res_srtp.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2005, Mikael Magnusson
 *
 * Mikael Magnusson <mikma@users.sourceforge.net>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 *
 * Builds on libSRTP http://srtp.sourceforge.net
 */

/*! \file res_srtp.c
 *
 * \brief Secure RTP (SRTP)
 *
 * Secure RTP (SRTP)
 * Specified in RFC 3711.
 *
 * \author Mikael Magnusson <mikma@users.sourceforge.net>
 */

/*** MODULEINFO
   <depend>srtp</depend>
   <support_level>core</support_level>
***/

/* See https://wiki.asterisk.org/wiki/display/AST/Secure+Calling */

#include "asterisk.h"

ASTERISK_FILE_VERSION(__FILE__, "$Revision: 356650 $")

#include <srtp/srtp.h>

#include "asterisk/lock.h"
#include "asterisk/sched.h"
#include "asterisk/module.h"
#include "asterisk/options.h"
#include "asterisk/rtp_engine.h"
#include "asterisk/astobj2.h"

struct ast_srtp {
   struct ast_rtp_instance *rtp;
   struct ao2_container *policies;
   srtp_t session;
   const struct ast_srtp_cb *cb;
   void *data;
   int warned;
   unsigned char buf[8192 + AST_FRIENDLY_OFFSET];
   unsigned char rtcpbuf[8192 + AST_FRIENDLY_OFFSET];
};

struct ast_srtp_policy {
   srtp_policy_t sp;
};

/*! Tracks whether or not we've initialized the libsrtp library */
static int g_initialized = 0;

/* SRTP functions */
static int ast_srtp_create(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy);
static int ast_srtp_replace(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy);
static void ast_srtp_destroy(struct ast_srtp *srtp);
static int ast_srtp_add_stream(struct ast_srtp *srtp, struct ast_srtp_policy *policy);
static int ast_srtp_change_source(struct ast_srtp *srtp, unsigned int from_ssrc, unsigned int to_ssrc);

static int ast_srtp_unprotect(struct ast_srtp *srtp, void *buf, int *len, int rtcp);
static int ast_srtp_protect(struct ast_srtp *srtp, void **buf, int *len, int rtcp);
static void ast_srtp_set_cb(struct ast_srtp *srtp, const struct ast_srtp_cb *cb, void *data);
static int ast_srtp_get_random(unsigned char *key, size_t len);

/* Policy functions */
static struct ast_srtp_policy *ast_srtp_policy_alloc(void);
static void ast_srtp_policy_destroy(struct ast_srtp_policy *policy);
static int ast_srtp_policy_set_suite(struct ast_srtp_policy *policy, enum ast_srtp_suite suite);
static int ast_srtp_policy_set_master_key(struct ast_srtp_policy *policy, const unsigned char *key, size_t key_len, const unsigned char *salt, size_t salt_len);
static void ast_srtp_policy_set_ssrc(struct ast_srtp_policy *policy, unsigned long ssrc, int inbound);

static struct ast_srtp_res srtp_res = {
   .create = ast_srtp_create,
   .replace = ast_srtp_replace,
   .destroy = ast_srtp_destroy,
   .add_stream = ast_srtp_add_stream,
   .change_source = ast_srtp_change_source,
   .set_cb = ast_srtp_set_cb,
   .unprotect = ast_srtp_unprotect,
   .protect = ast_srtp_protect,
   .get_random = ast_srtp_get_random
};

static struct ast_srtp_policy_res policy_res = {
   .alloc = ast_srtp_policy_alloc,
   .destroy = ast_srtp_policy_destroy,
   .set_suite = ast_srtp_policy_set_suite,
   .set_master_key = ast_srtp_policy_set_master_key,
   .set_ssrc = ast_srtp_policy_set_ssrc
};

static const char *srtp_errstr(int err)
{
   switch(err) {
   case err_status_ok:
      return "nothing to report";
   case err_status_fail:
      return "unspecified failure";
   case err_status_bad_param:
      return "unsupported parameter";
   case err_status_alloc_fail:
      return "couldn't allocate memory";
   case err_status_dealloc_fail:
      return "couldn't deallocate properly";
   case err_status_init_fail:
      return "couldn't initialize";
   case err_status_terminus:
      return "can't process as much data as requested";
   case err_status_auth_fail:
      return "authentication failure";
   case err_status_cipher_fail:
      return "cipher failure";
   case err_status_replay_fail:
      return "replay check failed (bad index)";
   case err_status_replay_old:
      return "replay check failed (index too old)";
   case err_status_algo_fail:
      return "algorithm failed test routine";
   case err_status_no_such_op:
      return "unsupported operation";
   case err_status_no_ctx:
      return "no appropriate context found";
   case err_status_cant_check:
      return "unable to perform desired validation";
   case err_status_key_expired:
      return "can't use key any more";
   default:
      return "unknown";
   }
}

