path: root/src/auths/gsasl_exim.c

/*************************************************
*     Exim - an Internet mail transport agent    *
*************************************************/

/* Copyright (c) University of Cambridge 1995 - 2018 */
/* See the file NOTICE for conditions of use and distribution. */

/* Copyright (c) Twitter Inc 2012
   Author: Phil Pennock <pdp@exim.org> */
/* Copyright (c) Phil Pennock 2012 */

/* Interface to GNU SASL library for generic authentication. */

/* Trade-offs:

GNU SASL does not provide authentication data itself, so we have to expose
that decision to configuration.  For some mechanisms, we need to act much
like plaintext.  For others, we only need to be able to provide some
evaluated data on demand.  There's no abstracted way (ie, without hardcoding
knowledge of authenticators here) to know which need what properties; we
can't query a session or the library for "we will need these for mechanism X".

So: we always require server_condition, even if sometimes it will just be
set as "yes".  We do provide a number of other hooks, which might not make
sense in all contexts.  For some, we can do checks at init time.
*/

#include "../exim.h"

#ifndef AUTH_GSASL
/* dummy function to satisfy compilers when we link in an "empty" file. */
static void dummy(int x);
static void dummy2(int x) { dummy(x-1); }
static void dummy(int x) { dummy2(x-1); }
#else

#include <gsasl.h>
#include "gsasl_exim.h"

/* Authenticator-specific options. */
/* I did have server_*_condition options for various mechanisms, but since
we only ever handle one mechanism at a time, I didn't see the point in keeping
that.  In case someone sees a point, I've left the condition_check() API
alone. */
optionlist auth_gsasl_options[] = {
  { "server_channelbinding", opt_bool,
      (void *)(offsetof(auth_gsasl_options_block, server_channelbinding)) },
  { "server_hostname",      opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_hostname)) },
  { "server_mech",          opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_mech)) },
  { "server_password",      opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_password)) },
  { "server_realm",         opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_realm)) },
  { "server_scram_iter",    opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_scram_iter)) },
  { "server_scram_salt",    opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_scram_salt)) },
  { "server_service",       opt_stringptr,
      (void *)(offsetof(auth_gsasl_options_block, server_service)) }
};
/* GSASL_SCRAM_SALTED_PASSWORD documented only for client, so not implementing
hooks to avoid cleartext passwords in the Exim server. */

int auth_gsasl_options_count =
  sizeof(auth_gsasl_options)/sizeof(optionlist);

/* Defaults for the authenticator-specific options. */
auth_gsasl_options_block auth_gsasl_option_defaults = {
  US"smtp",                 /* server_service */
  US"$primary_hostname",    /* server_hostname */
  NULL,                     /* server_realm */
  NULL,                     /* server_mech */
  NULL,                     /* server_password */
  NULL,                     /* server_scram_iter */
  NULL,                     /* server_scram_salt */
  FALSE                     /* server_channelbinding */
};


#ifdef MACRO_PREDEF

/* Dummy values */
void auth_gsasl_init(auth_instance *ablock) {}
int auth_gsasl_server(auth_instance *ablock, uschar *data) {return 0;}
int auth_gsasl_client(auth_instance *ablock, smtp_inblock * sx,
  int timeout, uschar *buffer, int buffsize) {return 0;}
void auth_gsasl_version_report(FILE *f) {}

#else   /*!MACRO_PREDEF*/



/* "Globals" for managing the gsasl interface. */

static Gsasl *gsasl_ctx = NULL;
static int
  main_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop);
static int
  server_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop, auth_instance *ablock);
static int
  client_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop, auth_instance *ablock);

static BOOL sasl_error_should_defer = FALSE;
static Gsasl_property callback_loop = 0;
static BOOL checked_server_condition = FALSE;

enum { CURRENTLY_SERVER = 1, CURRENTLY_CLIENT = 2 };

struct callback_exim_state {
  auth_instance *ablock;
  int currently;
};


/*************************************************
*          Initialization entry point            *
*************************************************/

