x20,0xf0,0x66, /* initial block */ 463 0x1a,0x82,0xda,0xb6,0xba,0x90,0x42,0x19, 464 0x39,0xc2,0x4e,0x4d,0xaf,0xbc,0x67,0xcf, 465 0xe3,0xe4,0xe2,0x80,0x38,0x80,0x8e,0x53, 466 0x19,0x25,0x37,0x67,0x66,0x57,0x7c,0x78, 467 0xac,0xb3,0x8b,0x97,0x54,0x20,0xc4,0x46, 468 0xff,0x90,0x76,0x56,0xcc,0xde,0xe5,0xb9, 469 0xdf,0x82,0x8c,0x05,0x9d,0xf0,0x69,0x99, 470 0x42,0x53,0x74,0x5e,0x80,0x81,0xdb,0x9b, 471 0xb1, /* final block */ 472 # else 473 # error crypto_core_ROUNDS other than 20 left as exercise for reader. 474 # endif 475 #else 476 # error Byte order must be little-endian or big-endian. 477 #endif 478 0xcc, /* guard byte */ 479 }; 480 uint8_t seed[crypto_prng_SEEDBYTES]; 481 uint8_t output[70] __aligned(4); 482 unsigned i; 483 484 for (i = 0; i < __arraycount(seed); i++) 485 seed[i] = i; 486 output[0] = 0x5a; 487 output[69] = 0xcc; 488 crypto_onetimestream(seed, output + 1, 68); 489 if (memcmp(output, expected, 70) != 0) 490 return EIO; 491 return 0; 492 } 493 494 /* 495 * entropy_epoch() 496 * 497 * Return the current entropy epoch, from the sysctl node 498 * kern.entropy.epoch. 499 * 500 * The entropy epoch is never zero. Initially, or on error, it is 501 * (unsigned)-1. It may wrap around but it skips (unsigned)-1 and 502 * 0 when it does. Changes happen less than once per second, so 503 * wraparound will only affect systems after 136 years of uptime. 504 * 505 * XXX This should get it from a page shared read-only by kernel 506 * with userland, but until we implement such a mechanism, this 507 * sysctl -- incurring the cost of a syscall -- will have to 508 * serve. 509 */ 510 static unsigned 511 entropy_epoch(void) 512 { 513 const int mib[] = { CTL_KERN, KERN_ENTROPY, KERN_ENTROPY_EPOCH }; 514 unsigned epoch = (unsigned)-1; 515 size_t epochlen = sizeof(epoch); 516 517 if (sysctl(mib, __arraycount(mib), &epoch, &epochlen, NULL, 0) == -1) 518 return (unsigned)-1; 519 if (epochlen != sizeof(epoch)) 520 return (unsigned)-1; 521 522 return epoch; 523 } 524 525 /* arc4random state: per-thread, per-process (zeroed in child on fork) */ 526 527 static void 528 arc4random_prng_addrandom(struct arc4random_prng *prng, const void *data, 529 size_t datalen) 530 { 531 const int mib[] = { CTL_KERN, KERN_ARND }; 532 SHA256_CTX ctx; 533 uint8_t buf[crypto_prng_SEEDBYTES]; 534 size_t buflen = sizeof buf; 535 unsigned epoch = entropy_epoch(); 536 537 __CTASSERT(sizeof buf == SHA256_DIGEST_LENGTH); 538 539 SHA256_Init(&ctx); 540 541 crypto_prng_buf(&prng->arc4_prng, buf, sizeof buf); 542 SHA256_Update(&ctx, buf, sizeof buf); 543 544 if (sysctl(mib, (u_int)__arraycount(mib), buf, &buflen, NULL, 0) == -1) 545 abort(); 546 if (buflen != sizeof buf) 547 abort(); 548 SHA256_Update(&ctx, buf, sizeof buf); 549 550 if (data != NULL) 551 SHA256_Update(&ctx, data, datalen); 552 553 SHA256_Final(buf, &ctx); 554 (void)explicit_memset(&ctx, 0, sizeof ctx); 555 556 /* reseed(SHA256(prng() || sysctl(KERN_ARND) || data)) */ 557 crypto_prng_seed(&prng->arc4_prng, buf); 558 (void)explicit_memset(buf, 0, sizeof buf); 559 prng->arc4_epoch = epoch; 560 } 561 562 #ifdef _REENTRANT 563 static struct arc4random_prng * 564 arc4random_prng_create(void) 565 { 566 struct arc4random_prng *prng; 567 const size_t size = roundup(sizeof(*prng), sysconf(_SC_PAGESIZE)); 568 569 prng = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 570 0); 571 if (prng == MAP_FAILED) 572 goto fail0; 573 if (minherit(prng, size, MAP_INHERIT_ZERO) == -1) 574 goto fail1; 575 576 return prng; 577 578 fail1: (void)munmap(prng, size); 579 fail0: return NULL; 580 } 581 #endif 582 583 #ifdef _REENTRANT 584 static void 585 arc4random_prng_destroy(struct arc4random_prng *prng) 586 { 587 const size_t size = roundup(sizeof(*prng), sysconf(_SC_PAGESIZE)); 588 589 (void)explicit_memset(prng, 0, sizeof(*prng)); 590 (void)munmap(prng, size); 591 } 592 #endif 593 594 /* Library state */ 595 596 struct arc4random_global_state arc4random_global = { 597 #ifdef _REENTRANT 598 .lock = MUTEX_INITIALIZER, 599 #endif 600 .once = ONCE_INITIALIZER, 601 }; 602 603 static void 604 arc4random_atfork_prepare(void) 605 { 606 607 mutex_lock(&arc4random_global.lock); 608 (void)explicit_memset(&arc4random_global.prng, 0, 609 sizeof arc4random_global.prng); 610 } 611 612 static void 613 arc4random_atfork_parent(void) 614 { 615 616 mutex_unlock(&arc4random_global.lock); 617 } 618 619 static void 620 arc4random_atfork_child(void) 621 { 622 623 mutex_unlock(&arc4random_global.lock); 624 } 625 626 #ifdef _REENTRANT 627 static void 628 arc4random_tsd_destructor(void *p) 629 { 630 struct arc4random_prng *const prng = p; 631 632 arc4random_prng_destroy(prng); 633 } 634 #endif 635 636 static void 637 arc4random_initialize(void) 638 { 639 640 /* 641 * If the crypto software is broken, abort -- something is 642 * severely wrong with this process image. 643 */ 644 if (crypto_core_selftest() != 0 || 645 crypto_prng_selftest() != 0 || 646 crypto_onetimestream_selftest() != 0) 647 abort(); 648 649 /* 650 * Set up a pthread_atfork handler to lock the global state 651 * around fork so that if forked children can't use the 652 * per-thread state, they can take the lock and use the global 653 * state without deadlock. If this fails, we will fall back to 654 * PRNG state on the stack reinitialized from the kernel 655 * entropy pool at every call. 656 */ 657 if (pthread_atfork(&arc4random_atfork_prepare, 658 &arc4random_atfork_parent, &arc4random_atfork_child) 659 == 0) 660 arc4random_global.forksafe = true; 661 662 /* 663 * For multithreaded builds, try to allocate a per-thread PRNG 664 * state to avoid contention due to arc4random. 665 */ 666 #ifdef _REENTRANT 667 if (thr_keycreate(&arc4random_global.thread_key, 668 &arc4random_tsd_destructor) == 0) 669 arc4random_global.per_thread = true; 670 #endif 671 672 /* 673 * Note that the arc4random library state has been initialized 674 * for the sake of automatic tests. 675 */ 676 arc4random_global.initialized = true; 677 } 678 679 static struct arc4random_prng * 680 arc4random_prng_get(struct arc4random_prng *fallback) 681 { 682 struct arc4random_prng *prng = NULL; 683 684 /* Make sure the library is initialized. */ 685 thr_once(&arc4random_global.once, &arc4random_initialize); 686 687 #ifdef _REENTRANT 688 /* Get or create the per-thread PRNG state. */ 689 prng = __predict_true(arc4random_global.per_thread) 690 ? thr_getspecific(arc4random_global.thread_key) 691 : NULL; 692 if (__predict_false(prng == NULL) && arc4random_global.per_thread) { 693 prng = arc4random_prng_create(); 694 thr_setspecific(arc4random_global.thread_key, prng); 695 } 696 #endif 697 698 /* 699 * If we can't create it, fall back to the global PRNG -- or an 700 * on-stack PRNG, in the unlikely event that pthread_atfork 701 * failed, which we have to seed from scratch each time 702 * (suboptimal, but unlikely, so not worth optimizing). 