static int policy_hash_fn(const void *obj, const int flags)
{
   const struct ast_srtp_policy *policy = obj;

   return policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type;
}

static int policy_cmp_fn(void *obj, void *arg, int flags)
{
   const struct ast_srtp_policy *one = obj, *two = arg;

   return one->sp.ssrc.type == two->sp.ssrc.type && one->sp.ssrc.value == two->sp.ssrc.value;
}

static struct ast_srtp_policy *find_policy(struct ast_srtp *srtp, const srtp_policy_t *policy, int flags)
{
   struct ast_srtp_policy tmp = {
      .sp = {
         .ssrc.type = policy->ssrc.type,
         .ssrc.value = policy->ssrc.value,
      },
   };

   return ao2_t_find(srtp->policies, &tmp, flags, "Looking for policy");
}

static struct ast_srtp *res_srtp_new(void)
{
   struct ast_srtp *srtp;

   if (!(srtp = ast_calloc(1, sizeof(*srtp)))) {
      ast_log(LOG_ERROR, "Unable to allocate memory for srtp\n");
      return NULL;
   }

   if (!(srtp->policies = ao2_t_container_alloc(5, policy_hash_fn, policy_cmp_fn, "SRTP policy container"))) {
      ast_free(srtp);
      return NULL;
   }
   
   srtp->warned = 1;

   return srtp;
}

/*
  struct ast_srtp_policy
*/
static void srtp_event_cb(srtp_event_data_t *data)
{
   switch (data->event) {
   case event_ssrc_collision:
      ast_debug(1, "SSRC collision\n");
      break;
   case event_key_soft_limit:
      ast_debug(1, "event_key_soft_limit\n");
      break;
   case event_key_hard_limit:
      ast_debug(1, "event_key_hard_limit\n");
      break;
   case event_packet_index_limit:
      ast_debug(1, "event_packet_index_limit\n");
      break;
   }
}

static void ast_srtp_policy_set_ssrc(struct ast_srtp_policy *policy,
      unsigned long ssrc, int inbound)
{
   if (ssrc) {
      policy->sp.ssrc.type = ssrc_specific;
      policy->sp.ssrc.value = ssrc;
   } else {
      policy->sp.ssrc.type = inbound ? ssrc_any_inbound : ssrc_any_outbound;
   }
}

static void policy_destructor(void *obj)
{
   struct ast_srtp_policy *policy = obj;

   if (policy->sp.key) {
      ast_free(policy->sp.key);
      policy->sp.key = NULL;
   }
}

static struct ast_srtp_policy *ast_srtp_policy_alloc()
{
   struct ast_srtp_policy *tmp;

   if (!(tmp = ao2_t_alloc(sizeof(*tmp), policy_destructor, "Allocating policy"))) {
      ast_log(LOG_ERROR, "Unable to allocate memory for srtp_policy\n");
   }

   return tmp;
}

static void ast_srtp_policy_destroy(struct ast_srtp_policy *policy)
{
   ao2_t_ref(policy, -1, "Destroying policy");
}

static int policy_set_suite(crypto_policy_t *p, enum ast_srtp_suite suite)
{
   switch (suite) {
   case AST_AES_CM_128_HMAC_SHA1_80:
      p->cipher_type = AES_128_ICM;
      p->cipher_key_len = 30;
      p->auth_type = HMAC_SHA1;
      p->auth_key_len = 20;
      p->auth_tag_len = 10;
      p->sec_serv = sec_serv_conf_and_auth;
      return 0;

   case AST_AES_CM_128_HMAC_SHA1_32:
      p->cipher_type = AES_128_ICM;
      p->cipher_key_len = 30;
      p->auth_type = HMAC_SHA1;
      p->auth_key_len = 20;
      p->auth_tag_len = 4;
      p->sec_serv = sec_serv_conf_and_auth;
      return 0;

   default:
      ast_log(LOG_ERROR, "Invalid crypto suite: %d\n", suite);
      return -1;
   }
}

static int ast_srtp_policy_set_suite(struct ast_srtp_policy *policy, enum ast_srtp_suite suite)
{
   return policy_set_suite(&policy->sp.rtp, suite) | policy_set_suite(&policy->sp.rtcp, suite);
}

static int ast_srtp_policy_set_master_key(struct ast_srtp_policy *policy, const unsigned char *key, size_t key_len, const unsigned char *salt, size_t salt_len)
{
   size_t size = key_len + salt_len;
   unsigned char *master_key;

   if (policy->sp.key) {
      ast_free(policy->sp.key);
      policy->sp.key = NULL;
   }

