[exim.git] / src / src / tls-gnu.c

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

/* Copyright (c) University of Cambridge 1995 - 2012 */
/* See the file NOTICE for conditions of use and distribution. */

/* Copyright (c) Phil Pennock 2012 */

/* This file provides TLS/SSL support for Exim using the GnuTLS library,
one of the available supported implementations.  This file is #included into
tls.c when USE_GNUTLS has been set.

The code herein is a revamp of GnuTLS integration using the current APIs; the
original tls-gnu.c was based on a patch which was contributed by Nikos
Mavroyanopoulos.  The revamp is partially a rewrite, partially cut&paste as
appropriate.

APIs current as of GnuTLS 2.12.18; note that the GnuTLS manual is for GnuTLS 3,
which is not widely deployed by OS vendors.  Will note issues below, which may
assist in updating the code in the future.  Another sources of hints is
mod_gnutls for Apache (SNI callback registration and handling).

Keeping client and server variables more split than before and is currently
the norm, in anticipation of TLS in ACL callouts.

I wanted to switch to gnutls_certificate_set_verify_function() so that
certificate rejection could happen during handshake where it belongs, rather
than being dropped afterwards, but that was introduced in 2.10.0 and Debian
(6.0.5) is still on 2.8.6.  So for now we have to stick with sub-par behaviour.

(I wasn't looking for libraries quite that old, when updating to get rid of
compiler warnings of deprecated APIs.  If it turns out that a lot of the rest
require current GnuTLS, then we'll drop support for the ancient libraries).
*/

#include <gnutls/gnutls.h>
/* needed for cert checks in verification and DN extraction: */
#include <gnutls/x509.h>
/* man-page is incorrect, gnutls_rnd() is not in gnutls.h: */
#include <gnutls/crypto.h>

/* GnuTLS 2 vs 3

GnuTLS 3 only:
  gnutls_global_set_audit_log_function()

Changes:
  gnutls_certificate_verify_peers2(): is new, drop the 2 for old version
*/

/* Local static variables for GnuTLS */

/* Values for verify_requirement */

enum peer_verify_requirement { VERIFY_NONE, VERIFY_OPTIONAL, VERIFY_REQUIRED };

/* This holds most state for server or client; with this, we can set up an
outbound TLS-enabled connection in an ACL callout, while not stomping all
over the TLS variables available for expansion.

Some of these correspond to variables in globals.c; those variables will
be set to point to content in one of these instances, as appropriate for
the stage of the process lifetime.

Not handled here: globals tls_active, tls_bits, tls_cipher, tls_peerdn,
tls_certificate_verified, tls_channelbinding_b64, tls_sni.
*/

typedef struct exim_gnutls_state {
  gnutls_session_t session;
  gnutls_certificate_credentials_t x509_cred;
  gnutls_priority_t priority_cache;
  enum peer_verify_requirement verify_requirement;
  int fd_in;
  int fd_out;
  BOOL peer_cert_verified;
  BOOL trigger_sni_changes;
  const struct host_item *host;
  uschar *peerdn;
  uschar *received_sni;

  const uschar *tls_certificate;
  const uschar *tls_privatekey;
  const uschar *tls_sni; /* client send only, not received */
  const uschar *tls_verify_certificates;
  const uschar *tls_crl;
  const uschar *tls_require_ciphers;
  uschar *exp_tls_certificate;
  uschar *exp_tls_privatekey;
  uschar *exp_tls_sni;
  uschar *exp_tls_verify_certificates;
  uschar *exp_tls_crl;
  uschar *exp_tls_require_ciphers;

  uschar *xfer_buffer;
  int xfer_buffer_lwm;
  int xfer_buffer_hwm;
  int xfer_eof;
  int xfer_error;

  uschar cipherbuf[256];
} exim_gnutls_state_st;

static const exim_gnutls_state_st exim_gnutls_state_init = {
  NULL, NULL, NULL, VERIFY_NONE, -1, -1, FALSE, FALSE,
  NULL, NULL, NULL,
  NULL, NULL, NULL, NULL, NULL, NULL,
  NULL, NULL, NULL, NULL, NULL, NULL,
  NULL, 0, 0, 0, 0,
  ""
};

/* Not only do we have our own APIs which don't pass around state, assuming
it's held in globals, GnuTLS doesn't appear to let us register callback data
for callbacks, or as part of the session, so we have to keep a "this is the
context we're currently dealing with" pointer and rely upon being
single-threaded to keep from processing data on an inbound TLS connection while
talking to another TLS connection for an outbound check.  This does mean that
there's no way for heart-beats to be responded to, for the duration of the
second connection. */

static exim_gnutls_state_st state_server, state_client;
static exim_gnutls_state_st *current_global_tls_state;

/* dh_params are initialised once within the lifetime of a process using TLS;
if we used TLS in a long-lived daemon, we'd have to reconsider this.  But we
don't want to repeat this. */

static gnutls_dh_params_t dh_server_params = NULL;

/* No idea how this value was chosen; preserving it.  Default is 3600. */

static const int ssl_session_timeout = 200;

static const char * const exim_default_gnutls_priority = "NORMAL";

/* Guard library core initialisation */

static BOOL exim_gnutls_base_init_done = FALSE;


/* ------------------------------------------------------------------------ */
/* macros */

#define MAX_HOST_LEN 255

/* Set this to control gnutls_global_set_log_level(); values 0 to 9 will setup
the library logging; a value less than 0 disables the calls to set up logging
callbacks. */
#define EXIM_GNUTLS_LIBRARY_LOG_LEVEL -1

#define EXIM_CLIENT_DH_MIN_BITS 1024

/* With GnuTLS 2.12.x+ we have gnutls_sec_param_to_pk_bits() with which we
can ask for a bit-strength.  Without that, we stick to the constant we had
before, for now. */
#define EXIM_SERVER_DH_BITS_PRE2_12 1024

#define exim_gnutls_err_check(Label) do { \
  if (rc != GNUTLS_E_SUCCESS) { return tls_error((Label), gnutls_strerror(rc), host); } } while (0)

