selinux.c 17.1 KB
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/* selinux.c  SELinux security checks for D-BUS
 *
 * Author: Matthew Rickard <mjricka@epoch.ncsc.mil>
 *
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 * Licensed under the Academic Free License version 2.1
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 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 */
#include <dbus/dbus-internals.h>
#include <dbus/dbus-string.h>
#include "selinux.h"
#include "services.h"
#include "policy.h"
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#include "utils.h"
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#include "config-parser.h"

#ifdef HAVE_SELINUX
#include <errno.h>
#include <syslog.h>
#include <selinux/selinux.h>
#include <selinux/avc.h>
#include <selinux/av_permissions.h>
#include <selinux/flask.h>
#include <stdarg.h>
#endif /* HAVE_SELINUX */

#define BUS_SID_FROM_SELINUX(sid)  ((BusSELinuxID*) (sid))
#define SELINUX_SID_FROM_BUS(sid)  ((security_id_t) (sid))

#ifdef HAVE_SELINUX
/* Store the value telling us if SELinux is enabled in the kernel. */
static dbus_bool_t selinux_enabled = FALSE;

/* Store an avc_entry_ref to speed AVC decisions. */
static struct avc_entry_ref aeref;

static security_id_t bus_sid = SECSID_WILD;
#endif /* HAVE_SELINUX */

/**
 * Log callback to log denial messages from the AVC.
 * This is used in avc_init.  Logs to both standard
 * error and syslogd.
 *
 * @param fmt the format string
 * @param variable argument list
 */
#ifdef HAVE_SELINUX
static void 
log_callback (const char *fmt, ...) 
{
  va_list ap;
  va_start(ap, fmt);
  vsyslog (LOG_INFO, fmt, ap);
  va_end(ap);
}
#endif /* HAVE_SELINUX */


/**
 * Initialize the user space access vector cache (AVC) for D-BUS and set up
 * logging callbacks.
 */
dbus_bool_t
bus_selinux_init (void)
{
#ifdef HAVE_SELINUX
  struct avc_log_callback log_cb = {(void*)log_callback, NULL};
  int r;
  char *bus_context;

  _dbus_assert (bus_sid == SECSID_WILD);
  
  /* Determine if we are running an SELinux kernel. */
  r = is_selinux_enabled ();
  if (r < 0)
    {
      _dbus_warn ("Could not tell if SELinux is enabled: %s\n",
                  _dbus_strerror (errno));
      return FALSE;
    }

  selinux_enabled = r != 0;

  if (!selinux_enabled)
    {
      _dbus_verbose ("SELinux not enabled in this kernel.\n");
      return TRUE;
    }

  _dbus_verbose ("SELinux is enabled in this kernel.\n");

  avc_entry_ref_init (&aeref);
  if (avc_init ("avc", NULL, &log_cb, NULL, NULL) < 0)
    {
      _dbus_warn ("Failed to start Access Vector Cache (AVC).\n");
      return FALSE;
    }
  else
    {
      openlog ("dbus", LOG_PERROR, LOG_USER);
      _dbus_verbose ("Access Vector Cache (AVC) started.\n");
    }

  bus_context = NULL;
  bus_sid = SECSID_WILD;

  if (getcon (&bus_context) < 0)
    {
      _dbus_verbose ("Error getting context of bus: %s\n",
                     _dbus_strerror (errno));
      return FALSE;
    }
      
  if (avc_context_to_sid (bus_context, &bus_sid) < 0)
    {
      _dbus_verbose ("Error getting SID from bus context: %s\n",
                     _dbus_strerror (errno));
      freecon (bus_context);
      return FALSE;
    }

  freecon (bus_context);
  
  return TRUE;
#else
  return TRUE;
#endif /* HAVE_SELINUX */
}


/**
 * Decrement SID reference count.
 * 
 * @param sid the SID to decrement
 */
void
bus_selinux_id_unref (BusSELinuxID *sid)
{
#ifdef HAVE_SELINUX
  if (!selinux_enabled)
    return;

  _dbus_assert (sid != NULL);
  
  sidput (SELINUX_SID_FROM_BUS (sid));
#endif /* HAVE_SELINUX */
}

void
bus_selinux_id_ref (BusSELinuxID *sid)
{
#ifdef HAVE_SELINUX
  if (!selinux_enabled)
    return;