   if (!(master_key = ast_calloc(1, size))) {
      return -1;
   }

   memcpy(master_key, key, key_len);
   memcpy(master_key + key_len, salt, salt_len);

   policy->sp.key = master_key;

   return 0;
}

static int ast_srtp_get_random(unsigned char *key, size_t len)
{
   return crypto_get_random(key, len) != err_status_ok ? -1: 0;
}

static void ast_srtp_set_cb(struct ast_srtp *srtp, const struct ast_srtp_cb *cb, void *data)
{
   if (!srtp) {
      return;
   }

   srtp->cb = cb;
   srtp->data = data;
}

/* Vtable functions */
static int ast_srtp_unprotect(struct ast_srtp *srtp, void *buf, int *len, int rtcp)
{
   int res = 0;
   int i;
   int retry = 0;
   struct ast_rtp_instance_stats stats = {0,};

tryagain:

   for (i = 0; i < 2; i++) {
      res = rtcp ? srtp_unprotect_rtcp(srtp->session, buf, len) : srtp_unprotect(srtp->session, buf, len);
      if (res != err_status_no_ctx) {
         break;
      }

      if (srtp->cb && srtp->cb->no_ctx) {
         if (ast_rtp_instance_get_stats(srtp->rtp, &stats, AST_RTP_INSTANCE_STAT_REMOTE_SSRC)) {
            break;
         }
         if (srtp->cb->no_ctx(srtp->rtp, stats.remote_ssrc, srtp->data) < 0) {
            break;
         }
      } else {
         break;
      }
   }

   if (retry == 0  && res == err_status_replay_old) {
      ast_log(AST_LOG_NOTICE, "SRTP unprotect failed with %s, retrying\n", srtp_errstr(res));

      if (srtp->session) {
         struct ast_srtp_policy *policy;
         struct ao2_iterator it;
         int policies_count;

         /* dealloc first */
         ast_debug(5, "SRTP destroy before re-create\n");
         srtp_dealloc(srtp->session);

         /* get the count */
         policies_count = ao2_container_count(srtp->policies);

         /* get the first to build up */
         it = ao2_iterator_init(srtp->policies, 0);
         policy = ao2_iterator_next(&it);

         ast_debug(5, "SRTP try to re-create\n");
         if (policy) {
            if (srtp_create(&srtp->session, &policy->sp) == err_status_ok) {
               ast_debug(5, "SRTP re-created with first policy\n");
               ao2_t_ref(policy, -1, "Unreffing first policy for re-creating srtp session");

               /* if we have more than one policy, add them */
               if (policies_count > 1) {
                  ast_debug(5, "Add all the other %d policies\n",
                     policies_count - 1);
                  while ((policy = ao2_iterator_next(&it))) {
                     srtp_add_stream(srtp->session, &policy->sp);
                     ao2_t_ref(policy, -1, "Unreffing n-th policy for re-creating srtp session");
                  }
               }

               retry++;
               ao2_iterator_destroy(&it);
               goto tryagain;
            }
            ao2_t_ref(policy, -1, "Unreffing first policy after srtp_create failed");
         }
         ao2_iterator_destroy(&it);
      }
   }

   if (res != err_status_ok && res != err_status_replay_fail ) {
      if ((srtp->warned >= 10) && !((srtp->warned - 10) % 100)) {
         ast_log(AST_LOG_WARNING, "SRTP unprotect failed with: %s %d\n", srtp_errstr(res), srtp->warned);
         srtp->warned = 11;
      } else {
         srtp->warned++;
      }
      errno = EAGAIN;
      return -1;
   }

   return *len;
}

static int ast_srtp_protect(struct ast_srtp *srtp, void **buf, int *len, int rtcp)
{
   int res;
   unsigned char *localbuf;

   if ((*len + SRTP_MAX_TRAILER_LEN) > sizeof(srtp->buf)) {
      return -1;
   }
   
   localbuf = rtcp ? srtp->rtcpbuf : srtp->buf;

   memcpy(localbuf, *buf, *len);

   if ((res = rtcp ? srtp_protect_rtcp(srtp->session, localbuf, len) : srtp_protect(srtp->session, localbuf, len)) != err_status_ok && res != err_status_replay_fail) {
      ast_log(LOG_WARNING, "SRTP protect: %s\n", srtp_errstr(res));
      return -1;
   }