#define expand_check_tlsvar(Varname) expand_check(state->Varname, US #Varname, &state->exp_##Varname)

#if GNUTLS_VERSION_NUMBER >= 0x020c00
#define HAVE_GNUTLS_SESSION_CHANNEL_BINDING
#define HAVE_GNUTLS_SEC_PARAM_CONSTANTS
#define HAVE_GNUTLS_RND
#endif




/* ------------------------------------------------------------------------ */
/* Callback declarations */

#if EXIM_GNUTLS_LIBRARY_LOG_LEVEL >= 0
static void exim_gnutls_logger_cb(int level, const char *message);
#endif

static int exim_sni_handling_cb(gnutls_session_t session);




/* ------------------------------------------------------------------------ */
/* Static functions */

/*************************************************
*               Handle TLS error                 *
*************************************************/

/* Called from lots of places when errors occur before actually starting to do
the TLS handshake, that is, while the session is still in clear. Always returns
DEFER for a server and FAIL for a client so that most calls can use "return
tls_error(...)" to do this processing and then give an appropriate return. A
single function is used for both server and client, because it is called from
some shared functions.

Argument:
  prefix    text to include in the logged error
  msg       additional error string (may be NULL)
            usually obtained from gnutls_strerror()
  host      NULL if setting up a server;
            the connected host if setting up a client

Returns:    OK/DEFER/FAIL
*/

static int
tls_error(const uschar *prefix, const char *msg, const host_item *host)
{
if (host)
  {
  log_write(0, LOG_MAIN, "TLS error on connection to %s [%s] (%s)%s%s",
      host->name, host->address, prefix, msg ? ": " : "", msg ? msg : "");
  return FAIL;
  }
else
  {
  uschar *conn_info = smtp_get_connection_info();
  if (Ustrncmp(conn_info, US"SMTP ", 5) == 0)
    conn_info += 5;
  log_write(0, LOG_MAIN, "TLS error on %s (%s)%s%s",
      conn_info, prefix, msg ? ": " : "", msg ? msg : "");
  return DEFER;
  }
}




/*************************************************
*    Deal with logging errors during I/O         *
*************************************************/

/* We have to get the identity of the peer from saved data.

Argument:
  state    the current GnuTLS exim state container
  rc       the GnuTLS error code, or 0 if it's a local error
  when     text identifying read or write
  text     local error text when ec is 0

Returns:   nothing
*/

static void
record_io_error(exim_gnutls_state_st *state, int rc, uschar *when, uschar *text)
{
const char *msg;

if (rc == GNUTLS_E_FATAL_ALERT_RECEIVED)
  msg = CS string_sprintf("%s: %s", US gnutls_strerror(rc),
    US gnutls_alert_get_name(gnutls_alert_get(state->session)));
else
  msg = gnutls_strerror(rc);

tls_error(when, msg, state->host);
}




/*************************************************
*        Set various Exim expansion vars         *
*************************************************/

/* We set various Exim global variables from the state, once a session has
been established.  With TLS callouts, may need to change this to stack
variables, or just re-call it with the server state after client callout
has finished.

Make sure anything set here is inset in tls_getc().

Sets:
  tls_active                fd
  tls_bits                  strength indicator
  tls_certificate_verified  bool indicator
  tls_channelbinding_b64    for some SASL mechanisms
  tls_cipher                a string
  tls_peerdn                a string
  tls_sni                   a (UTF-8) string
Also:
  current_global_tls_state  for API limitations

Argument:
  state      the relevant exim_gnutls_state_st *
*/

static void
extract_exim_vars_from_tls_state(exim_gnutls_state_st *state)
{
gnutls_protocol_t protocol;
gnutls_cipher_algorithm_t cipher;
gnutls_kx_algorithm_t kx;
gnutls_mac_algorithm_t mac;
uschar *p;
#ifdef HAVE_GNUTLS_SESSION_CHANNEL_BINDING
int old_pool;
int rc;
gnutls_datum_t channel;
#endif

current_global_tls_state = state;

tls_active = state->fd_out;

cipher = gnutls_cipher_get(state->session);
/* returns size in "bytes" */
tls_bits = gnutls_cipher_get_key_size(cipher) * 8;

if (!*state->cipherbuf)
  {
  protocol = gnutls_protocol_get_version(state->session);
  mac = gnutls_mac_get(state->session);
  kx = gnutls_kx_get(state->session);

  string_format(state->cipherbuf, sizeof(state->cipherbuf),
      "%s:%s:%u",
      gnutls_protocol_get_name(protocol),
      gnutls_cipher_suite_get_name(kx, cipher, mac),
      tls_bits);

  /* I don't see a way that spaces could occur, in the current GnuTLS
  code base, but it was a concern in the old code and perhaps older GnuTLS
  releases did return "TLS 1.0"; play it safe, just in case. */
  for (p = state->cipherbuf; *p != '\0'; ++p)
    if (isspace(*p))
      *p = '-';
  }
tls_cipher = state->cipherbuf;

DEBUG(D_tls) debug_printf("cipher: %s\n", tls_cipher);

tls_certificate_verified = state->peer_cert_verified;

/* note that tls_channelbinding_b64 is not saved to the spool file, since it's
only available for use for authenticators while this TLS session is running. */

tls_channelbinding_b64 = NULL;
#ifdef HAVE_GNUTLS_SESSION_CHANNEL_BINDING
channel.data = NULL;
channel.size = 0;
rc = gnutls_session_channel_binding(state->session, GNUTLS_CB_TLS_UNIQUE, &channel);
if (rc) {
  DEBUG(D_tls) debug_printf("Channel binding error: %s\n", gnutls_strerror(rc));
} else {
  old_pool = store_pool;
  store_pool = POOL_PERM;
  tls_channelbinding_b64 = auth_b64encode(channel.data, (int)channel.size);
  store_pool = old_pool;
  DEBUG(D_tls) debug_printf("Have channel bindings cached for possible auth usage.\n");
}
#endif

tls_peerdn = state->peerdn;

tls_sni = state->received_sni;
}




/*************************************************
*            Setup up DH parameters              *
*************************************************/

/* Generating the D-H parameters may take a long time. They only need to
be re-generated every so often, depending on security policy. What we do is to
keep these parameters in a file in the spool directory. If the file does not
exist, we generate them. This means that it is easy to cause a regeneration.