703 */ 704 if (__predict_false(prng == NULL)) { 705 if (__predict_true(arc4random_global.forksafe)) { 706 mutex_lock(&arc4random_global.lock); 707 prng = &arc4random_global.prng; 708 } else { 709 prng = fallback; 710 memset(prng, 0, sizeof(*prng)); 711 } 712 } 713 714 /* Guarantee the PRNG is seeded. */ 715 if (__predict_false(prng->arc4_epoch != entropy_epoch())) 716 arc4random_prng_addrandom(prng, NULL, 0); 717 718 return prng; 719 } 720 721 static void 722 arc4random_prng_put(struct arc4random_prng *prng, 723 struct arc4random_prng *fallback) 724 { 725 726 /* 727 * If we had to use a stack fallback, zero it before we return 728 * so that after we return we avoid leaving secrets on the 729 * stack that could recover the parent's future outputs in an 730 * unprivileged forked child (of course, we can't guarantee 731 * that the compiler hasn't spilled anything; this is 732 * best-effort, not a guarantee). 733 */ 734 if (__predict_false(prng == fallback)) 735 explicit_memset(fallback, 0, sizeof(*fallback)); 736 737 /* If we had fallen back to the global PRNG, unlock it. */ 738 if (__predict_false(prng == &arc4random_global.prng)) 739 mutex_unlock(&arc4random_global.lock); 740 } 741 742 /* Public API */ 743 744 uint32_t 745 arc4random(void) 746 { 747 struct arc4random_prng *prng, fallback; 748 uint32_t v; 749 750 prng = arc4random_prng_get(&fallback); 751 crypto_prng_buf(&prng->arc4_prng, &v, sizeof v); 752 arc4random_prng_put(prng, &fallback); 753 754 return v; 755 } 756 757 void 758 arc4random_buf(void *buf, size_t len) 759 { 760 struct arc4random_prng *prng, fallback; 761 762 if (len <= crypto_prng_MAXOUTPUTBYTES) { 763 prng = arc4random_prng_get(&fallback); 764 crypto_prng_buf(&prng->arc4_prng, buf, len); 765 arc4random_prng_put(prng, &fallback); 766 } else { 767 uint8_t seed[crypto_onetimestream_SEEDBYTES]; 768 769 prng = arc4random_prng_get(&fallback); 770 crypto_prng_buf(&prng->arc4_prng, seed, sizeof seed); 771 arc4random_prng_put(prng, &fallback); 772 773 crypto_onetimestream(seed, buf, len); 774 (void)explicit_memset(seed, 0, sizeof seed); 775 } 776 } 777 778 uint32_t 779 arc4random_uniform(uint32_t bound) 780 { 781 struct arc4random_prng *prng, fallback; 782 uint32_t minimum, r; 783 784 /* 785 * We want a uniform random choice in [0, n), and arc4random() 786 * makes a uniform random choice in [0, 2^32). If we reduce 787 * that modulo n, values in [0, 2^32 mod n) will be represented 788 * slightly more than values in [2^32 mod n, n). Instead we 789 * choose only from [2^32 mod n, 2^32) by rejecting samples in 790 * [0, 2^32 mod n), to avoid counting the extra representative 791 * of [0, 2^32 mod n). To compute 2^32 mod n, note that 792 * 793 * 2^32 mod n = 2^32 mod n - 0 794 * = 2^32 mod n - n mod n 795 * = (2^32 - n) mod n, 796 * 797 * the last of which is what we compute in 32-bit arithmetic. 798 */ 799 minimum = (-bound % bound); 800 801 prng = arc4random_prng_get(&fallback); 802 do crypto_prng_buf(&prng->arc4_prng, &r, sizeof r); 803 while (__predict_false(r < minimum)); 804 arc4random_prng_put(prng, &fallback); 805 806 return (r % bound); 807 } 808 809 void 810 arc4random_stir(void) 811 { 812 struct arc4random_prng *prng, fallback; 813 814 prng = arc4random_prng_get(&fallback); 815 arc4random_prng_addrandom(prng, NULL, 0); 816 arc4random_prng_put(prng, &fallback); 817 } 818 819 /* 820 * Silly signature here is for hysterical raisins. Should instead be 821 * const void *data and size_t datalen. 