1 /* $OpenBSD: arc4random.c,v 1.58 2022/07/31 13:41:45 tb Exp $ */
2
3 /*
4 * Copyright (c) 1996, David Mazieres <dm@uun.org>
5 * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
6 * Copyright (c) 2013, Markus Friedl <markus@openbsd.org>
7 * Copyright (c) 2014, Theo de Raadt <deraadt@openbsd.org>
8 *
9 * Permission to use, copy, modify, and distribute this software for any
10 * purpose with or without fee is hereby granted, provided that the above
11 * copyright notice and this permission notice appear in all copies.
12 *
13 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
14 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
15 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
16 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
17 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
18 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
19 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
20 */
21
22 /*
23 * ChaCha based random number generator for OpenBSD.
24 */
25 #define _BSD_SOURCE
26
27 #include <sys/cdefs.h>
28 #if defined(__FreeBSD__)
29 #include <assert.h>
30 #endif
31 #include <fcntl.h>
32 #include <limits.h>
33 #include <pthread.h>
34 #include <signal.h>
35 #include <stdint.h>
36 #include <stdlib.h>
37 #include <string.h>
38 #include <unistd.h>
39 #include <sys/types.h>
40 #include <sys/time.h>
41
42 /*
43 * Unfortunately, pthread_atfork() is broken on FreeBSD (at least 9 and 10) if
44 * a program does not link to -lthr. Callbacks registered with pthread_atfork()
45 * appear to fail silently. So, it is not always possible to detect a PID
46 * wraparound.
47 */
48 #define _ARC4_ATFORK(f) pthread_atfork(NULL, NULL, (f))
49
50 #define CHACHA_EMBED
51 #define KEYSTREAM_ONLY
52 #if defined(__FreeBSD__)
53 #define ARC4RANDOM_FXRNG 1
54 #else
55 #define ARC4RANDOM_FXRNG 0
56 #endif
57 #include "chacha_private.h"
58
59 #define minimum(a, b) ((a) < (b) ? (a) : (b))
60
61 #if defined(__GNUC__) || defined(_MSC_VER)
62 #define inline __inline
63 #else /* __GNUC__ || _MSC_VER */
64 #define inline
65 #endif /* !__GNUC__ && !_MSC_VER */
66
67 #define KEYSZ 32
68 #define IVSZ 8
69 #define BLOCKSZ 64
70 #define RSBUFSZ (16*BLOCKSZ)
71
72 #define REKEY_BASE (1024*1024) /* NB. should be a power of 2 */
73
74 /* Marked INHERIT_ZERO, so zero'd out in fork children. */
75 static struct _rs {
76 size_t rs_have; /* valid bytes at end of rs_buf */
77 size_t rs_count; /* bytes till reseed */
78 } *rs;
79
80 /* Maybe be preserved in fork children, if _rs_allocate() decides. */
81 static struct _rsx {
82 chacha_ctx rs_chacha; /* chacha context for random keystream */
83 u_char rs_buf[RSBUFSZ]; /* keystream blocks */
84 #ifdef __FreeBSD__
85 uint32_t rs_seed_generation; /* 32-bit userspace RNG version */
86 #endif
87 } *rsx;
88
89 static inline int _rs_allocate(struct _rs **, struct _rsx **);
90 static inline void _rs_forkdetect(void);
91 #include "arc4random.h"
92
93 static inline void _rs_rekey(u_char *dat, size_t datlen);
94
95 static inline void
_rs_init(u_char * buf,size_t n)96 _rs_init(u_char *buf, size_t n)
97 {
98 if (n < KEYSZ + IVSZ)
99 return;
100
101 if (rs == NULL) {
102 if (_rs_allocate(&rs, &rsx) == -1)
103 _exit(1);
104 }
105
106 chacha_keysetup(&rsx->rs_chacha, buf, KEYSZ * 8);
107 chacha_ivsetup(&rsx->rs_chacha, buf + KEYSZ);
108 }
109
110 static void
_rs_stir(void)111 _rs_stir(void)
112 {
113 u_char rnd[KEYSZ + IVSZ];
114 uint32_t rekey_fuzz = 0;
115
116 #if defined(__FreeBSD__)
117 bool need_init;
118
119 /*
120 * De-couple allocation (which locates the vdso_fxrngp pointer in
121 * auxinfo) from initialization. This allows us to read the root seed
122 * version before we fetch system entropy, maintaining the invariant
123 * that the PRF was seeded with entropy from rs_seed_generation or a
124 * later generation. But never seeded from an earlier generation.
125 * This invariant prevents us from missing a root reseed event.
126 */
127 need_init = false;
128 if (rs == NULL) {
129 if (_rs_allocate(&rs, &rsx) == -1)
130 abort();
131 need_init = true;
132 }
133 /*
134 * Transition period: new userspace on old kernel. This should become
135 * a hard error at some point, if the scheme is adopted.