The new file is written as a temporary file and renamed, so that an incomplete
file is never present. If two processes both compute some new parameters, you
waste a bit of effort, but it doesn't seem worth messing around with locking to
prevent this.

Argument:
  host       NULL for server, server for client (for error handling)

Returns:     OK/DEFER/FAIL
*/

static int
init_server_dh(void)
{
int fd, rc;
unsigned int dh_bits;
gnutls_datum m;
uschar filename[PATH_MAX];
size_t sz;
host_item *host = NULL; /* dummy for macros */

DEBUG(D_tls) debug_printf("Initialising GnuTLS server params.\n");

rc = gnutls_dh_params_init(&dh_server_params);
exim_gnutls_err_check(US"gnutls_dh_params_init");

#ifdef HAVE_GNUTLS_SEC_PARAM_CONSTANTS
/* If you change this constant, also change dh_param_fn_ext so that we can use a
different filename and ensure we have sufficient bits. */
dh_bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH, GNUTLS_SEC_PARAM_NORMAL);
if (!dh_bits)
  return tls_error(US"gnutls_sec_param_to_pk_bits() failed", NULL, NULL);
DEBUG(D_tls)
  debug_printf("GnuTLS tells us that for D-H PK, NORMAL is %d bits.\n",
      dh_bits);
#else
dh_bits = EXIM_SERVER_DH_BITS_PRE2_12;
DEBUG(D_tls)
  debug_printf("GnuTLS lacks gnutls_sec_param_to_pk_bits(), using %d bits.\n",
      dh_bits);
#endif

if (!string_format(filename, sizeof(filename),
      "%s/gnutls-params-%d", spool_directory, dh_bits))
  return tls_error(US"overlong filename", NULL, NULL);

/* Open the cache file for reading and if successful, read it and set up the
parameters. */

fd = Uopen(filename, O_RDONLY, 0);
if (fd >= 0)
  {
  struct stat statbuf;
  FILE *fp;
  int saved_errno;

  if (fstat(fd, &statbuf) < 0)  /* EIO */
    {
    saved_errno = errno;
    (void)close(fd);
    return tls_error(US"TLS cache stat failed", strerror(saved_errno), NULL);
    }
  if (!S_ISREG(statbuf.st_mode))
    {
    (void)close(fd);
    return tls_error(US"TLS cache not a file", NULL, NULL);
    }
  fp = fdopen(fd, "rb");
  if (!fp)
    {
    saved_errno = errno;
    (void)close(fd);
    return tls_error(US"fdopen(TLS cache stat fd) failed",
        strerror(saved_errno), NULL);
    }

  m.size = statbuf.st_size;
  m.data = malloc(m.size);
  if (m.data == NULL)
    {
    fclose(fp);
    return tls_error(US"malloc failed", strerror(errno), NULL);
    }
  sz = fread(m.data, m.size, 1, fp);
  if (!sz)
    {
    saved_errno = errno;
    fclose(fp);
    free(m.data);
    return tls_error(US"fread failed", strerror(saved_errno), NULL);
    }
  fclose(fp);

  rc = gnutls_dh_params_import_pkcs3(dh_server_params, &m, GNUTLS_X509_FMT_PEM);
  free(m.data);
  exim_gnutls_err_check(US"gnutls_dh_params_import_pkcs3");
  DEBUG(D_tls) debug_printf("read D-H parameters from file \"%s\"\n", filename);
  }

/* If the file does not exist, fall through to compute new data and cache it.
If there was any other opening error, it is serious. */

else if (errno == ENOENT)
  {
  rc = -1;
  DEBUG(D_tls)
    debug_printf("D-H parameter cache file \"%s\" does not exist\n", filename);
  }
else
  return tls_error(string_open_failed(errno, "\"%s\" for reading", filename),
      NULL, NULL);

/* If ret < 0, either the cache file does not exist, or the data it contains
is not useful. One particular case of this is when upgrading from an older
release of Exim in which the data was stored in a different format. We don't
try to be clever and support both formats; we just regenerate new data in this
case. */

if (rc < 0)
  {
  uschar *temp_fn;

  if ((PATH_MAX - Ustrlen(filename)) < 10)
    return tls_error(US"Filename too long to generate replacement",
        CS filename, NULL);

  temp_fn = string_copy(US "%s.XXXXXXX");
  fd = mkstemp(CS temp_fn); /* modifies temp_fn */
  if (fd < 0)
    return tls_error(US"Unable to open temp file", strerror(errno), NULL);
  (void)fchown(fd, exim_uid, exim_gid);   /* Probably not necessary */

  DEBUG(D_tls) debug_printf("generating %d bits Diffie-Hellman key ...\n", dh_bits);
  rc = gnutls_dh_params_generate2(dh_server_params, dh_bits);
  exim_gnutls_err_check(US"gnutls_dh_params_generate2");