  _dbus_assert (sid != NULL);
  
  sidget (SELINUX_SID_FROM_BUS (sid));
#endif /* HAVE_SELINUX */
}

/**
 * Determine if the SELinux security policy allows the given sender
 * security context to go to the given recipient security context.
 * This function determines if the requested permissions are to be
 * granted from the connection to the message bus or to another
 * optionally supplied security identifier (e.g. for a service
 * context).  Currently these permissions are either send_msg or
 * acquire_svc in the dbus class.
 *
 * @param sender_sid source security context
 * @param override_sid is the target security context.  If SECSID_WILD this will
 *        use the context of the bus itself (e.g. the default).
 * @param target_class is the target security class.
 * @param requested is the requested permissions.
 * @returns #TRUE if security policy allows the send.
 */
#ifdef HAVE_SELINUX
static dbus_bool_t
bus_selinux_check (BusSELinuxID        *sender_sid,
                   BusSELinuxID        *override_sid,
                   security_class_t     target_class,
                   access_vector_t      requested)
{
  if (!selinux_enabled)
    return TRUE;
  
  /* Make the security check.  AVC checks enforcing mode here as well. */
  if (avc_has_perm (SELINUX_SID_FROM_BUS (sender_sid),
                    override_sid ?
                    SELINUX_SID_FROM_BUS (override_sid) :
                    SELINUX_SID_FROM_BUS (bus_sid), 
                    target_class, requested, &aeref, NULL) < 0)
    {
      _dbus_verbose ("SELinux denying due to security policy.\n");
      return FALSE;
    }
  else
    return TRUE;
}
#endif /* HAVE_SELINUX */

/**
 * Returns true if the given connection can acquire a service,
 * assuming the given security ID is needed for that service.
 *
 * @param connection connection that wants to own the service
 * @param service_sid the SID of the service from the table
 * @returns #TRUE if acquire is permitted.
 */
dbus_bool_t
bus_selinux_allows_acquire_service (DBusConnection     *connection,
                                    BusSELinuxID       *service_sid)
{
#ifdef HAVE_SELINUX
  BusSELinuxID *connection_sid;
  
  if (!selinux_enabled)
    return TRUE;

  connection_sid = bus_connection_get_selinux_id (connection);
  
  return bus_selinux_check (connection_sid,
                            service_sid,
                            SECCLASS_DBUS,
                            DBUS__ACQUIRE_SVC);
#else
  return TRUE;
#endif /* HAVE_SELINUX */
}

/**
 * Check if SELinux security controls allow the message to be sent to a
 * particular connection based on the security context of the sender and
 * that of the receiver. The destination connection need not be the
 * addressed recipient, it could be an "eavesdropper"
 *
 * @param sender the sender of the message.
 * @param proposed_recipient the connection the message is to be sent to.
 * @returns whether to allow the send
 */
dbus_bool_t
bus_selinux_allows_send (DBusConnection     *sender,
                         DBusConnection     *proposed_recipient)
{
#ifdef HAVE_SELINUX
  BusSELinuxID *recipient_sid;
  BusSELinuxID *sender_sid;

  if (!selinux_enabled)
    return TRUE;

  sender_sid = bus_connection_get_selinux_id (sender);
  /* A NULL proposed_recipient means the bus itself. */
  if (proposed_recipient)
    recipient_sid = bus_connection_get_selinux_id (proposed_recipient);
  else
    recipient_sid = BUS_SID_FROM_SELINUX (bus_sid);

  return bus_selinux_check (sender_sid, recipient_sid,
                            SECCLASS_DBUS, DBUS__SEND_MSG);
#else
  return TRUE;
#endif /* HAVE_SELINUX */
}

/**
 * Gets the security context of a connection to the bus. It is up to
 * the caller to freecon() when they are done. 
 *
 * @param connection the connection to get the context of.
 * @param con the location to store the security context.
 * @returns #TRUE if context is successfully obtained.
 */
#ifdef HAVE_SELINUX
static dbus_bool_t
bus_connection_read_selinux_context (DBusConnection     *connection,
                                     char              **con)
{
  int fd;

  if (!selinux_enabled)
    return FALSE;

  _dbus_assert (connection != NULL);
  
  if (!dbus_connection_get_unix_fd (connection, &fd))
    {
      _dbus_verbose ("Failed to get file descriptor of socket.\n");
      return FALSE;
    }
  
  if (getpeercon (fd, con) < 0)
    {
      _dbus_verbose ("Error getting context of socket peer: %s\n",
                     _dbus_strerror (errno));
      return FALSE;
    }
  
  _dbus_verbose ("Successfully read connection context.\n");
  return TRUE;
}
#endif /* HAVE_SELINUX */

/**
 * Read the SELinux ID from the connection.
 *
 * @param connection the connection to read from
 * @returns the SID if successfully determined, #NULL otherwise.
 */
BusSELinuxID*
bus_selinux_init_connection_id (DBusConnection *connection,
                                DBusError      *error)
{
#ifdef HAVE_SELINUX
  char *con;
  security_id_t sid;
  
  if (!selinux_enabled)
    return NULL;

  if (!bus_connection_read_selinux_context (connection, &con))
    {
      dbus_set_error (error, DBUS_ERROR_FAILED,
                      "Failed to read an SELinux context from connection");
      _dbus_verbose ("Error getting peer context.\n");
      return NULL;
    }

