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 <stdio.h>
  13 #include <stdlib.h>
  14 #include <string.h>
  15 #include <time.h>
  16 #include <unistd.h>
  17
  18 #ifdef USE_OPENSSL
  19 #include <openssl/rand.h>
  20 #endif
  21 #ifdef HAVE_LINUX_RANDOM_H
  22 #include <sys/syscall.h>
  23 #include <linux/random.h>
  24 #endif
  25 #ifdef RNDGETENTCNT
  26 #include <sys/ioctl.h>
  27 #endif
  28
  29 /* Take some reasonable steps to attempt to *seed* random number generators with
  30  * cryptographically random data.  Some of these initialization routines may
  31  * block, and are intended to be called only at startup in lighttpd, or
  32  * immediately after fork() to start lighttpd workers.
  33  *
  34  * Note: results from li_rand() are not necessarily cryptographically random.
  35  *
  36  * https://wiki.openssl.org/index.php/Random_Numbers
  37  * https://wiki.openssl.org/index.php/Random_fork-safety
  38  *
  39  * openssl random number generators are not thread-safe by default
  40  * https://wiki.openssl.org/index.php/Manual:Threads(3)
  41  *
  42  * RFE: add more paranoid checks from the following to improve confidence:
  43  * http://insanecoding.blogspot.co.uk/2014/05/a-good-idea-with-bad-usage-devurandom.html
  44  * RFE: retry on EINTR
  45  * RFE: check RAND_status()
  46  */
  47
  48 static int li_getentropy (void *buf, size_t buflen)
  49 {
  50   #ifdef HAVE_GETENTROPY
  51     return getentropy(buf, buflen);
  52   #else
  53     /*(see NOTES section in 'man getrandom' on Linux)*/
  54    #if defined(HAVE_GETRANDOM) || defined(SYS_getrandom)
  55     if (buflen <= 256) {
  56       #ifdef HAVE_GETRANDOM /*(not implemented in glibc yet)*/
  57         int num = getrandom(buf, buflen, 0);
  58       #elif defined(SYS_getrandom)
  59         /* https://lwn.net/Articles/605828/ */
  60         /* https://bbs.archlinux.org/viewtopic.php?id=200039 */
  61         int num = (int)syscall(SYS_getrandom, buf, buflen, 0);
  62       #endif
  63         if (num == (int)buflen) return 0;
  64         if (num < 0)            return num; /* -1 */
  65     }
  66    #else
  67     UNUSED(buf);
  68     UNUSED(buflen);
  69    #endif
  70     errno = EIO;
  71     return -1;
  72   #endif
  73 }
  74
  75 static int li_rand_device_bytes (unsigned char *buf, int num)
  76 {
  77     /* randomness from these devices is cryptographically strong,
  78      * unless /dev/urandom is low on entropy */
  79
  80     static const char * const devices[] = {
  81       #ifdef __OpenBSD__
  82         "/dev/arandom",
  83       #endif
  84         "/dev/urandom",
  85         "/dev/random"
  86     };
  87
  88     /* device files might not be available in chroot environment,
  89      * so prefer syscall, if available */
  90     if (0 == li_getentropy(buf, (size_t)num)) return 1;
  91
  92     for (unsigned int u = 0; u < sizeof(devices)/sizeof(devices[0]); ++u) {
  93         /*(some systems might have symlink to another device; omit O_NOFOLLOW)*/
  94         int fd = fdevent_open_cloexec(devices[u], O_RDONLY, 0);
  95         if (fd >= 0) {
  96             ssize_t rd = 0;
  97           #ifdef RNDGETENTCNT
  98             int entropy;
  99             if (0 == ioctl(fd, RNDGETENTCNT, &entropy) && entropy >= num*8)
 100           #endif
 101                 rd = read(fd, buf, (size_t)num);
 102             close(fd);
 103             if (rd == num) {
 104                 return 1;
 105             }
 106         }
 107     }
 108
 109     return 0;
 110 }
 111
 112 static unsigned short xsubi[3];
 113
 114 void li_rand_reseed (void)
 115 {
 116     /* (intended to be called at init and after fork() in order to re-seed PRNG
 117      *  so that forked children, grandchildren, etc do not share PRNG seed)
 118      * https://github.com/ramsey/uuid/issues/80
 119      * https://www.agwa.name/blog/post/libressls_prng_is_unsafe_on_linux
 120      *   (issue in early version of libressl has since been fixed)
 121      * https://github.com/libressl-portable/portable/commit/32d9eeeecf4e951e1566d5f4a42b36ea37b60f35
 122      */
 123     unsigned int u;
 124     if (1 == li_rand_device_bytes((unsigned char *)xsubi, (int)sizeof(xsubi))) {
 125         u = ((unsigned int)xsubi[0] << 16) | xsubi[1];
 126         srand(u); /*(initialize just in case rand() used elsewhere)*/
 127     }
 128     else {
 129       #ifdef HAVE_ARC4RANDOM
 130         srand(arc4random()); /*(initialize just in case rand() used elsewhere)*/
 131         arc4random_buf(xsubi, sizeof(xsubi));
 132       #else
 133         /* NOTE: not cryptographically random !!! */
 134         srand((unsigned int)(time(NULL) ^ getpid()));
 135         for (u = 0; u < sizeof(unsigned short); ++u)
 136             xsubi[u] = (unsigned short)(rand() & 0xFFFF);
 137       #endif
 138     }
 139   #ifdef USE_OPENSSL
 140     RAND_poll();
 141     RAND_seed(xsubi, (int)sizeof(xsubi));
 142   #endif
 143 }
 144
 145 int li_rand (void)
 146 {
 147     /* randomness *is not* cryptographically strong */
 148     /* (attempt to use better mechanisms to replace the more portable rand()) */
 149   #ifdef USE_OPENSSL
 150     int i;
 151     if (-1 != RAND_pseudo_bytes((unsigned char *)&i, sizeof(i))) return i;
 152   #endif
 153   #ifdef HAVE_ARC4RANDOM
 154     return (int)arc4random();
 155   #endif
 156   #ifdef HAVE_JRAND48
 157     /*(FYI: jrand48() reentrant, but use of file-scoped static xsubi[] is not)*/
 158     return (int)jrand48(xsubi);
 159   #else
 160     return rand();
 161   #endif
 162 }
 163
 164 int li_rand_bytes (unsigned char *buf, int num)
 165 {
 166   #ifdef USE_OPENSSL
 167     int rc = RAND_bytes(buf, num);
 168     if (-1 != rc) {
 169         return rc;
 170     }
 171   #endif
 172     if (1 == li_rand_device_bytes(buf, num)) {
 173         return 1;
 174     }
 175     else {
 176         /* NOTE: not cryptographically random !!! */
 177         for (int i = 0; i < num; ++i)
 178             buf[i] = li_rand() & 0xFF;
 179         /*(openssl RAND_pseudo_bytes rc for non-cryptographically random data)*/
 180         return 0;
 181     }
 182 }
 183
 184 void li_rand_cleanup (void)
 185 {
 186   #ifdef USE_OPENSSL
 187     RAND_cleanup();
 188   #endif
 189     safe_memclear(xsubi, sizeof(xsubi));
 190 }