/* Called for each instance, after its options have been read, to
enable consistency checks to be done, or anything else that needs
to be set up. */

void
auth_gsasl_init(auth_instance *ablock)
{
  char *p;
  int rc, supported;
  auth_gsasl_options_block *ob =
    (auth_gsasl_options_block *)(ablock->options_block);

  /* As per existing Cyrus glue, use the authenticator's public name as
  the default for the mechanism name; we don't handle multiple mechanisms
  in one authenticator, but the same driver can be used multiple times. */

  if (ob->server_mech == NULL)
    ob->server_mech = string_copy(ablock->public_name);

  /* Can get multiple session contexts from one library context, so just
  initialise the once. */
  if (gsasl_ctx == NULL) {
    rc = gsasl_init(&gsasl_ctx);
    if (rc != GSASL_OK) {
      log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
                "couldn't initialise GNU SASL library: %s (%s)",
                ablock->name, gsasl_strerror_name(rc), gsasl_strerror(rc));
    }
    gsasl_callback_set(gsasl_ctx, main_callback);
  }

  /* We don't need this except to log it for debugging. */
  rc = gsasl_server_mechlist(gsasl_ctx, &p);
  if (rc != GSASL_OK)
    log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
              "failed to retrieve list of mechanisms: %s (%s)",
              ablock->name,  gsasl_strerror_name(rc), gsasl_strerror(rc));
  HDEBUG(D_auth) debug_printf("GNU SASL supports: %s\n", p);

  supported = gsasl_client_support_p(gsasl_ctx, CCS ob->server_mech);
  if (!supported)
    log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
              "GNU SASL does not support mechanism \"%s\"",
              ablock->name, ob->server_mech);

  if ((ablock->server_condition == NULL) &&
      (streqic(ob->server_mech, US"EXTERNAL") ||
       streqic(ob->server_mech, US"ANONYMOUS") ||
       streqic(ob->server_mech, US"PLAIN") ||
       streqic(ob->server_mech, US"LOGIN")))
    log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
              "Need server_condition for %s mechanism",
              ablock->name, ob->server_mech);

  /* This does *not* scale to new SASL mechanisms.  Need a better way to ask
  which properties will be needed. */
  if ((ob->server_realm == NULL) &&
      streqic(ob->server_mech, US"DIGEST-MD5"))
    log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
              "Need server_realm for %s mechanism",
              ablock->name, ob->server_mech);

  /* At present, for mechanisms we don't panic on absence of server_condition;
  need to figure out the most generically correct approach to deciding when
  it's critical and when it isn't.  Eg, for simple validation (PLAIN mechanism,
  etc) it clearly is critical.

  So don't activate without server_condition, this might be relaxed in the future.
  */
  if (ablock->server_condition != NULL) ablock->server = TRUE;
  ablock->client = FALSE;
}


/* GNU SASL uses one top-level callback, registered at library level.
We dispatch to client and server functions instead. */

static int
main_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop)
{
  int rc = 0;
  struct callback_exim_state *cb_state =
    (struct callback_exim_state *)gsasl_session_hook_get(sctx);

  HDEBUG(D_auth)
    debug_printf("GNU SASL Callback entered, prop=%d (loop prop=%d)\n",
        prop, callback_loop);

  if (cb_state == NULL) {
    HDEBUG(D_auth) debug_printf("  not from our server/client processing.\n");
    return GSASL_NO_CALLBACK;
  }

  if (callback_loop > 0) {
    /* Most likely is that we were asked for property foo, and to
    expand the string we asked for property bar to put into an auth
    variable, but property bar is not supplied for this mechanism. */
    HDEBUG(D_auth)
      debug_printf("Loop, asked for property %d while handling property %d\n",
          prop, callback_loop);
    return GSASL_NO_CALLBACK;
  }
  callback_loop = prop;

  if (cb_state->currently == CURRENTLY_CLIENT)
    rc = client_callback(ctx, sctx, prop, cb_state->ablock);
  else if (cb_state->currently == CURRENTLY_SERVER)
    rc = server_callback(ctx, sctx, prop, cb_state->ablock);
  else {
    log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
        "unhandled callback state, bug in Exim", cb_state->ablock->name);
    /* NOTREACHED */
  }

  callback_loop = 0;
  return rc;
}


/*************************************************
*             Server entry point                 *
*************************************************/

