Commit message: snmp plugin: Fix double free of request PDU

snmp_sess_synch_response() always frees request PDU, in both case of request
error and success. If error condition occurs inside of `while (status == 0)`
loop, double free of `req` happens.

Issue: #2291
Signed-off-by: Florian Forster <octo@collectd.org>
CVE: CVE-2017-16820
CWE: CWE-415
Vulnerability description: The csnmp_read_table function in snmp.c in the SNMP plugin in collectd before 5.6.3 is susceptible to a double free in a certain error case, which could lead to a crash (or potentially have other impact).

Project: collectd
File: snmp.c

static int csnmp_read_table(host_definition_t *host, data_definition_t *data) {
  struct snmp_pdu *req;
  struct snmp_pdu *res = NULL;
  struct variable_list *vb;

  const data_set_t *ds;

  size_t oid_list_len = data->values_len + 1;
  /* Holds the last OID returned by the device. We use this in the GETNEXT
   * request to proceed. */
  oid_t oid_list[oid_list_len];
  /* Set to false when an OID has left its subtree so we don't re-request it
   * again. */
  _Bool oid_list_todo[oid_list_len];

  int status;
  size_t i;

  /* `value_list_head' and `value_list_tail' implement a linked list for each
   * value. `instance_list_head' and `instance_list_tail' implement a linked
   * list of
   * instance names. This is used to jump gaps in the table. */
  csnmp_list_instances_t *instance_list_head;
  csnmp_list_instances_t *instance_list_tail;
  csnmp_table_values_t **value_list_head;
  csnmp_table_values_t **value_list_tail;

  DEBUG("snmp plugin: csnmp_read_table (host = %s, data = %s)", host->name,
        data->name);

  if (host->sess_handle == NULL) {
    DEBUG("snmp plugin: csnmp_read_table: host->sess_handle == NULL");
    return (-1);
  }

  ds = plugin_get_ds(data->type);
  if (!ds) {
    ERROR("snmp plugin: DataSet `%s' not defined.", data->type);
    return (-1);
  }

  if (ds->ds_num != data->values_len) {
    ERROR("snmp plugin: DataSet `%s' requires %zu values, but config talks "
          "about %zu",
          data->type, ds->ds_num, data->values_len);
    return (-1);
  }
  assert(data->values_len > 0);

  /* We need a copy of all the OIDs, because GETNEXT will destroy them. */
  memcpy(oid_list, data->values, data->values_len * sizeof(oid_t));
  if (data->instance.oid.oid_len > 0)
    memcpy(oid_list + data->values_len, &data->instance.oid, sizeof(oid_t));
  else /* no InstanceFrom option specified. */
    oid_list_len--;

  for (i = 0; i < oid_list_len; i++)
    oid_list_todo[i] = 1;

  /* We're going to construct n linked lists, one for each "value".
   * value_list_head will contain pointers to the heads of these linked lists,
   * value_list_tail will contain pointers to the tail of the lists. */
  value_list_head = calloc(data->values_len, sizeof(*value_list_head));
  value_list_tail = calloc(data->values_len, sizeof(*value_list_tail));
  if ((value_list_head == NULL) || (value_list_tail == NULL)) {
    ERROR("snmp plugin: csnmp_read_table: calloc failed.");
    sfree(value_list_head);
    sfree(value_list_tail);
    return (-1);
  }

  instance_list_head = NULL;
  instance_list_tail = NULL;

  status = 0;
  while (status == 0) {
    int oid_list_todo_num;

    req = snmp_pdu_create(SNMP_MSG_GETNEXT);
    if (req == NULL) {
      ERROR("snmp plugin: snmp_pdu_create failed.");
      status = -1;
      break;
    }

    oid_list_todo_num = 0;
    for (i = 0; i < oid_list_len; i++) {
      /* Do not rerequest already finished OIDs */
      if (!oid_list_todo[i])
        continue;
      oid_list_todo_num++;
      snmp_add_null_var(req, oid_list[i].oid, oid_list[i].oid_len);
    }

     if (oid_list_todo_num == 0) {
       /* The request is still empty - so we are finished */
       DEBUG("snmp plugin: all variables have left their subtree");
      snmp_free_pdu(req);
       status = 0;
       break;
     }
 
     res = NULL;
    /* snmp_sess_synch_response always frees our req PDU */
     status = snmp_sess_synch_response(host->sess_handle, req, &res);
     if ((status != STAT_SUCCESS) || (res == NULL)) {
       char *errstr = NULL;

      snmp_sess_error(host->sess_handle, NULL, NULL, &errstr);