  /* gnutls_dh_params_export_pkcs3() will tell us the exact size, every time,
  and I confirmed that a NULL call to get the size first is how the GnuTLS
  sample apps handle this. */

  sz = 0;
  m.data = NULL;
  rc = gnutls_dh_params_export_pkcs3(dh_server_params, GNUTLS_X509_FMT_PEM,
      m.data, &sz);
  if (rc != GNUTLS_E_SHORT_MEMORY_BUFFER)
    exim_gnutls_err_check(US"gnutls_dh_params_export_pkcs3(NULL) sizing");
  m.size = sz;
  m.data = malloc(m.size);
  if (m.data == NULL)
    return tls_error(US"memory allocation failed", strerror(errno), NULL);
  rc = gnutls_dh_params_export_pkcs3(dh_server_params, GNUTLS_X509_FMT_PEM,
      m.data, &sz);
  if (rc != GNUTLS_E_SUCCESS)
    {
    free(m.data);
    exim_gnutls_err_check(US"gnutls_dh_params_export_pkcs3() real");
    }

  sz = write_to_fd_buf(fd, m.data, (size_t) m.size);
  if (sz != m.size)
    {
    free(m.data);
    return tls_error(US"TLS cache write D-H params failed",
        strerror(errno), NULL);
    }
  free(m.data);
  sz = write_to_fd_buf(fd, US"\n", 1);
  if (sz != 1)
    return tls_error(US"TLS cache write D-H params final newline failed",
        strerror(errno), NULL);

  rc = close(fd);
  if (rc)
    return tls_error(US"TLS cache write close() failed",
        strerror(errno), NULL);

  if (Urename(temp_fn, filename) < 0)
    return tls_error(string_sprintf("failed to rename \"%s\" as \"%s\"",
          temp_fn, filename), strerror(errno), NULL);

  DEBUG(D_tls) debug_printf("wrote D-H parameters to file \"%s\"\n", filename);
  }

DEBUG(D_tls) debug_printf("initialized server D-H parameters\n");
return OK;
}




/*************************************************
*       Variables re-expanded post-SNI           *
*************************************************/

/* Called from both server and client code, via tls_init(), and also from
the SNI callback after receiving an SNI, if tls_certificate includes "tls_sni".

We can tell the two apart by state->received_sni being non-NULL in callback.

The callback should not call us unless state->trigger_sni_changes is true,
which we are responsible for setting on the first pass through.

Arguments:
  state           exim_gnutls_state_st *

Returns:          OK/DEFER/FAIL
*/

static int
tls_expand_session_files(exim_gnutls_state_st *state)
{
struct stat statbuf;
int rc;
const host_item *host = state->host;  /* macro should be reconsidered? */
uschar *saved_tls_certificate = NULL;
uschar *saved_tls_privatekey = NULL;
uschar *saved_tls_verify_certificates = NULL;
uschar *saved_tls_crl = NULL;
int cert_count;

/* We check for tls_sni *before* expansion. */
if (!state->host)
  {
  if (!state->received_sni)
    {
    if (Ustrstr(state->tls_certificate, US"tls_sni"))
      {
      DEBUG(D_tls) debug_printf("We will re-expand TLS session files if we receive SNI.\n");
      state->trigger_sni_changes = TRUE;
      }
    }
  else
    {
    /* useful for debugging */
    saved_tls_certificate = state->exp_tls_certificate;
    saved_tls_privatekey = state->exp_tls_privatekey;
    saved_tls_verify_certificates = state->exp_tls_verify_certificates;
    saved_tls_crl = state->exp_tls_crl;
    }
  }

rc = gnutls_certificate_allocate_credentials(&state->x509_cred);
exim_gnutls_err_check(US"gnutls_certificate_allocate_credentials");

/* remember: expand_check_tlsvar() is expand_check() but fiddling with
state members, assuming consistent naming; and expand_check() returns
false if expansion failed, unless expansion was forced to fail. */

/* check if we at least have a certificate, before doing expensive
D-H generation. */

if (!expand_check_tlsvar(tls_certificate))
  return DEFER;

/* certificate is mandatory in server, optional in client */

if ((state->exp_tls_certificate == NULL) ||
    (*state->exp_tls_certificate == '\0'))
  {
  if (state->host == NULL)
    return tls_error(US"no TLS server certificate is specified", NULL, NULL);
  else
    DEBUG(D_tls) debug_printf("TLS: no client certificate specified; okay\n");
  }

if (state->tls_privatekey && !expand_check_tlsvar(tls_privatekey))
  return DEFER;

/* tls_privatekey is optional, defaulting to same file as certificate */

if (state->tls_privatekey == NULL || *state->tls_privatekey == '\0')
  {
  state->tls_privatekey = state->tls_certificate;
  state->exp_tls_privatekey = state->exp_tls_certificate;
  }


if (state->exp_tls_certificate && *state->exp_tls_certificate)
  {
  DEBUG(D_tls) debug_printf("certificate file = %s\nkey file = %s\n",
      state->exp_tls_certificate, state->exp_tls_privatekey);

  if (state->received_sni)
    {
    if ((Ustrcmp(state->exp_tls_certificate, saved_tls_certificate) == 0) &&
        (Ustrcmp(state->exp_tls_privatekey, saved_tls_privatekey) == 0))
      {
      DEBUG(D_tls) debug_printf("TLS SNI: cert and key unchanged\n");
      }
    else
      {
      DEBUG(D_tls) debug_printf("TLS SNI: have a changed cert/key pair.\n");
      }
    }

  rc = gnutls_certificate_set_x509_key_file(state->x509_cred,
      CS state->exp_tls_certificate, CS state->exp_tls_privatekey,
      GNUTLS_X509_FMT_PEM);
  exim_gnutls_err_check(
      string_sprintf("cert/key setup: cert=%s key=%s",
        state->exp_tls_certificate, state->exp_tls_privatekey));
  DEBUG(D_tls) debug_printf("TLS: cert/key registered\n");
  } /* tls_certificate */