  _dbus_verbose ("Converting context to SID to store on connection\n");

  if (avc_context_to_sid (con, &sid) < 0)
    {
      if (errno == ENOMEM)
        BUS_SET_OOM (error);
      else
        dbus_set_error (error, DBUS_ERROR_FAILED,
                        "Error getting SID from context: %s\n",
                        _dbus_strerror (errno));
      
      _dbus_warn ("Error getting SID from context: %s\n",
                  _dbus_strerror (errno));
      
      freecon (con);
      return NULL;
    }
 
  freecon (con); 
  return BUS_SID_FROM_SELINUX (sid);
#else
  return NULL;
#endif /* HAVE_SELINUX */
}


/* Function for freeing hash table data.  These SIDs
 * should no longer be referenced.
 */
static void
bus_selinux_id_table_free_value (BusSELinuxID *sid)
{
#ifdef HAVE_SELINUX
  /* NULL sometimes due to how DBusHashTable works */
  if (sid)
    bus_selinux_id_unref (sid);
#endif /* HAVE_SELINUX */
}

/**
 * Creates a new table mapping service names to security ID.
 * A security ID is a "compiled" security context, a security
 * context is just a string.
 *
 * @returns the new table or #NULL if no memory
 */
DBusHashTable*
bus_selinux_id_table_new (void)
{
  return _dbus_hash_table_new (DBUS_HASH_STRING,
                               (DBusFreeFunction) dbus_free,
                               (DBusFreeFunction) bus_selinux_id_table_free_value);
}

/** 
 * Hashes a service name and service context into the service SID
 * table as a string and a SID.
 *
 * @param service_name is the name of the service.
 * @param service_context is the context of the service.
 * @param service_table is the table to hash them into.
 * @return #FALSE if not enough memory
 */
dbus_bool_t
bus_selinux_id_table_insert (DBusHashTable *service_table,
                             const char    *service_name,
                             const char    *service_context)
{
#ifdef HAVE_SELINUX
  dbus_bool_t retval;
  security_id_t sid;
  char *key;

  if (!selinux_enabled)
    return TRUE;

  sid = SECSID_WILD;
  retval = FALSE;

  key = _dbus_strdup (service_name);
  if (key == NULL)
    return retval;
  
  if (avc_context_to_sid (service_context, &sid) < 0)
    {
      _dbus_assert (errno == ENOMEM);
      goto out;
    }

  if (!_dbus_hash_table_insert_string (service_table,
                                       key,
                                       BUS_SID_FROM_SELINUX (sid)))
    goto out;

  _dbus_verbose ("Parsed \tservice: %s \n\t\tcontext: %s\n",
                  key, 
                  sid->ctx);

  /* These are owned by the hash, so clear them to avoid unref */
  key = NULL;
  sid = SECSID_WILD;

  retval = TRUE;
  
 out:
  if (sid != SECSID_WILD)
    sidput (sid);

  if (key)
    dbus_free (key);

  return retval;
#else
  return TRUE;
#endif /* HAVE_SELINUX */
}


/**
 * Find the security identifier associated with a particular service
 * name.  Return a pointer to this SID, or #NULL/SECSID_WILD if the
 * service is not found in the hash table.  This should be nearly a
 * constant time operation.  If SELinux support is not available,
 * always return NULL.
 *
 * @todo This should return a const security_id_t since we don't
 *       want the caller to mess with it.
 *
 * @param service_table the hash table to check for service name.
 * @param service_name the name of the service to look for.
 * @returns the SELinux ID associated with the service
 */
BusSELinuxID*
bus_selinux_id_table_lookup (DBusHashTable    *service_table,
                             const DBusString *service_name)
{
#ifdef HAVE_SELINUX
  security_id_t sid;

  sid = SECSID_WILD;     /* default context */

  if (!selinux_enabled)
    return NULL;
  
  _dbus_verbose ("Looking up service SID for %s\n",
                 _dbus_string_get_const_data (service_name));

  sid = _dbus_hash_table_lookup_string (service_table,
                                        _dbus_string_get_const_data (service_name));

  if (sid == SECSID_WILD)
    _dbus_verbose ("Service %s not found\n", 
                   _dbus_string_get_const_data (service_name));
  else
    _dbus_verbose ("Service %s found\n", 
                   _dbus_string_get_const_data (service_name));

  return BUS_SID_FROM_SELINUX (sid);
#endif /* HAVE_SELINUX */
  return NULL;
}

#ifdef HAVE_SELINUX
static dbus_bool_t
bus_selinux_id_table_copy_over (DBusHashTable    *dest,
                                DBusHashTable    *override)
{
  const char *key;
  char *key_copy;
  BusSELinuxID *sid;
  DBusHashIter iter;
  