/* For interface, see auths/README */

int
auth_gsasl_server(auth_instance *ablock, uschar *initial_data)
{
  char *tmps;
  char *to_send, *received;
  Gsasl_session *sctx = NULL;
  auth_gsasl_options_block *ob =
    (auth_gsasl_options_block *)(ablock->options_block);
  struct callback_exim_state cb_state;
  int rc, auth_result, exim_error, exim_error_override;

  HDEBUG(D_auth)
    debug_printf("GNU SASL: initialising session for %s, mechanism %s.\n",
        ablock->name, ob->server_mech);

  rc = gsasl_server_start(gsasl_ctx, CCS ob->server_mech, &sctx);
  if (rc != GSASL_OK) {
    auth_defer_msg = string_sprintf("GNU SASL: session start failure: %s (%s)",
        gsasl_strerror_name(rc), gsasl_strerror(rc));
    HDEBUG(D_auth) debug_printf("%s\n", auth_defer_msg);
    return DEFER;
  }
  /* Hereafter: gsasl_finish(sctx) please */

  gsasl_session_hook_set(sctx, (void *)ablock);
  cb_state.ablock = ablock;
  cb_state.currently = CURRENTLY_SERVER;
  gsasl_session_hook_set(sctx, (void *)&cb_state);

  tmps = CS expand_string(ob->server_service);
  gsasl_property_set(sctx, GSASL_SERVICE, tmps);
  tmps = CS expand_string(ob->server_hostname);
  gsasl_property_set(sctx, GSASL_HOSTNAME, tmps);
  if (ob->server_realm) {
    tmps = CS expand_string(ob->server_realm);
    if (tmps && *tmps) {
      gsasl_property_set(sctx, GSASL_REALM, tmps);
    }
  }
  /* We don't support protection layers. */
  gsasl_property_set(sctx, GSASL_QOPS, "qop-auth");
#ifdef SUPPORT_TLS
  if (tls_channelbinding_b64) {
    /* Some auth mechanisms can ensure that both sides are talking within the
    same security context; for TLS, this means that even if a bad certificate
    has been accepted, they remain MitM-proof because both sides must be within
    the same negotiated session; if someone is terminating one session and
    proxying data on within a second, authentication will fail.

    We might not have this available, depending upon TLS implementation,
    ciphersuite, phase of moon ...

    If we do, it results in extra SASL mechanisms being available; here,
    Exim's one-mechanism-per-authenticator potentially causes problems.
    It depends upon how GNU SASL will implement the PLUS variants of GS2
    and whether it automatically mandates a switch to the bound PLUS
    if the data is available.  Since default-on, despite being more secure,
    would then result in mechanism name changes on a library update, we
    have little choice but to default it off and let the admin choose to
    enable it.  *sigh*
    */
    if (ob->server_channelbinding) {
      HDEBUG(D_auth) debug_printf("Auth %s: Enabling channel-binding\n",
          ablock->name);
      gsasl_property_set(sctx, GSASL_CB_TLS_UNIQUE,
          CCS  tls_channelbinding_b64);
    } else {
      HDEBUG(D_auth)
        debug_printf("Auth %s: Not enabling channel-binding (data available)\n",
            ablock->name);
    }
  } else {
    HDEBUG(D_auth)
      debug_printf("Auth %s: no channel-binding data available\n",
          ablock->name);
  }
#endif

  checked_server_condition = FALSE;

  received = CS initial_data;
  to_send = NULL;
  exim_error = exim_error_override = OK;

  do {
    rc = gsasl_step64(sctx, received, &to_send);