/* Set the trusted CAs file if one is provided, and then add the CRL if one is
provided. Experiment shows that, if the certificate file is empty, an unhelpful
error message is provided. However, if we just refrain from setting anything up
in that case, certificate verification fails, which seems to be the correct
behaviour. */

if (state->tls_verify_certificates && *state->tls_verify_certificates)
  {
  if (!expand_check_tlsvar(tls_verify_certificates))
    return DEFER;
  if (state->tls_crl && *state->tls_crl)
    if (!expand_check_tlsvar(tls_crl))
      return DEFER;

  if (!(state->exp_tls_verify_certificates &&
        *state->exp_tls_verify_certificates))
    {
    DEBUG(D_tls)
      debug_printf("TLS: tls_verify_certificates expanded empty, ignoring\n");
    /* With no tls_verify_certificates, we ignore tls_crl too */
    return OK;
    }
  }
else
  {
  DEBUG(D_tls)
    debug_printf("TLS: tls_verify_certificates not set or empty, ignoring\n");
  return OK;
  }

if (Ustat(state->exp_tls_verify_certificates, &statbuf) < 0)
  {
  log_write(0, LOG_MAIN|LOG_PANIC, "could not stat %s "
      "(tls_verify_certificates): %s", state->exp_tls_verify_certificates,
      strerror(errno));
  return DEFER;
  }

if (!S_ISREG(statbuf.st_mode))
  {
  DEBUG(D_tls)
    debug_printf("verify certificates path is not a file: \"%s\"\n%s\n",
        state->exp_tls_verify_certificates,
        S_ISDIR(statbuf.st_mode)
          ? " it's a directory, that's OpenSSL, this is GnuTLS"
          : " (not a directory either)");
  log_write(0, LOG_MAIN|LOG_PANIC,
      "tls_verify_certificates \"%s\" is not a file",
      state->exp_tls_verify_certificates);
  return DEFER;
  }

DEBUG(D_tls) debug_printf("verify certificates = %s size=" OFF_T_FMT "\n",
        state->exp_tls_verify_certificates, statbuf.st_size);

if (statbuf.st_size == 0)
  {
  DEBUG(D_tls)
    debug_printf("cert file empty, no certs, no verification, ignoring any CRL\n");
  return OK;
  }

cert_count = gnutls_certificate_set_x509_trust_file(state->x509_cred,
    CS state->exp_tls_verify_certificates, GNUTLS_X509_FMT_PEM);
if (cert_count < 0)
  {
  rc = cert_count;
  exim_gnutls_err_check(US"gnutls_certificate_set_x509_trust_file");
  }
DEBUG(D_tls) debug_printf("Added %d certificate authorities.\n", cert_count);

if (state->tls_crl && *state->tls_crl)
  {
  if (state->exp_tls_crl && *state->exp_tls_crl)
    {
    DEBUG(D_tls) debug_printf("loading CRL file = %s\n", state->exp_tls_crl);
    rc = gnutls_certificate_set_x509_crl_file(state->x509_cred,
        CS state->exp_tls_crl, GNUTLS_X509_FMT_PEM);
    exim_gnutls_err_check(US"gnutls_certificate_set_x509_crl_file");
    }
  }

return OK;
}




/*************************************************
*          Set X.509 state variables             *
*************************************************/

/* In GnuTLS, the registered cert/key are not replaced by a later
set of a cert/key, so for SNI support we need a whole new x509_cred
structure.  Which means various other non-re-expanded pieces of state
need to be re-set in the new struct, so the setting logic is pulled
out to this.

Arguments:
  state           exim_gnutls_state_st *

Returns:          OK/DEFER/FAIL
*/

static int
tls_set_remaining_x509(exim_gnutls_state_st *state)
{
int rc;
const host_item *host = state->host;  /* macro should be reconsidered? */

/* Create D-H parameters, or read them from the cache file. This function does
its own SMTP error messaging. This only happens for the server, TLS D-H ignores
client-side params. */

if (!state->host)
  {
  if (!dh_server_params)
    {
    rc = init_server_dh();
    if (rc != OK) return rc;
    }
  gnutls_certificate_set_dh_params(state->x509_cred, dh_server_params);
  }

/* Link the credentials to the session. */

rc = gnutls_credentials_set(state->session, GNUTLS_CRD_CERTIFICATE, state->x509_cred);
exim_gnutls_err_check(US"gnutls_credentials_set");

return OK;
}

/*************************************************
*            Initialize for GnuTLS               *
*************************************************/

/* Called from both server and client code. In the case of a server, errors
before actual TLS negotiation return DEFER.

Arguments:
  host            connected host, if client; NULL if server
  certificate     certificate file
  privatekey      private key file
  sni             TLS SNI to send, sometimes when client; else NULL
  cas             CA certs file
  crl             CRL file
  require_ciphers tls_require_ciphers setting

Returns:          OK/DEFER/FAIL
*/

static int
tls_init(
    const host_item *host,
    const uschar *certificate,
    const uschar *privatekey,
    const uschar *sni,
    const uschar *cas,
    const uschar *crl,
    const uschar *require_ciphers,
    exim_gnutls_state_st **caller_state)
{
exim_gnutls_state_st *state;
int rc;
size_t sz;
const char *errpos;
uschar *p;
BOOL want_default_priorities;

if (!exim_gnutls_base_init_done)
  {
  DEBUG(D_tls) debug_printf("GnuTLS global init required.\n");

  rc = gnutls_global_init();
  exim_gnutls_err_check(US"gnutls_global_init");

#if EXIM_GNUTLS_LIBRARY_LOG_LEVEL >= 0
  DEBUG(D_tls)
    {
    gnutls_global_set_log_function(exim_gnutls_logger_cb);
    /* arbitrarily chosen level; bump upto 9 for more */
    gnutls_global_set_log_level(EXIM_GNUTLS_LIBRARY_LOG_LEVEL);
    }
#endif

  exim_gnutls_base_init_done = TRUE;
  }

if (host)
  {
  state = &state_client;
  memcpy(state, &exim_gnutls_state_init, sizeof(exim_gnutls_state_init));
  DEBUG(D_tls) debug_printf("initialising GnuTLS client session\n");
  rc = gnutls_init(&state->session, GNUTLS_CLIENT);
  }
else
  {
  state = &state_server;
  memcpy(state, &exim_gnutls_state_init, sizeof(exim_gnutls_state_init));
  DEBUG(D_tls) debug_printf("initialising GnuTLS server session\n");
  rc = gnutls_init(&state->session, GNUTLS_SERVER);
  }
exim_gnutls_err_check(US"gnutls_init");

state->host = host;

state->tls_certificate = certificate;
state->tls_privatekey = privatekey;
state->tls_sni = sni;
state->tls_verify_certificates = cas;
state->tls_crl = crl;

