hs_dos.c

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/* Copyright (c) 2019-2021, The Tor Project, Inc. */
/* See LICENSE for licensing information */
 
/**
 * \file hs_dos.c
 * \brief Implement denial of service mitigation for the onion service
 *        subsystem.
 *
 * This module defenses:
 *
 * - Introduction Rate Limiting: If enabled by the consensus, an introduction
 *   point will rate limit client introduction towards the service (INTRODUCE2
 *   cells). It uses a token bucket model with a rate and burst per second.
 *
 *   Proposal 305 will expand this module by allowing an operator to define
 *   these values into the ESTABLISH_INTRO cell. Not yet implemented.
 **/
 
#define HS_DOS_PRIVATE
 
#include "core/or/or.h"
#include "app/config/config.h"
 
#include "core/or/circuitlist.h"
 
#include "feature/hs/hs_circuitmap.h"
#include "feature/nodelist/networkstatus.h"
#include "feature/relay/routermode.h"
 
#include "lib/evloop/token_bucket.h"
 
#include "feature/hs/hs_dos.h"
 
/** Default value of the allowed INTRODUCE2 cell rate per second. Above that
 * value per second, the introduction is denied. */
#define HS_DOS_INTRODUCE_DEFAULT_CELL_RATE_PER_SEC 25
 
/** Default value of the allowed INTRODUCE2 cell burst per second. This is the
 * maximum value a token bucket has per second. We thus allow up to this value
 * of INTRODUCE2 cell per second but the bucket is refilled by the rate value
 * but never goes above that burst value. */
#define HS_DOS_INTRODUCE_DEFAULT_CELL_BURST_PER_SEC 200
 
/** Default value of the consensus parameter enabling or disabling the
 * introduction DoS defense. Disabled by default. */
#define HS_DOS_INTRODUCE_ENABLED_DEFAULT 0
 
/** INTRODUCE2 rejected request counter. */
static uint64_t intro2_rejected_count = 0;
 
/* Consensus parameters. The ESTABLISH_INTRO DoS cell extension have higher
 * priority than these values. If no extension is sent, these are used only by
 * the introduction point. */
static uint32_t consensus_param_introduce_rate_per_sec =
  HS_DOS_INTRODUCE_DEFAULT_CELL_RATE_PER_SEC;
static uint32_t consensus_param_introduce_burst_per_sec =
  HS_DOS_INTRODUCE_DEFAULT_CELL_BURST_PER_SEC;
static uint32_t consensus_param_introduce_defense_enabled =
  HS_DOS_INTRODUCE_ENABLED_DEFAULT;
 
STATIC uint32_t
get_intro2_enable_consensus_param(const networkstatus_t *ns)
{
  return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSDefense",
                                 HS_DOS_INTRODUCE_ENABLED_DEFAULT, 0, 1);
}
 
/** Return the parameter for the introduction rate per sec. */
STATIC uint32_t
get_intro2_rate_consensus_param(const networkstatus_t *ns)
{
  return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSRatePerSec",
                                 HS_DOS_INTRODUCE_DEFAULT_CELL_RATE_PER_SEC,
                                 0, INT32_MAX);
}
 
/** Return the parameter for the introduction burst per sec. */
STATIC uint32_t
get_intro2_burst_consensus_param(const networkstatus_t *ns)
{
  return networkstatus_get_param(ns, "HiddenServiceEnableIntroDoSBurstPerSec",
                                 HS_DOS_INTRODUCE_DEFAULT_CELL_BURST_PER_SEC,
                                 0, INT32_MAX);
}
 
/** Go over all introduction circuit relay side and adjust their rate/burst
 * values using the global parameters. This is called right after the
 * consensus parameters might have changed. */
static void
update_intro_circuits(void)
{
  /* Returns all HS version intro circuits. */
  smartlist_t *intro_circs = hs_circuitmap_get_all_intro_circ_relay_side();
 
