1 /*-------------------------------------------------------------------------
2 *
3 * pg_strong_random.c
4 * generate a cryptographically secure random number
5 *
6 * Our definition of "strong" is that it's suitable for generating random
7 * salts and query cancellation keys, during authentication.
8 *
9 * Copyright (c) 1996-2017, PostgreSQL Global Development Group
10 *
11 * IDENTIFICATION
12 * src/port/pg_strong_random.c
13 *
14 *-------------------------------------------------------------------------
15 */
16
17 #ifndef FRONTEND
18 #include "postgres.h"
19 #else
20 #include "postgres_fe.h"
21 #endif
22
23 #include <fcntl.h>
24 #include <unistd.h>
25 #include <sys/time.h>
26
27 #ifdef USE_OPENSSL
28 #include <openssl/rand.h>
29 #endif
30 #ifdef WIN32
31 #include <wincrypt.h>
32 #endif
33
34 #ifdef WIN32
35 /*
36 * Cache a global crypto provider that only gets freed when the process
37 * exits, in case we need random numbers more than once.
38 */
39 static HCRYPTPROV hProvider = 0;
40 #endif
41
42 #if defined(USE_DEV_URANDOM)
43 /*
44 * Read (random) bytes from a file.
45 */
46 static bool
random_from_file(char * filename,void * buf,size_t len)47 random_from_file(char *filename, void *buf, size_t len)
48 {
49 int f;
50 char *p = buf;
51 ssize_t res;
52
53 f = open(filename, O_RDONLY, 0);
54 if (f == -1)
55 return false;
56
57 while (len)
58 {
59 res = read(f, p, len);
60 if (res <= 0)
61 {
62 if (errno == EINTR)
63 continue; /* interrupted by signal, just retry */
64
65 close(f);
66 return false;
67 }
68
69 p += res;
70 len -= res;
71 }
72
73 close(f);
74 return true;
75 }
76 #endif
77
78 /*
79 * pg_strong_random
80 *
81 * Generate requested number of random bytes. The returned bytes are
82 * cryptographically secure, suitable for use e.g. in authentication.
83 *
84 * We rely on system facilities for actually generating the numbers.
85 * We support a number of sources:
86 *
87 * 1. OpenSSL's RAND_bytes()
88 * 2. Windows' CryptGenRandom() function
89 * 3. /dev/urandom
90 *
91 * The configure script will choose which one to use, and set
92 * a USE_*_RANDOM flag accordingly.
93 *
94 * Returns true on success, and false if none of the sources
95 * were available. NB: It is important to check the return value!
96 * Proceeding with key generation when no random data was available
97 * would lead to predictable keys and security issues.
98 */
99 bool
pg_strong_random(void * buf,size_t len)100 pg_strong_random(void *buf, size_t len)
101 {
102 /*
103 * When built with OpenSSL, use OpenSSL's RAND_bytes function.
104 */
105 #if defined(USE_OPENSSL_RANDOM)
106 int i;
107
108 /*
109 * Check that OpenSSL's CSPRNG has been sufficiently seeded, and if not
110 * add more seed data using RAND_poll(). With some older versions of
111 * OpenSSL, it may be necessary to call RAND_poll() a number of times.
112 */
113 #define NUM_RAND_POLL_RETRIES 8
114
115 for (i = 0; i < NUM_RAND_POLL_RETRIES; i++)
116 {
117 if (RAND_status() == 1)
118 {
119 /* The CSPRNG is sufficiently seeded */
120 break;
121 }
122
123 if (RAND_poll() == 0)
124 {
125 /*
126 * RAND_poll() failed to generate any seed data, which means that
127 * RAND_bytes() will probably fail. For now, just fall through
128 * and let that happen. XXX: maybe we could seed it some other
129 * way.
130 */
131 break;
132 }
133 }
134
135 if (RAND_bytes(buf, len) == 1)
136 return true;
137 return false;
138
139 /*
140 * Windows has CryptoAPI for strong cryptographic numbers.
141 */
142 #elif defined(USE_WIN32_RANDOM)
143 if (hProvider == 0)
144 {
145 if (!CryptAcquireContext(&hProvider,
146 NULL,
147 MS_DEF_PROV,
148 PROV_RSA_FULL,
149 CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
150 {
151 /*
152 * On failure, set back to 0 in case the value was for some reason
153 * modified.
154 */
155 hProvider = 0;
156 }
157 }
158 /* Re-check in case we just retrieved the provider */
159 if (hProvider != 0)
160 {
161 if (CryptGenRandom(hProvider, len, buf))
162 return true;
163 }
164 return false;
165
166 /*
167 * Read /dev/urandom ourselves.
168 */
169 #elif defined(USE_DEV_URANDOM)
170 if (random_from_file("/dev/urandom", buf, len))
171 return true;
172 return false;
173
174 #else
175 /* The autoconf script should not have allowed this */
176 #error no source of random numbers configured
177 #endif
178 }
179