136 */
137 if (vdso_fxrngp != NULL)
138 rsx->rs_seed_generation =
139 fxrng_load_acq_generation(&vdso_fxrngp->fx_generation32);
140 #endif
141
142 if (getentropy(rnd, sizeof rnd) == -1)
143 _getentropy_fail();
144
145 #if !defined(__FreeBSD__)
146 if (!rs)
147 _rs_init(rnd, sizeof(rnd));
148 #else /* __FreeBSD__ */
149 assert(rs != NULL);
150 if (need_init)
151 _rs_init(rnd, sizeof(rnd));
152 #endif
153 else
154 _rs_rekey(rnd, sizeof(rnd));
155 explicit_bzero(rnd, sizeof(rnd)); /* discard source seed */
156
157 /* invalidate rs_buf */
158 rs->rs_have = 0;
159 memset(rsx->rs_buf, 0, sizeof(rsx->rs_buf));
160
161 /* rekey interval should not be predictable */
162 chacha_encrypt_bytes(&rsx->rs_chacha, (uint8_t *)&rekey_fuzz,
163 (uint8_t *)&rekey_fuzz, sizeof(rekey_fuzz));
164 rs->rs_count = REKEY_BASE + (rekey_fuzz % REKEY_BASE);
165 }
166
167 static inline void
_rs_stir_if_needed(size_t len)168 _rs_stir_if_needed(size_t len)
169 {
170 _rs_forkdetect();
171 if (!rs || rs->rs_count <= len)
172 _rs_stir();
173 if (rs->rs_count <= len)
174 rs->rs_count = 0;
175 else
176 rs->rs_count -= len;
177 }
178
179 static inline void
_rs_rekey(u_char * dat,size_t datlen)180 _rs_rekey(u_char *dat, size_t datlen)
181 {
182 #ifndef KEYSTREAM_ONLY
183 memset(rsx->rs_buf, 0, sizeof(rsx->rs_buf));
184 #endif
185 /* fill rs_buf with the keystream */
186 chacha_encrypt_bytes(&rsx->rs_chacha, rsx->rs_buf,
187 rsx->rs_buf, sizeof(rsx->rs_buf));
188 /* mix in optional user provided data */
189 if (dat) {
190 size_t i, m;
191
192 m = minimum(datlen, KEYSZ + IVSZ);
193 for (i = 0; i < m; i++)
194 rsx->rs_buf[i] ^= dat[i];
195 }
196 /* immediately reinit for backtracking resistance */
197 _rs_init(rsx->rs_buf, KEYSZ + IVSZ);
198 memset(rsx->rs_buf, 0, KEYSZ + IVSZ);
199 rs->rs_have = sizeof(rsx->rs_buf) - KEYSZ - IVSZ;
200 }
201
202 static inline void
_rs_random_buf(void * _buf,size_t n)203 _rs_random_buf(void *_buf, size_t n)
204 {
205 u_char *buf = (u_char *)_buf;
206 u_char *keystream;
207 size_t m;
208
209 _rs_stir_if_needed(n);
210 while (n > 0) {
211 if (rs->rs_have > 0) {
212 m = minimum(n, rs->rs_have);
213 keystream = rsx->rs_buf + sizeof(rsx->rs_buf)
214 - rs->rs_have;
215 memcpy(buf, keystream, m);
216 memset(keystream, 0, m);
217 buf += m;
218 n -= m;
219 rs->rs_have -= m;
220 }
221 if (rs->rs_have == 0)
222 _rs_rekey(NULL, 0);
223 }
224 }
225
226 static inline void
_rs_random_u32(uint32_t * val)227 _rs_random_u32(uint32_t *val)
228 {
229 u_char *keystream;
230
231 _rs_stir_if_needed(sizeof(*val));
232 if (rs->rs_have < sizeof(*val))
233 _rs_rekey(NULL, 0);
234 keystream = rsx->rs_buf + sizeof(rsx->rs_buf) - rs->rs_have;
235 memcpy(val, keystream, sizeof(*val));
236 memset(keystream, 0, sizeof(*val));
237 rs->rs_have -= sizeof(*val);
238 }
239
240 uint32_t
arc4random(void)241 arc4random(void)
242 {
243 uint32_t val;
244
245 _ARC4_LOCK();
246 _rs_random_u32(&val);
247 _ARC4_UNLOCK();
248 return val;
249 }
250
251 void
arc4random_buf(void * buf,size_t n)252 arc4random_buf(void *buf, size_t n)
253 {
254 _ARC4_LOCK();
255 _rs_random_buf(buf, n);
256 _ARC4_UNLOCK();
257 }
258