/* This handles the variables that might get re-expanded after TLS SNI;
that's tls_certificate, tls_privatekey, tls_verify_certificates, tls_crl */

DEBUG(D_tls)
  debug_printf("Expanding various TLS configuration options for session credentials.\n");
rc = tls_expand_session_files(state);
if (rc != OK) return rc;

/* These are all other parts of the x509_cred handling, since SNI in GnuTLS
requires a new structure afterwards. */

rc = tls_set_remaining_x509(state);
if (rc != OK) return rc;

/* set SNI in client, only */
if (host)
  {
  if (!expand_check_tlsvar(tls_sni))
    return DEFER;
  if (state->exp_tls_sni && *state->exp_tls_sni)
    {
    DEBUG(D_tls)
      debug_printf("Setting TLS client SNI to \"%s\"\n", state->exp_tls_sni);
    sz = Ustrlen(state->exp_tls_sni);
    rc = gnutls_server_name_set(state->session,
        GNUTLS_NAME_DNS, state->exp_tls_sni, sz);
    exim_gnutls_err_check(US"gnutls_server_name_set");
    }
  }
else if (state->tls_sni)
  DEBUG(D_tls) debug_printf("*** PROBABLY A BUG *** " \
      "have an SNI set for a client [%s]\n", state->tls_sni);

/* This is the priority string support,
http://www.gnu.org/software/gnutls/manual/html_node/Priority-Strings.html
and replaces gnutls_require_kx, gnutls_require_mac & gnutls_require_protocols.
This was backwards incompatible, but means Exim no longer needs to track
all algorithms and provide string forms for them. */

want_default_priorities = TRUE;

if (state->tls_require_ciphers && *state->tls_require_ciphers)
  {
  if (!expand_check_tlsvar(tls_require_ciphers))
    return DEFER;
  if (state->exp_tls_require_ciphers && *state->exp_tls_require_ciphers)
    {
    DEBUG(D_tls) debug_printf("GnuTLS session cipher/priority \"%s\"\n",
        state->exp_tls_require_ciphers);

    rc = gnutls_priority_init(&state->priority_cache,
        CS state->exp_tls_require_ciphers, &errpos);
    want_default_priorities = FALSE;
    p = state->exp_tls_require_ciphers;
    }
  }
if (want_default_priorities)
  {
  DEBUG(D_tls)
    debug_printf("GnuTLS using default session cipher/priority \"%s\"\n",
        exim_default_gnutls_priority);
  rc = gnutls_priority_init(&state->priority_cache,
      exim_default_gnutls_priority, &errpos);
  p = US exim_default_gnutls_priority;
  }

exim_gnutls_err_check(string_sprintf(
      "gnutls_priority_init(%s) failed at offset %ld, \"%.6s..\"",
      p, errpos - CS p, errpos));

rc = gnutls_priority_set(state->session, state->priority_cache);
exim_gnutls_err_check(US"gnutls_priority_set");

gnutls_db_set_cache_expiration(state->session, ssl_session_timeout);

/* Reduce security in favour of increased compatibility, if the admin
decides to make that trade-off. */
if (gnutls_compat_mode)
  {
#if LIBGNUTLS_VERSION_NUMBER >= 0x020104
  DEBUG(D_tls) debug_printf("lowering GnuTLS security, compatibility mode\n");
  gnutls_session_enable_compatibility_mode(state->session);
#else
  DEBUG(D_tls) debug_printf("Unable to set gnutls_compat_mode - GnuTLS version too old\n");
#endif
  }

*caller_state = state;
/* needs to happen before callbacks during handshake */
current_global_tls_state = state;
return OK;
}




/*************************************************
*            Extract peer information            *
*************************************************/

/* Called from both server and client code.
Only this is allowed to set state->peerdn and we use that to detect double-calls.

Arguments:
  state           exim_gnutls_state_st *

Returns:          OK/DEFER/FAIL
*/

static int
peer_status(exim_gnutls_state_st *state)
{
const gnutls_datum *cert_list;
int rc;
unsigned int cert_list_size = 0;
gnutls_certificate_type_t ct;
gnutls_x509_crt_t crt;
uschar *dn_buf;
size_t sz;

if (state->peerdn)
  return OK;

state->peerdn = US"unknown";

cert_list = gnutls_certificate_get_peers(state->session, &cert_list_size);

if (cert_list == NULL || cert_list_size == 0)
  {
  state->peerdn = US"unknown (no certificate)";
  DEBUG(D_tls) debug_printf("TLS: no certificate from peer (%p & %d)\n",
      cert_list, cert_list_size);
  if (state->verify_requirement == VERIFY_REQUIRED)
    return tls_error(US"certificate verification failed",
        "no certificate received from peer", state->host);
  return OK;
  }

ct = gnutls_certificate_type_get(state->session);
if (ct != GNUTLS_CRT_X509)
  {
  const char *ctn = gnutls_certificate_type_get_name(ct);
  state->peerdn = string_sprintf("unknown (type %s)", ctn);
  DEBUG(D_tls)
    debug_printf("TLS: peer cert not X.509 but instead \"%s\"\n", ctn);
  if (state->verify_requirement == VERIFY_REQUIRED)
    return tls_error(US"certificate verification not possible, unhandled type",
        ctn, state->host);
  return OK;
  }