822 */ 823 void 824 arc4random_addrandom(u_char *data, int datalen) 825 { 826 struct arc4random_prng *prng, fallback; 827 828 _DIAGASSERT(0 <= datalen); 829 830 prng = arc4random_prng_get(&fallback); 831 arc4random_prng_addrandom(prng, data, datalen); 832 arc4random_prng_put(prng, &fallback); 833 } 834 835 #ifdef _ARC4RANDOM_TEST 836 837 #include <sys/wait.h> 838 839 #include <err.h> 840 #include <stdio.h> 841 842 int 843 main(int argc __unused, char **argv __unused) 844 { 845 unsigned char gubbish[] = "random gubbish"; 846 const uint8_t zero64[64] = {0}; 847 uint8_t buf[2048]; 848 unsigned i, a, n; 849 850 /* Test arc4random: should not be deterministic. */ 851 if (printf("arc4random: %08"PRIx32"\n", arc4random()) < 0) 852 err(1, "printf"); 853 854 /* Test stirring: should definitely not be deterministic. */ 855 arc4random_stir(); 856 857 /* Test small buffer. */ 858 arc4random_buf(buf, 8); 859 if (printf("arc4randombuf small:") < 0) 860 err(1, "printf"); 861 for (i = 0; i < 8; i++) 862 if (printf(" %02x", buf[i]) < 0) 863 err(1, "printf"); 864 if (printf("\n") < 0) 865 err(1, "printf"); 866 867 /* Test addrandom: should not make the rest deterministic. */ 868 arc4random_addrandom(gubbish, sizeof gubbish); 869 870 /* Test large buffer. */ 871 arc4random_buf(buf, sizeof buf); 872 if (printf("arc4randombuf_large:") < 0) 873 err(1, "printf"); 874 for (i = 0; i < sizeof buf; i++) 875 if (printf(" %02x", buf[i]) < 0) 876 err(1, "printf"); 877 if (printf("\n") < 0) 878 err(1, "printf"); 879 880 /* Test misaligned small and large. */ 881 for (a = 0; a < 64; a++) { 882 for (n = a; n < sizeof buf; n++) { 883 (void)memset(buf, 0, sizeof buf); 884 arc4random_buf(buf, n - a); 885 if (memcmp(buf + n - a, zero64, a) != 0) 886 errx(1, "arc4random buffer overflow 0"); 887 888 (void)memset(buf, 0, sizeof buf); 889 arc4random_buf(buf + a, n - a); 890 if (memcmp(buf, zero64, a) != 0) 891 errx(1, "arc4random buffer overflow 1"); 892 893 if ((2*a) <= n) { 894 (void)memset(buf, 0, sizeof buf); 895 arc4random_buf(buf + a, n - a - a); 896 if (memcmp(buf + n - a, zero64, a) != 0) 897 errx(1, 898 "arc4random buffer overflow 2"); 899 } 900 } 901 } 902 903 /* Test fork-safety. */ 904 { 905 pid_t pid, rpid; 906 int status; 907 908 pid = fork(); 909 switch (pid) { 910 case -1: 911 err(1, "fork"); 912 case 0: { 913 /* 914 * Verify the epoch has been set to zero by fork. 915 */ 916 struct arc4random_prng *prng = NULL; 917 #ifdef _REENTRANT 918 prng = arc4random_global.per_thread 919 ? thr_getspecific(arc4random_global.thread_key) 920 : NULL; 921 #endif 922 if (prng == NULL) 923 prng = &arc4random_global.prng; 924 _exit(prng->arc4_epoch != 0); 925 } 926 default: 927 rpid = waitpid(pid, &status, 0); 928 if (rpid == -1) 929 err(1, "waitpid"); 930 if (rpid != pid) 931 errx(1, "waitpid returned wrong pid" 932 ": %"PRIdMAX" != %"PRIdMAX, 933 (intmax_t)rpid, 934 (intmax_t)pid); 935 if (WIFEXITED(status)) { 936 if (WEXITSTATUS(status) != 0) 937 errx(1, "child exited with %d", 938 WEXITSTATUS(status)); 939 } else if (WIFSIGNALED(status)) { 940 errx(1, "child terminated on signal %d", 941 WTERMSIG(status)); 942 } else { 943 errx(1, "child died mysteriously: %d", status); 944 } 945 } 946 } 947 948 /* XXX Test multithreaded fork safety...? */ 949 950 return 0; 951 } 952 #endif 953
Indexes created Wed Sep 24 05:09:52 GMT 2025