    switch (rc) {
      case GSASL_OK:
        if (!to_send)
          goto STOP_INTERACTION;
        break;

      case GSASL_NEEDS_MORE:
        break;

      case GSASL_AUTHENTICATION_ERROR:
      case GSASL_INTEGRITY_ERROR:
      case GSASL_NO_AUTHID:
      case GSASL_NO_ANONYMOUS_TOKEN:
      case GSASL_NO_AUTHZID:
      case GSASL_NO_PASSWORD:
      case GSASL_NO_PASSCODE:
      case GSASL_NO_PIN:
      case GSASL_BASE64_ERROR:
        HDEBUG(D_auth) debug_printf("GNU SASL permanent error: %s (%s)\n",
            gsasl_strerror_name(rc), gsasl_strerror(rc));
        log_write(0, LOG_REJECT, "%s authenticator (%s):\n  "
            "GNU SASL permanent failure: %s (%s)",
            ablock->name, ob->server_mech,
            gsasl_strerror_name(rc), gsasl_strerror(rc));
        if (rc == GSASL_BASE64_ERROR)
          exim_error_override = BAD64;
        goto STOP_INTERACTION;

      default:
        auth_defer_msg = string_sprintf("GNU SASL temporary error: %s (%s)",
            gsasl_strerror_name(rc), gsasl_strerror(rc));
        HDEBUG(D_auth) debug_printf("%s\n", auth_defer_msg);
        exim_error_override = DEFER;
        goto STOP_INTERACTION;
    }

    if ((rc == GSASL_NEEDS_MORE) ||
        (to_send && *to_send))
      exim_error =
        auth_get_no64_data((uschar **)&received, US to_send);

    if (to_send) {
      free(to_send);
      to_send = NULL;
    }

    if (exim_error)
      break; /* handles * cancelled check */

  } while (rc == GSASL_NEEDS_MORE);

STOP_INTERACTION:
  auth_result = rc;

  gsasl_finish(sctx);

  /* Can return: OK DEFER FAIL CANCELLED BAD64 UNEXPECTED */

  if (exim_error != OK)
    return exim_error;

  if (auth_result != GSASL_OK) {
    HDEBUG(D_auth) debug_printf("authentication returned %s (%s)\n",
        gsasl_strerror_name(auth_result), gsasl_strerror(auth_result));
    if (exim_error_override != OK)
      return exim_error_override; /* might be DEFER */
    if (sasl_error_should_defer) /* overriding auth failure SASL error */
      return DEFER;
    return FAIL;
  }

  /* Auth succeeded, check server_condition unless already done in callback */
  return checked_server_condition ? OK : auth_check_serv_cond(ablock);
}

/* returns the GSASL status of expanding the Exim string given */
static int
condition_check(auth_instance *ablock, uschar *label, uschar *condition_string)
{
  int exim_rc;

  exim_rc = auth_check_some_cond(ablock, label, condition_string, FAIL);

  if (exim_rc == OK) {
    return GSASL_OK;
  } else if (exim_rc == DEFER) {
    sasl_error_should_defer = TRUE;
    return GSASL_AUTHENTICATION_ERROR;
  } else if (exim_rc == FAIL) {
    return GSASL_AUTHENTICATION_ERROR;
  }

  log_write(0, LOG_PANIC_DIE|LOG_CONFIG_FOR, "%s authenticator:  "
            "Unhandled return from checking %s: %d",
            ablock->name, label, exim_rc);
  /* NOTREACHED */
  return GSASL_AUTHENTICATION_ERROR;
}

static int
server_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop, auth_instance *ablock)
{
  char *tmps;
  uschar *propval;
  int cbrc = GSASL_NO_CALLBACK;
  int i;
  auth_gsasl_options_block *ob =
    (auth_gsasl_options_block *)(ablock->options_block);