#define exim_gnutls_peer_err(Label) do { \
  if (rc != GNUTLS_E_SUCCESS) { \
    DEBUG(D_tls) debug_printf("TLS: peer cert problem: %s: %s\n", (Label), gnutls_strerror(rc)); \
    if (state->verify_requirement == VERIFY_REQUIRED) { return tls_error((Label), gnutls_strerror(rc), state->host); } \
    return OK; } } while (0)

rc = gnutls_x509_crt_init(&crt);
exim_gnutls_peer_err(US"gnutls_x509_crt_init (crt)");

rc = gnutls_x509_crt_import(crt, &cert_list[0], GNUTLS_X509_FMT_DER);
exim_gnutls_peer_err(US"failed to import certificate [gnutls_x509_crt_import(cert 0)]");
sz = 0;
rc = gnutls_x509_crt_get_dn(crt, NULL, &sz);
if (rc != GNUTLS_E_SHORT_MEMORY_BUFFER)
  {
  exim_gnutls_peer_err(US"getting size for cert DN failed");
  return FAIL; /* should not happen */
  }
dn_buf = store_get_perm(sz);
rc = gnutls_x509_crt_get_dn(crt, CS dn_buf, &sz);
exim_gnutls_peer_err(US"failed to extract certificate DN [gnutls_x509_crt_get_dn(cert 0)]");
state->peerdn = dn_buf;

return OK;
#undef exim_gnutls_peer_err
}




/*************************************************
*            Verify peer certificate             *
*************************************************/

/* Called from both server and client code.
*Should* be using a callback registered with
gnutls_certificate_set_verify_function() to fail the handshake if we dislike
the peer information, but that's too new for some OSes.

Arguments:
  state           exim_gnutls_state_st *
  error           where to put an error message

Returns:
  FALSE     if the session should be rejected
  TRUE      if the cert is okay or we just don't care
*/

static BOOL
verify_certificate(exim_gnutls_state_st *state, const char **error)
{
int rc;
unsigned int verify;

*error = NULL;

rc = peer_status(state);
if (rc != OK)
  {
  verify = GNUTLS_CERT_INVALID;
  *error = "not supplied";
  }
else
  {
  rc = gnutls_certificate_verify_peers2(state->session, &verify);
  }

/* Handle the result of verification. INVALID seems to be set as well
as REVOKED, but leave the test for both. */

if ((rc < 0) || (verify & (GNUTLS_CERT_INVALID|GNUTLS_CERT_REVOKED)) != 0)
  {
  state->peer_cert_verified = FALSE;
  if (*error == NULL)
    *error = ((verify & GNUTLS_CERT_REVOKED) != 0) ? "revoked" : "invalid";

  DEBUG(D_tls)
    debug_printf("TLS certificate verification failed (%s): peerdn=%s\n",
        *error, state->peerdn);

  if (state->verify_requirement == VERIFY_REQUIRED)
    {
    gnutls_alert_send(state->session, GNUTLS_AL_FATAL, GNUTLS_A_BAD_CERTIFICATE);
    return FALSE;
    }
  DEBUG(D_tls)
    debug_printf("TLS verify failure overriden (host in tls_try_verify_hosts)\n");
  }
else
  {
  state->peer_cert_verified = TRUE;
  DEBUG(D_tls) debug_printf("TLS certificate verified: peerdn=%s\n", state->peerdn);
  }

tls_peerdn = state->peerdn;

return TRUE;
}




/* ------------------------------------------------------------------------ */
/* Callbacks */

/* Logging function which can be registered with
 *   gnutls_global_set_log_function()
 *   gnutls_global_set_log_level() 0..9
 */
#if EXIM_GNUTLS_LIBRARY_LOG_LEVEL >= 0
static void
exim_gnutls_logger_cb(int level, const char *message)
{
  DEBUG(D_tls) debug_printf("GnuTLS<%d>: %s\n", level, message);
}
#endif


/* Called after client hello, should handle SNI work.
This will always set tls_sni (state->received_sni) if available,
and may trigger presenting different certificates,
if state->trigger_sni_changes is TRUE.

Should be registered with
  gnutls_handshake_set_post_client_hello_function()

"This callback must return 0 on success or a gnutls error code to terminate the
handshake.".

For inability to get SNI information, we return 0.
We only return non-zero if re-setup failed.
*/

static int
exim_sni_handling_cb(gnutls_session_t session)
{
char sni_name[MAX_HOST_LEN];
size_t data_len = MAX_HOST_LEN;
exim_gnutls_state_st *state = current_global_tls_state;
unsigned int sni_type;
int rc, old_pool;

rc = gnutls_server_name_get(session, sni_name, &data_len, &sni_type, 0);
if (rc != GNUTLS_E_SUCCESS)
  {
  DEBUG(D_tls) {
    if (rc == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE)
      debug_printf("TLS: no SNI presented in handshake.\n");
    else
      debug_printf("TLS failure: gnutls_server_name_get(): %s [%d]\n",
        gnutls_strerror(rc), rc);
  };
  return 0;
  }

if (sni_type != GNUTLS_NAME_DNS)
  {
  DEBUG(D_tls) debug_printf("TLS: ignoring SNI of unhandled type %u\n", sni_type);
  return 0;
  }

/* We now have a UTF-8 string in sni_name */
old_pool = store_pool;
store_pool = POOL_PERM;
state->received_sni = string_copyn(US sni_name, data_len);
store_pool = old_pool;

/* We set this one now so that variable expansions below will work */
tls_sni = state->received_sni;

DEBUG(D_tls) debug_printf("Received TLS SNI \"%s\"%s\n", sni_name,
    state->trigger_sni_changes ? "" : " (unused for certificate selection)");

if (!state->trigger_sni_changes)
  return 0;

rc = tls_expand_session_files(state);
if (rc != OK)
  {
  /* If the setup of certs/etc failed before handshake, TLS would not have
  been offered.  The best we can do now is abort. */
  return GNUTLS_E_APPLICATION_ERROR_MIN;
  }

rc = tls_set_remaining_x509(state);
if (rc != OK) return GNUTLS_E_APPLICATION_ERROR_MIN;

return 0;
}




/* ------------------------------------------------------------------------ */
/* Exported functions */




/*************************************************
*       Start a TLS session in a server          *
*************************************************/

/* This is called when Exim is running as a server, after having received
the STARTTLS command. It must respond to that command, and then negotiate
a TLS session.