   *buf = localbuf;
   return *len;
}

static int ast_srtp_create(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy)
{
   struct ast_srtp *temp;

   if (!(temp = res_srtp_new())) {
      return -1;
   }

   if (srtp_create(&temp->session, &policy->sp) != err_status_ok) {
      return -1;
   }

   ast_module_ref(ast_module_info->self);
   temp->rtp = rtp;
   *srtp = temp;

   ao2_t_link((*srtp)->policies, policy, "Created initial policy");

   return 0;
}

static int ast_srtp_replace(struct ast_srtp **srtp, struct ast_rtp_instance *rtp, struct ast_srtp_policy *policy)
{
   if ((*srtp) != NULL) {
      ast_srtp_destroy(*srtp);
   }
   return ast_srtp_create(srtp, rtp, policy);
}

static void ast_srtp_destroy(struct ast_srtp *srtp)
{
   if (srtp->session) {
      srtp_dealloc(srtp->session);
   }

   ao2_t_callback(srtp->policies, OBJ_UNLINK | OBJ_NODATA | OBJ_MULTIPLE, NULL, NULL, "Unallocate policy");
   ao2_t_ref(srtp->policies, -1, "Destroying container");

   ast_free(srtp);
   ast_module_unref(ast_module_info->self);
}

static int ast_srtp_add_stream(struct ast_srtp *srtp, struct ast_srtp_policy *policy)
{
   struct ast_srtp_policy *match;

   /* For existing streams, replace if its an SSRC stream, or bail if its a wildcard */
   if ((match = find_policy(srtp, &policy->sp, OBJ_POINTER))) {
      if (policy->sp.ssrc.type != ssrc_specific) {
         ast_log(AST_LOG_WARNING, "Cannot replace an existing wildcard policy");
         ao2_t_ref(match, -1, "Unreffing already existing policy");
         return -1;
      } else {
         if (srtp_remove_stream(srtp->session, match->sp.ssrc.value) != err_status_ok) {
            ast_log(AST_LOG_WARNING, "Failed to remove SRTP stream for SSRC %d\n", match->sp.ssrc.value);
         }
         ao2_t_unlink(srtp->policies, match, "Remove existing match policy");
         ao2_t_ref(match, -1, "Unreffing already existing policy");
      }
   }

   ast_debug(3, "Adding new policy for %s %d\n",
      policy->sp.ssrc.type == ssrc_specific ? "SSRC" : "type",
      policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type);
   if (srtp_add_stream(srtp->session, &policy->sp) != err_status_ok) {
      ast_log(AST_LOG_WARNING, "Failed to add SRTP stream for %s %d\n",
         policy->sp.ssrc.type == ssrc_specific ? "SSRC" : "type",
         policy->sp.ssrc.type == ssrc_specific ? policy->sp.ssrc.value : policy->sp.ssrc.type);
      return -1;
   }

   ao2_t_link(srtp->policies, policy, "Added additional stream");

   return 0;
}

static int ast_srtp_change_source(struct ast_srtp *srtp, unsigned int from_ssrc, unsigned int to_ssrc)
{
   struct ast_srtp_policy *match;
   struct srtp_policy_t sp = {
      .ssrc.type = ssrc_specific,
      .ssrc.value = from_ssrc,
   };
   err_status_t status;

   /* If we find a match, return and unlink it from the container so we
    * can change the SSRC (which is part of the hash) and then have
    * ast_srtp_add_stream link it back in if all is well */
   if ((match = find_policy(srtp, &sp, OBJ_POINTER | OBJ_UNLINK))) {
      match->sp.ssrc.value = to_ssrc;
      if (ast_srtp_add_stream(srtp, match)) {
         ast_log(LOG_WARNING, "Couldn't add stream\n");
      } else if ((status = srtp_remove_stream(srtp->session, from_ssrc))) {
         ast_debug(3, "Couldn't remove stream (%d)\n", status);
      }
      ao2_t_ref(match, -1, "Unreffing found policy in change_source");
   }

   return 0;
}

static void res_srtp_shutdown(void)
{
   srtp_install_event_handler(NULL);
   ast_rtp_engine_unregister_srtp();
   g_initialized = 0;
}

static int res_srtp_init(void)
{
   if (g_initialized) {
      return 0;
   }

   if (srtp_init() != err_status_ok) {
      ast_log(AST_LOG_WARNING, "Failed to initialize libsrtp\n");
      return -1;
   }

   srtp_install_event_handler(srtp_event_cb);

   if (ast_rtp_engine_register_srtp(&srtp_res, &policy_res)) {
      ast_log(AST_LOG_WARNING, "Failed to register SRTP with rtp engine\n");
      res_srtp_shutdown();
      return -1;
   }

   g_initialized = 1;
   return 0;
}

/*
 * Exported functions
 */

static int load_module(void)
{
   return res_srtp_init();
}

static int unload_module(void)
{
   res_srtp_shutdown();
   return 0;
}

AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_GLOBAL_SYMBOLS | AST_MODFLAG_LOAD_ORDER, "Secure RTP (SRTP)",
   .load = load_module,
   .unload = unload_module,
   .load_pri = AST_MODPRI_CHANNEL_DEPEND,
);