  HDEBUG(D_auth)
    debug_printf("GNU SASL callback %d for %s/%s as server\n",
        prop, ablock->name, ablock->public_name);

  for (i = 0; i < AUTH_VARS; i++) auth_vars[i] = NULL;
  expand_nmax = 0;

  switch (prop) {
    case GSASL_VALIDATE_SIMPLE:
      /* GSASL_AUTHID, GSASL_AUTHZID, and GSASL_PASSWORD */
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHID);
      auth_vars[0] = expand_nstring[1] = propval ? propval : US"";
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHZID);
      auth_vars[1] = expand_nstring[2] = propval ? propval : US"";
      propval = US  gsasl_property_fast(sctx, GSASL_PASSWORD);
      auth_vars[2] = expand_nstring[3] = propval ? propval : US"";
      expand_nmax = 3;
      for (i = 1; i <= 3; ++i)
        expand_nlength[i] = Ustrlen(expand_nstring[i]);

      cbrc = condition_check(ablock, US"server_condition", ablock->server_condition);
      checked_server_condition = TRUE;
      break;

    case GSASL_VALIDATE_EXTERNAL:
      if (ablock->server_condition == NULL) {
        HDEBUG(D_auth) debug_printf("No server_condition supplied, to validate EXTERNAL.\n");
        cbrc = GSASL_AUTHENTICATION_ERROR;
        break;
      }
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHZID);
      /* We always set $auth1, even if only to empty string. */
      auth_vars[0] = expand_nstring[1] = propval ? propval : US"";
      expand_nlength[1] = Ustrlen(expand_nstring[1]);
      expand_nmax = 1;

      cbrc = condition_check(ablock,
          US"server_condition (EXTERNAL)", ablock->server_condition);
      checked_server_condition = TRUE;
      break;

    case GSASL_VALIDATE_ANONYMOUS:
      if (ablock->server_condition == NULL) {
        HDEBUG(D_auth) debug_printf("No server_condition supplied, to validate ANONYMOUS.\n");
        cbrc = GSASL_AUTHENTICATION_ERROR;
        break;
      }
      propval = US  gsasl_property_fast(sctx, GSASL_ANONYMOUS_TOKEN);
      /* We always set $auth1, even if only to empty string. */
      auth_vars[0] = expand_nstring[1] = propval ? propval : US"";
      expand_nlength[1] = Ustrlen(expand_nstring[1]);
      expand_nmax = 1;

      cbrc = condition_check(ablock,
          US"server_condition (ANONYMOUS)", ablock->server_condition);
      checked_server_condition = TRUE;
      break;

    case GSASL_VALIDATE_GSSAPI:
      /* GSASL_AUTHZID and GSASL_GSSAPI_DISPLAY_NAME
      The display-name is authenticated as part of GSS, the authzid is claimed
      by the SASL integration after authentication; protected against tampering
      (if the SASL mechanism supports that, which Kerberos does) but is
      unverified, same as normal for other mechanisms.

      First coding, we had these values swapped, but for consistency and prior
      to the first release of Exim with this authenticator, they've been
      switched to match the ordering of GSASL_VALIDATE_SIMPLE. */
      propval = US  gsasl_property_fast(sctx, GSASL_GSSAPI_DISPLAY_NAME);
      auth_vars[0] = expand_nstring[1] = propval ? propval : US"";
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHZID);
      auth_vars[1] = expand_nstring[2] = propval ? propval : US"";
      expand_nmax = 2;
      for (i = 1; i <= 2; ++i)
        expand_nlength[i] = Ustrlen(expand_nstring[i]);