      c_complain(LOG_ERR, &host->complaint,
                 "snmp plugin: host %s: snmp_sess_synch_response failed: %s",
                 host->name, (errstr == NULL) ? "Unknown problem" : errstr);

      if (res != NULL)
         snmp_free_pdu(res);
       res = NULL;
 
       sfree(errstr);
       csnmp_host_close_session(host);
 
      status = -1;
      break;
    }

    status = 0;
    assert(res != NULL);
    c_release(LOG_INFO, &host->complaint,
              "snmp plugin: host %s: snmp_sess_synch_response successful.",
              host->name);

    vb = res->variables;
    if (vb == NULL) {
      status = -1;
      break;
    }

    for (vb = res->variables, i = 0; (vb != NULL);
         vb = vb->next_variable, i++) {
      /* Calculate value index from todo list */
      while ((i < oid_list_len) && !oid_list_todo[i])
        i++;

      /* An instance is configured and the res variable we process is the
       * instance value (last index) */
      if ((data->instance.oid.oid_len > 0) && (i == data->values_len)) {
        if ((vb->type == SNMP_ENDOFMIBVIEW) ||
            (snmp_oid_ncompare(
                 data->instance.oid.oid, data->instance.oid.oid_len, vb->name,
                 vb->name_length, data->instance.oid.oid_len) != 0)) {
          DEBUG("snmp plugin: host = %s; data = %s; Instance left its subtree.",
                host->name, data->name);
          oid_list_todo[i] = 0;
          continue;
        }

        /* Allocate a new `csnmp_list_instances_t', insert the instance name and
         * add it to the list */
        if (csnmp_instance_list_add(&instance_list_head, &instance_list_tail,
                                    res, host, data) != 0) {
          ERROR("snmp plugin: host %s: csnmp_instance_list_add failed.",
                host->name);
          status = -1;
          break;
        }
      } else /* The variable we are processing is a normal value */
      {
        csnmp_table_values_t *vt;
        oid_t vb_name;
        oid_t suffix;
        int ret;

        csnmp_oid_init(&vb_name, vb->name, vb->name_length);

        /* Calculate the current suffix. This is later used to check that the
         * suffix is increasing. This also checks if we left the subtree */
        ret = csnmp_oid_suffix(&suffix, &vb_name, data->values + i);
        if (ret != 0) {
          DEBUG("snmp plugin: host = %s; data = %s; i = %zu; "
                "Value probably left its subtree.",
                host->name, data->name, i);
          oid_list_todo[i] = 0;
          continue;
        }

        /* Make sure the OIDs returned by the agent are increasing. Otherwise
         * our
         * table matching algorithm will get confused. */
        if ((value_list_tail[i] != NULL) &&
            (csnmp_oid_compare(&suffix, &value_list_tail[i]->suffix) <= 0)) {
          DEBUG("snmp plugin: host = %s; data = %s; i = %zu; "
                "Suffix is not increasing.",
                host->name, data->name, i);
          oid_list_todo[i] = 0;
          continue;
        }

        vt = calloc(1, sizeof(*vt));
        if (vt == NULL) {
          ERROR("snmp plugin: calloc failed.");
          status = -1;
          break;
        }

        vt->value =
            csnmp_value_list_to_value(vb, ds->ds[i].type, data->scale,
                                      data->shift, host->name, data->name);
        memcpy(&vt->suffix, &suffix, sizeof(vt->suffix));
        vt->next = NULL;

        if (value_list_tail[i] == NULL)
          value_list_head[i] = vt;
        else
          value_list_tail[i]->next = vt;
        value_list_tail[i] = vt;
      }

      /* Copy OID to oid_list[i] */
      memcpy(oid_list[i].oid, vb->name, sizeof(oid) * vb->name_length);
      oid_list[i].oid_len = vb->name_length;

    } /* for (vb = res->variables ...) */

    if (res != NULL)
      snmp_free_pdu(res);
    res = NULL;
  } /* while (status == 0) */

  if (res != NULL)
     snmp_free_pdu(res);
   res = NULL;
 
 
   if (status == 0)
     csnmp_dispatch_table(host, data, instance_list_head, value_list_head);

  /* Free all allocated variables here */
  while (instance_list_head != NULL) {
    csnmp_list_instances_t *next = instance_list_head->next;
    sfree(instance_list_head);
    instance_list_head = next;
  }

  for (i = 0; i < data->values_len; i++) {
    while (value_list_head[i] != NULL) {
      csnmp_table_values_t *next = value_list_head[i]->next;
      sfree(value_list_head[i]);
      value_list_head[i] = next;
    }
  }

  sfree(value_list_head);
  sfree(value_list_tail);

  return (0);
} /* int csnmp_read_table */