src/rand.c

   1 #include "first.h"
   2
   3 #include "rand.h"
   4 #include "base.h"
   5 #include "fdevent.h"
   6 #include "safe_memclear.h"
   7
   8 #include <sys/types.h>
   9 #include <sys/stat.h>
  10 #include <errno.h>
  11 #include <fcntl.h>
  12 #include <stdlib.h>
  13 #include <string.h>
  14 #include <time.h>
  15 #include <unistd.h>
  16
  17 #include "sys-crypto.h"
  18 #ifdef USE_OPENSSL_CRYPTO
  19 #include <openssl/opensslv.h> /* OPENSSL_VERSION_NUMBER */
  20 #include <openssl/rand.h>
  21 #endif
  22 #ifdef HAVE_GETENTROPY
  23 #include <sys/random.h>
  24 #endif
  25 #ifdef HAVE_LINUX_RANDOM_H
  26 #include <sys/syscall.h>
  27 #include <linux/random.h>
  28 #endif
  29 #ifdef RNDGETENTCNT
  30 #include <sys/ioctl.h>
  31 #endif
  32
  33 /* Take some reasonable steps to attempt to *seed* random number generators with
  34  * cryptographically random data.  Some of these initialization routines may
  35  * block, and are intended to be called only at startup in lighttpd, or
  36  * immediately after fork() to start lighttpd workers.
  37  *
  38  * Update: li_rand_init() is now deferred until first use so that installations
  39  * that do not use modules which use these routines do need to potentially block
  40  * at startup.  Current use by core lighttpd modules is in mod_auth HTTP Digest
  41  * auth and in mod_usertrack.  Deferring collection of random data until first
  42  * use may allow sufficient entropy to be collected by kernel before first use,
  43  * helping reduce or avoid situations in low-entropy-generating embedded devices
  44  * which might otherwise block lighttpd for minutes at device startup.
  45  * Further discussion in https://redmine.lighttpd.net/boards/2/topics/6981
  46  *
  47  * Note: results from li_rand_pseudo_bytes() are not necessarily
  48  * cryptographically random and must not be used for purposes such
  49  * as key generation which require cryptographic randomness.
  50  *
  51  * https://wiki.openssl.org/index.php/Random_Numbers
  52  * https://wiki.openssl.org/index.php/Random_fork-safety
  53  *
  54  * openssl random number generators are not thread-safe by default
  55  * https://wiki.openssl.org/index.php/Manual:Threads(3)
  56  *
  57  * RFE: add more paranoid checks from the following to improve confidence:
  58  * http://insanecoding.blogspot.co.uk/2014/05/a-good-idea-with-bad-usage-devurandom.html
  59  * RFE: retry on EINTR
  60  * RFE: check RAND_status()
  61  */
  62
  63 static int li_getentropy (void *buf, size_t buflen)
  64 {
  65   #ifdef HAVE_GETENTROPY
  66     return getentropy(buf, buflen);
  67   #else
  68     /*(see NOTES section in 'man getrandom' on Linux)*/
  69    #if defined(HAVE_GETRANDOM) || defined(SYS_getrandom)
  70     if (buflen <= 256) {
  71       #ifdef HAVE_GETRANDOM /*(not implemented in glibc yet)*/
  72         int num = getrandom(buf, buflen, 0);
  73       #elif defined(SYS_getrandom)
  74         /* https://lwn.net/Articles/605828/ */
  75         /* https://bbs.archlinux.org/viewtopic.php?id=200039 */
  76         int num = (int)syscall(SYS_getrandom, buf, buflen, 0);
  77       #endif
  78         if (num == (int)buflen) return 0;
  79         if (num < 0)            return num; /* -1 */
  80     }
  81    #else
  82     UNUSED(buf);
  83     UNUSED(buflen);
  84    #endif
  85     errno = EIO;
  86     return -1;
  87   #endif
  88 }
  89
  90 static int li_rand_device_bytes (unsigned char *buf, int num)
  91 {
  92     /* randomness from these devices is cryptographically strong,
  93      * unless /dev/urandom is low on entropy */
  94
  95     static const char * const devices[] = {
  96       #ifdef __OpenBSD__
  97         "/dev/arandom",
  98       #endif
  99         "/dev/urandom",
 100         "/dev/random"
 101     };
 102
 103     /* device files might not be available in chroot environment,
 104      * so prefer syscall, if available */
 105     if (0 == li_getentropy(buf, (size_t)num)) return 1;
 106
 107     for (unsigned int u = 0; u < sizeof(devices)/sizeof(devices[0]); ++u) {
 108         /*(some systems might have symlink to another device; omit O_NOFOLLOW)*/
 109         int fd = fdevent_open_cloexec(devices[u], 0, O_RDONLY, 0);