  SMARTLIST_FOREACH_BEGIN(intro_circs, circuit_t *, circ) {
    /* Ignore circuit if the defenses were set explicitly through the
     * ESTABLISH_INTRO cell DoS extension. */
    if (TO_OR_CIRCUIT(circ)->introduce2_dos_defense_explicit) {
      continue;
    }
    /* Defenses might have been enabled or disabled. */
    TO_OR_CIRCUIT(circ)->introduce2_dos_defense_enabled =
      consensus_param_introduce_defense_enabled;
    /* Adjust the rate/burst value that might have changed. */
    token_bucket_ctr_adjust(&TO_OR_CIRCUIT(circ)->introduce2_bucket,
                            consensus_param_introduce_rate_per_sec,
                            consensus_param_introduce_burst_per_sec);
  } SMARTLIST_FOREACH_END(circ);
 
  smartlist_free(intro_circs);
}
 
/** Set consensus parameters. */
static void
set_consensus_parameters(const networkstatus_t *ns)
{
  consensus_param_introduce_rate_per_sec =
    get_intro2_rate_consensus_param(ns);
  consensus_param_introduce_burst_per_sec =
    get_intro2_burst_consensus_param(ns);
  consensus_param_introduce_defense_enabled =
    get_intro2_enable_consensus_param(ns);
 
  /* The above might have changed which means we need to go through all
   * introduction circuits (relay side) and update the token buckets. */
  update_intro_circuits();
}
 
/*
 * Public API.
 */
 
/** Initialize the INTRODUCE2 token bucket for the DoS defenses using the
 * consensus/default values. We might get a cell extension that changes those
 * later but if we don't, the default or consensus parameters are used. */
void
hs_dos_setup_default_intro2_defenses(or_circuit_t *circ)
{
  tor_assert(circ);
 
  circ->introduce2_dos_defense_enabled =
    consensus_param_introduce_defense_enabled;
  token_bucket_ctr_init(&circ->introduce2_bucket,
                        consensus_param_introduce_rate_per_sec,
                        consensus_param_introduce_burst_per_sec,
                        (uint32_t) approx_time());
}
 
/** Called when the consensus has changed. We might have new consensus
 * parameters to look at. */
void
hs_dos_consensus_has_changed(const networkstatus_t *ns)
{
  /* No point on updating these values if we are not a public relay that can
   * be picked to be an introduction point. */
  if (!public_server_mode(get_options())) {
    return;
  }
 
  set_consensus_parameters(ns);
}
 
/** Return true iff an INTRODUCE2 cell can be sent on the given service
 * introduction circuit. */
bool
hs_dos_can_send_intro2(or_circuit_t *s_intro_circ)
{
  tor_assert(s_intro_circ);
 
  /* Allow to send the cell if the DoS defenses are disabled on the circuit.
   * This can be set by the consensus, the ESTABLISH_INTRO cell extension or
   * the hardcoded values in tor code. */
  if (!s_intro_circ->introduce2_dos_defense_enabled) {
    goto allow;
  }
 
  /* Should not happen but if so, scream loudly. */
  if (BUG(TO_CIRCUIT(s_intro_circ)->purpose != CIRCUIT_PURPOSE_INTRO_POINT)) {
    goto disallow;
  }
 
  /* This is called just after we got a valid and parsed INTRODUCE1 cell. The
   * service has been found and we have its introduction circuit.
   *
   * First, the INTRODUCE2 bucket will be refilled (if any). Then, decremented
   * because we are about to send or not the cell we just got. Finally,
   * evaluate if we can send it based on our token bucket state. */
 
  /* Refill INTRODUCE2 bucket. */
  token_bucket_ctr_refill(&s_intro_circ->introduce2_bucket,
                          (uint32_t) approx_time());
 
  /* Decrement the bucket for this valid INTRODUCE1 cell we just got. Don't
   * underflow else we end up with a too big of a bucket. */
  if (token_bucket_ctr_get(&s_intro_circ->introduce2_bucket) > 0) {
    token_bucket_ctr_dec(&s_intro_circ->introduce2_bucket, 1);
  }
 
  /* Finally, we can send a new INTRODUCE2 if there are still tokens. */
  if (token_bucket_ctr_get(&s_intro_circ->introduce2_bucket) > 0) {
    goto allow;
  }
 
  /* If we reach this point, then it means the bucket has reached zero, and
     we're going to disallow. */
 
 disallow:
  /* Increment stats counter, we are rejecting the INTRO2 cell. */
  intro2_rejected_count++;
  return false;
 
 allow:
  return true;
}
 
/** Return rolling count of rejected INTRO2. */
uint64_t
hs_dos_get_intro2_rejected_count(void)
{
  return intro2_rejected_count;
}
 
/** Initialize the onion service Denial of Service subsystem. */
void
hs_dos_init(void)
{
  set_consensus_parameters(NULL);
}