src/or/replaycache.c

   1  /* Copyright (c) 2012-2017, The Tor Project, Inc. */
   2  /* See LICENSE for licensing information */
   3
   4 /**
   5  * \file replaycache.c
   6  *
   7  * \brief Self-scrubbing replay cache for rendservice.c
   8  *
   9  * To prevent replay attacks, hidden services need to recognize INTRODUCE2
  10  * cells that they've already seen, and drop them.  If they didn't, then
  11  * sending the same INTRODUCE2 cell over and over would force the hidden
  12  * service to make a huge number of circuits to the same rendezvous
  13  * point, aiding traffic analysis.
  14  *
  15  * (It's not that simple, actually.  We only check for replays in the
  16  * RSA-encrypted portion of the handshake, since the rest of the handshake is
  17  * malleable.)
  18  *
  19  * This module is used from rendservice.c.
  20  */
  21
  22 #define REPLAYCACHE_PRIVATE
  23
  24 #include "or.h"
  25 #include "replaycache.h"
  26
  27 /** Free the replaycache r and all of its entries.
  28  */
  29 void
  30 replaycache_free_(replaycache_t *r)
  31 {
  32   if (!r) {
  33     log_info(LD_BUG, "replaycache_free() called on NULL");
  34     return;
  35   }
  36
  37   if (r->digests_seen) digest256map_free(r->digests_seen, tor_free_);
  38
  39   tor_free(r);
  40 }
  41
  42 /** Allocate a new, empty replay detection cache, where horizon is the time
  43  * for entries to age out and interval is the time after which the cache
  44  * should be scrubbed for old entries.
  45  */
  46 replaycache_t *
  47 replaycache_new(time_t horizon, time_t interval)
  48 {
  49   replaycache_t *r = NULL;
  50
  51   if (horizon < 0) {
  52     log_info(LD_BUG, "replaycache_new() called with negative"
  53         " horizon parameter");
  54     goto err;
  55   }
  56
  57   if (interval < 0) {
  58     log_info(LD_BUG, "replaycache_new() called with negative interval"
  59         " parameter");
  60     interval = 0;
  61   }
  62
  63   r = tor_malloc(sizeof(*r));
  64   r->scrub_interval = interval;
  65   r->scrubbed = 0;
  66   r->horizon = horizon;
  67   r->digests_seen = digest256map_new();
  68
  69  err:
  70   return r;
  71 }
  72
  73 /** See documentation for replaycache_add_and_test().
  74  */
  75 STATIC int
  76 replaycache_add_and_test_internal(
  77     time_t present, replaycache_t *r, const void *data, size_t len,
  78     time_t *elapsed)
  79 {
  80   int rv = 0;
  81   uint8_t digest[DIGEST256_LEN];
  82   time_t *access_time;
  83
  84   /* sanity check */
  85   if (present <= 0 || !r || !data || len == 0) {
  86     log_info(LD_BUG, "replaycache_add_and_test_internal() called with stupid"
  87         " parameters; please fix this.");
  88     goto done;
  89   }
  90
  91   /* compute digest */
  92   crypto_digest256((char *)digest, (const char *)data, len, DIGEST_SHA256);
  93
  94   /* check map */
  95   access_time = digest256map_get(r->digests_seen, digest);
  96
  97   /* seen before? */
  98   if (access_time != NULL) {
  99     /*
 100      * If it's far enough in the past, no hit.  If the horizon is zero, we
 101      * never expire.
 102      */
 103     if (*access_time >= present - r->horizon || r->horizon == 0) {
 104       /* replay cache hit, return 1 */
 105       rv = 1;
 106       /* If we want to output an elapsed time, do so */
 107       if (elapsed) {
 108         if (present >= *access_time) {
 109           *elapsed = present - *access_time;
 110         } else {
 111           /* We shouldn't really be seeing hits from the future, but... */
 112           *elapsed = 0;
 113         }
 114       }
 115     }
 116     /*
 117      * If it's ahead of the cached time, update
 118      */
 119     if (*access_time < present) {
 120       *access_time = present;
 121     }
 122   } else {
 123     /* No, so no hit and update the digest map with the current time */
 124     access_time = tor_malloc(sizeof(*access_time));
 125     *access_time = present;
 126     digest256map_set(r->digests_seen, digest, access_time);
 127   }
 128
 129   /* now scrub the cache if it's time */
 130   replaycache_scrub_if_needed_internal(present, r);
 131
 132  done:
 133   return rv;
 134 }
 135
 136 /** See documentation for replaycache_scrub_if_needed().
 137  */
 138 STATIC void
 139 replaycache_scrub_if_needed_internal(time_t present, replaycache_t *r)
 140 {
 141   digest256map_iter_t *itr = NULL;
 142   const uint8_t *digest;
 143   void *valp;
 144   time_t *access_time;
 145
 146   /* sanity check */
 147   if (!r || !(r->digests_seen)) {
 148     log_info(LD_BUG, "replaycache_scrub_if_needed_internal() called with"
 149         " stupid parameters; please fix this.");
 150     return;
 151   }
 152
 153   /* scrub time yet? (scrubbed == 0 indicates never scrubbed before) */
 154   if (present - r->scrubbed < r->scrub_interval && r->scrubbed > 0) return;
 155
 156   /* if we're never expiring, don't bother scrubbing */
 157   if (r->horizon == 0) return;
 158
 159   /* okay, scrub time */
 160   itr = digest256map_iter_init(r->digests_seen);
 161   while (!digest256map_iter_done(itr)) {
 162     digest256map_iter_get(itr, &digest, &valp);
 163     access_time = (time_t *)valp;
 164     /* aged out yet? */
 165     if (*access_time < present - r->horizon) {
 166       /* Advance the iterator and remove this one */
 167       itr = digest256map_iter_next_rmv(r->digests_seen, itr);
 168       /* Free the value removed */
 169       tor_free(access_time);
 170     } else {
 171       /* Just advance the iterator */
 172       itr = digest256map_iter_next(r->digests_seen, itr);
 173     }
 174   }
 175
 176   /* update scrubbed timestamp */
 177   if (present > r->scrubbed) r->scrubbed = present;
 178 }
 179
 180 /** Test the buffer of length len point to by data against the replay cache r;
 181  * the digest of the buffer will be added to the cache at the current time,
 182  * and the function will return 1 if it was already seen within the cache's
 183  * horizon, or 0 otherwise.
 184  */
 185 int
 186 replaycache_add_and_test(replaycache_t *r, const void *data, size_t len)
 187 {
 188   return replaycache_add_and_test_internal(time(NULL), r, data, len, NULL);
 189 }
 190
 191 /** Like replaycache_add_and_test(), but if it's a hit also return the time
 192  * elapsed since this digest was last seen.
 193  */
 194 int
 195 replaycache_add_test_and_elapsed(
 196     replaycache_t *r, const void *data, size_t len, time_t *elapsed)
 197 {
 198   return replaycache_add_and_test_internal(time(NULL), r, data, len, elapsed);
 199 }
 200
 201 /** Scrub aged entries out of r if sufficiently long has elapsed since r was
 202  * last scrubbed.
 203  */
 204 void
 205 replaycache_scrub_if_needed(replaycache_t *r)
 206 {
 207   replaycache_scrub_if_needed_internal(time(NULL), r);
 208 }
 209