 110         if (fd >= 0) {
 111             ssize_t rd = 0;
 112           #ifdef RNDGETENTCNT
 113             int entropy;
 114             if (0 == ioctl(fd, (unsigned long)(RNDGETENTCNT), &entropy)
 115                 && entropy >= num*8)
 116           #endif
 117                 rd = read(fd, buf, (size_t)num);
 118             close(fd);
 119             if (rd == num) {
 120                 return 1;
 121             }
 122         }
 123     }
 124
 125     return 0;
 126 }
 127
 128 static int li_rand_inited;
 129 static unsigned short xsubi[3];
 130
 131 static void li_rand_init (void)
 132 {
 133     /* (intended to be called at init and after fork() in order to re-seed PRNG
 134      *  so that forked children, grandchildren, etc do not share PRNG seed)
 135      * https://github.com/ramsey/uuid/issues/80
 136      * https://www.agwa.name/blog/post/libressls_prng_is_unsafe_on_linux
 137      *   (issue in early version of libressl has since been fixed)
 138      * https://github.com/libressl-portable/portable/commit/32d9eeeecf4e951e1566d5f4a42b36ea37b60f35
 139      */
 140     unsigned int u;
 141     li_rand_inited = 1;
 142     if (1 == li_rand_device_bytes((unsigned char *)xsubi, (int)sizeof(xsubi))) {
 143         u = ((unsigned int)xsubi[0] << 16) | xsubi[1];
 144     }
 145     else {
 146       #ifdef HAVE_ARC4RANDOM_BUF
 147         u = arc4random();
 148         arc4random_buf(xsubi, sizeof(xsubi));
 149       #else
 150         /* NOTE: not cryptographically random !!! */
 151         srand((unsigned int)(time(NULL) ^ getpid()));
 152         for (u = 0; u < sizeof(unsigned short); ++u)
 153             /* coverity[dont_call : FALSE] */
 154             xsubi[u] = (unsigned short)(rand() & 0xFFFF);
 155         u = ((unsigned int)xsubi[0] << 16) | xsubi[1];
 156       #endif
 157     }
 158     srand(u);   /*(initialize just in case rand() used elsewhere)*/
 159   #ifdef HAVE_SRANDOM
 160     srandom(u); /*(initialize just in case random() used elsewhere)*/
 161   #endif
 162   #ifdef USE_OPENSSL_CRYPTO
 163     RAND_poll();
 164     RAND_seed(xsubi, (int)sizeof(xsubi));
 165   #endif
 166 }
 167
 168 void li_rand_reseed (void)
 169 {
 170     if (li_rand_inited) li_rand_init();
 171 }
 172
 173 int li_rand_pseudo (void)
 174 {
 175     /* randomness *is not* cryptographically strong */
 176     /* (attempt to use better mechanisms to replace the more portable rand()) */
 177   #ifdef USE_OPENSSL_CRYPTO /* (openssl 1.1.0 deprecates RAND_pseudo_bytes()) */
 178   #if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 179     int i;
 180     if (-1 != RAND_pseudo_bytes((unsigned char *)&i, sizeof(i))) return i;
 181   #endif
 182   #endif
 183     if (!li_rand_inited) li_rand_init();
 184   #ifdef HAVE_ARC4RANDOM_BUF
 185     return (int)arc4random();
 186   #elif defined(HAVE_SRANDOM)
 187     /* coverity[dont_call : FALSE] */
 188     return (int)random();
 189   #elif defined(HAVE_JRAND48)
 190     /*(FYI: jrand48() reentrant, but use of file-scoped static xsubi[] is not)*/
 191     /* coverity[dont_call : FALSE] */
 192     return (int)jrand48(xsubi);
 193   #else
 194     /* coverity[dont_call : FALSE] */
 195     return rand();
 196   #endif
 197 }
 198
 199 void li_rand_pseudo_bytes (unsigned char *buf, int num)
 200 {
 201     for (int i = 0; i < num; ++i)
 202         buf[i] = li_rand_pseudo() & 0xFF;
 203 }
 204
 205 int li_rand_bytes (unsigned char *buf, int num)
 206 {
 207   #ifdef USE_OPENSSL_CRYPTO
 208     int rc = RAND_bytes(buf, num);
 209     if (-1 != rc) {
 210         return rc;
 211     }
 212   #endif
 213     if (1 == li_rand_device_bytes(buf, num)) {
 214         return 1;
 215     }
 216     else {
 217         /* NOTE: not cryptographically random !!! */
 218         li_rand_pseudo_bytes(buf, num);
 219         /*(openssl RAND_pseudo_bytes rc for non-cryptographically random data)*/
 220         return 0;
 221     }
 222 }
 223
 224 void li_rand_cleanup (void)
 225 {
 226   #ifdef USE_OPENSSL_CRYPTO
 227   #if OPENSSL_VERSION_NUMBER < 0x10100000L
 228     RAND_cleanup();
 229   #endif
 230   #endif
 231     safe_memclear(xsubi, sizeof(xsubi));
 232 }