  _dbus_hash_iter_init (override, &iter);
  while (_dbus_hash_iter_next (&iter))
    {
      key = _dbus_hash_iter_get_string_key (&iter);
      sid = _dbus_hash_iter_get_value (&iter);

      key_copy = _dbus_strdup (key);
      if (key_copy == NULL)
        return FALSE;

      if (!_dbus_hash_table_insert_string (dest,
                                           key_copy,
                                           sid))
        {
          dbus_free (key_copy);
          return FALSE;
        }

      bus_selinux_id_ref (sid);
    }

  return TRUE;
}
#endif /* HAVE_SELINUX */

/**
 * Creates the union of the two tables (each table maps a service
 * name to a security ID). In case of the same service name in
 * both tables, the security ID from "override" will be used.
 *
 * @param base the base table
 * @param override the table that takes precedence in the merge
 * @returns the new table, or #NULL if out of memory
 */
DBusHashTable*
bus_selinux_id_table_union (DBusHashTable    *base,
                            DBusHashTable    *override)
{
  DBusHashTable *combined_table;

  combined_table = bus_selinux_id_table_new ();

  if (combined_table == NULL)
    return NULL;
  
#ifdef HAVE_SELINUX 
  if (!selinux_enabled)
    return combined_table;

  if (!bus_selinux_id_table_copy_over (combined_table, base))
    {
      _dbus_hash_table_unref (combined_table);
      return NULL;
    }

  if (!bus_selinux_id_table_copy_over (combined_table, override))
    {
      _dbus_hash_table_unref (combined_table);
      return NULL;
    }
#endif /* HAVE_SELINUX */
  
  return combined_table;
}

/**
 * For debugging:  Print out the current hash table of service SIDs.
 */
void
bus_selinux_id_table_print (DBusHashTable *service_table)
{
#ifdef DBUS_ENABLE_VERBOSE_MODE
#ifdef HAVE_SELINUX
  DBusHashIter iter;

  if (!selinux_enabled)
    return;
  
  _dbus_verbose ("Service SID Table:\n");
  _dbus_hash_iter_init (service_table, &iter);
  while (_dbus_hash_iter_next (&iter))
    {
      const char *key = _dbus_hash_iter_get_string_key (&iter);
      security_id_t sid = _dbus_hash_iter_get_value (&iter);
      _dbus_verbose ("The key is %s\n", key);
      _dbus_verbose ("The context is %s\n", sid->ctx);
      _dbus_verbose ("The refcount is %d\n", sid->refcnt);
    }
#endif /* HAVE_SELINUX */
#endif /* DBUS_ENABLE_VERBOSE_MODE */
}


#ifdef DBUS_ENABLE_VERBOSE_MODE
#ifdef HAVE_SELINUX
/**
 * Print out some AVC statistics.
 */
static void
bus_avc_print_stats (void)
{
  struct avc_cache_stats cstats;

  if (!selinux_enabled)
    return;
  
  _dbus_verbose ("AVC Statistics:\n");
  avc_cache_stats (&cstats);
  avc_av_stats ();
  _dbus_verbose ("AVC Cache Statistics:\n");
  _dbus_verbose ("Entry lookups: %d\n", cstats.entry_lookups);
  _dbus_verbose ("Entry hits: %d\n", cstats.entry_hits);
  _dbus_verbose ("Entry misses %d\n", cstats.entry_misses);
  _dbus_verbose ("Entry discards: %d\n", cstats.entry_discards);
  _dbus_verbose ("CAV lookups: %d\n", cstats.cav_lookups);
  _dbus_verbose ("CAV hits: %d\n", cstats.cav_hits);
  _dbus_verbose ("CAV probes: %d\n", cstats.cav_probes);
  _dbus_verbose ("CAV misses: %d\n", cstats.cav_misses);
}
#endif /* HAVE_SELINUX */
#endif /* DBUS_ENABLE_VERBOSE_MODE */


/**
 * Destroy the AVC before we terminate.
 */
void
bus_selinux_shutdown (void)
{
#ifdef HAVE_SELINUX
  if (!selinux_enabled)
    return;

  sidput (bus_sid);
  bus_sid = SECSID_WILD;
  
#ifdef DBUS_ENABLE_VERBOSE_MODE
  bus_avc_print_stats ();
#endif /* DBUS_ENABLE_VERBOSE_MODE */

  avc_destroy ();
#endif /* HAVE_SELINUX */
}