      /* In this one case, it perhaps makes sense to default back open?
      But for consistency, let's just mandate server_condition here too. */
      cbrc = condition_check(ablock,
          US"server_condition (GSSAPI family)", ablock->server_condition);
      checked_server_condition = TRUE;
      break;

    case GSASL_PASSWORD:
      /* DIGEST-MD5: GSASL_AUTHID, GSASL_AUTHZID and GSASL_REALM
         CRAM-MD5: GSASL_AUTHID
         PLAIN: GSASL_AUTHID and GSASL_AUTHZID
         LOGIN: GSASL_AUTHID
       */
      if (ob->server_scram_iter) {
        tmps = CS expand_string(ob->server_scram_iter);
        gsasl_property_set(sctx, GSASL_SCRAM_ITER, tmps);
      }
      if (ob->server_scram_salt) {
        tmps = CS expand_string(ob->server_scram_salt);
        gsasl_property_set(sctx, GSASL_SCRAM_SALT, tmps);
      }
      /* Asking for GSASL_AUTHZID calls back into us if we use
      gsasl_property_get(), thus the use of gsasl_property_fast().
      Do we really want to hardcode limits per mechanism?  What happens when
      a new mechanism is added to the library.  It *shouldn't* result in us
      needing to add more glue, since avoiding that is a large part of the
      point of SASL. */
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHID);
      auth_vars[0] = expand_nstring[1] = propval ? propval : US"";
      propval = US  gsasl_property_fast(sctx, GSASL_AUTHZID);
      auth_vars[1] = expand_nstring[2] = propval ? propval : US"";
      propval = US  gsasl_property_fast(sctx, GSASL_REALM);
      auth_vars[2] = expand_nstring[3] = propval ? propval : US"";
      expand_nmax = 3;
      for (i = 1; i <= 3; ++i)
        expand_nlength[i] = Ustrlen(expand_nstring[i]);

      tmps = CS expand_string(ob->server_password);
      if (tmps == NULL) {
        sasl_error_should_defer = f.expand_string_forcedfail ? FALSE : TRUE;
        HDEBUG(D_auth) debug_printf("server_password expansion failed, so "
            "can't tell GNU SASL library the password for %s\n", auth_vars[0]);
        return GSASL_AUTHENTICATION_ERROR;
      }
      gsasl_property_set(sctx, GSASL_PASSWORD, tmps);
      /* This is inadequate; don't think Exim's store stacks are geared
      for memory wiping, so expanding strings will leave stuff laying around.
      But no need to compound the problem, so get rid of the one we can. */
      memset(tmps, '\0', strlen(tmps));
      cbrc = GSASL_OK;
      break;

    default:
      HDEBUG(D_auth) debug_printf("Unrecognised callback: %d\n", prop);
      cbrc = GSASL_NO_CALLBACK;
  }

  HDEBUG(D_auth) debug_printf("Returning %s (%s)\n",
      gsasl_strerror_name(cbrc), gsasl_strerror(cbrc));

  return cbrc;
}


/*************************************************
*              Client entry point                *
*************************************************/

/* For interface, see auths/README */

int
auth_gsasl_client(
  auth_instance *ablock,		/* authenticator block */
  smtp_inblock * sx,			/* connection */
  int timeout,				/* command timeout */
  uschar *buffer,			/* buffer for reading response */
  int buffsize)				/* size of buffer */
{
  HDEBUG(D_auth)
    debug_printf("Client side NOT IMPLEMENTED: you should not see this!\n");
  /* NOT IMPLEMENTED */
  return FAIL;
}

static int
client_callback(Gsasl *ctx, Gsasl_session *sctx, Gsasl_property prop, auth_instance *ablock)
{
  int cbrc = GSASL_NO_CALLBACK;
  HDEBUG(D_auth)
    debug_printf("GNU SASL callback %d for %s/%s as client\n",
        prop, ablock->name, ablock->public_name);

  HDEBUG(D_auth)
    debug_printf("Client side NOT IMPLEMENTED: you should not see this!\n");

  return cbrc;
}

/*************************************************
*                Diagnostic API                  *
*************************************************/

void
auth_gsasl_version_report(FILE *f)
{
  const char *runtime;
  runtime = gsasl_check_version(NULL);
  fprintf(f, "Library version: GNU SASL: Compile: %s\n"
             "                           Runtime: %s\n",
          GSASL_VERSION, runtime);
}

#endif   /*!MACRO_PREDEF*/
#endif  /* AUTH_GSASL */

/* End of gsasl_exim.c */