Arguments:
  require_ciphers  list of allowed ciphers or NULL

Returns:           OK on success
                   DEFER for errors before the start of the negotiation
                   FAIL for errors during the negotation; the server can't
                     continue running.
*/

int
tls_server_start(const uschar *require_ciphers)
{
int rc;
const char *error;
exim_gnutls_state_st *state = NULL;

/* Check for previous activation */
/* nb: this will not be TLS callout safe, needs reworking as part of that. */

if (tls_active >= 0)
  {
  tls_error(US"STARTTLS received after TLS started", "", NULL);
  smtp_printf("554 Already in TLS\r\n");
  return FAIL;
  }

/* Initialize the library. If it fails, it will already have logged the error
and sent an SMTP response. */

DEBUG(D_tls) debug_printf("initialising GnuTLS as a server\n");

rc = tls_init(NULL, tls_certificate, tls_privatekey,
    NULL, tls_verify_certificates, tls_crl,
    require_ciphers, &state);
if (rc != OK) return rc;

/* If this is a host for which certificate verification is mandatory or
optional, set up appropriately. */

if (verify_check_host(&tls_verify_hosts) == OK)
  {
  DEBUG(D_tls) debug_printf("TLS: a client certificate will be required.\n");
  state->verify_requirement = VERIFY_REQUIRED;
  gnutls_certificate_server_set_request(state->session, GNUTLS_CERT_REQUIRE);
  }
else if (verify_check_host(&tls_try_verify_hosts) == OK)
  {
  DEBUG(D_tls) debug_printf("TLS: a client certificate will be requested but not required.\n");
  state->verify_requirement = VERIFY_OPTIONAL;
  gnutls_certificate_server_set_request(state->session, GNUTLS_CERT_REQUEST);
  }
else
  {
  DEBUG(D_tls) debug_printf("TLS: a client certificate will not be requested.\n");
  state->verify_requirement = VERIFY_NONE;
  gnutls_certificate_server_set_request(state->session, GNUTLS_CERT_IGNORE);
  }

/* Register SNI handling; always, even if not in tls_certificate, so that the
expansion variable $tls_sni is always available. */

gnutls_handshake_set_post_client_hello_function(state->session,
    exim_sni_handling_cb);

/* Set context and tell client to go ahead, except in the case of TLS startup
on connection, where outputting anything now upsets the clients and tends to
make them disconnect. We need to have an explicit fflush() here, to force out
the response. Other smtp_printf() calls do not need it, because in non-TLS
mode, the fflush() happens when smtp_getc() is called. */

if (!tls_on_connect)
  {
  smtp_printf("220 TLS go ahead\r\n");
  fflush(smtp_out);
  }

/* Now negotiate the TLS session. We put our own timer on it, since it seems
that the GnuTLS library doesn't. */

gnutls_transport_set_ptr2(state->session,
    (gnutls_transport_ptr)fileno(smtp_in),
    (gnutls_transport_ptr)fileno(smtp_out));
state->fd_in = fileno(smtp_in);
state->fd_out = fileno(smtp_out);

sigalrm_seen = FALSE;
if (smtp_receive_timeout > 0) alarm(smtp_receive_timeout);
do
  {
  rc = gnutls_handshake(state->session);
  } while ((rc == GNUTLS_E_AGAIN) || (rc == GNUTLS_E_INTERRUPTED));
alarm(0);

if (rc != GNUTLS_E_SUCCESS)
  {
  tls_error(US"gnutls_handshake",
      sigalrm_seen ? "timed out" : gnutls_strerror(rc), NULL);
  /* It seems that, except in the case of a timeout, we have to close the
  connection right here; otherwise if the other end is running OpenSSL it hangs
  until the server times out. */

  if (!sigalrm_seen)
    {
    (void)fclose(smtp_out);
    (void)fclose(smtp_in);
    }

  return FAIL;
  }

DEBUG(D_tls) debug_printf("gnutls_handshake was successful\n");

/* Verify after the fact */

if (state->verify_requirement != VERIFY_NONE)
  {
  if (!verify_certificate(state, &error))
    {
    if (state->verify_requirement == VERIFY_OPTIONAL)
      {
      DEBUG(D_tls)
        debug_printf("TLS: continuing on only because verification was optional, after: %s\n",
            error);
      }
    else
      {
      tls_error(US"certificate verification failed", error, NULL);
      return FAIL;
      }
    }
  }

/* Figure out peer DN, and if authenticated, etc. */

rc = peer_status(state);
if (rc != OK) return rc;

/* Sets various Exim expansion variables; always safe within server */

extract_exim_vars_from_tls_state(state);

/* TLS has been set up. Adjust the input functions to read via TLS,
and initialize appropriately. */

state->xfer_buffer = store_malloc(ssl_xfer_buffer_size);

receive_getc = tls_getc;
receive_ungetc = tls_ungetc;
receive_feof = tls_feof;
receive_ferror = tls_ferror;
receive_smtp_buffered = tls_smtp_buffered;

return OK;
}




/*************************************************
*    Start a TLS session in a client             *
*************************************************/

/* Called from the smtp transport after STARTTLS has been accepted.

Arguments:
  fd                the fd of the connection
  host              connected host (for messages)
  addr              the first address (not used)
  dhparam           DH parameter file (ignored, we're a client)
  certificate       certificate file
  privatekey        private key file
  sni               TLS SNI to send to remote host
  verify_certs      file for certificate verify
  verify_crl        CRL for verify
  require_ciphers   list of allowed ciphers or NULL
  timeout           startup timeout

Returns:            OK/DEFER/FAIL (because using common functions),
                    but for a client, DEFER and FAIL have the same meaning
*/

int
tls_client_start(int fd, host_item *host,
    address_item *addr ARG_UNUSED, uschar *dhparam ARG_UNUSED,
    uschar *certificate, uschar *privatekey, uschar *sni,
    uschar *verify_certs, uschar *verify_crl,
    uschar *require_ciphers, int timeout)
{
int rc;
const char *error;
exim_gnutls_state_st *state = NULL;

DEBUG(D_tls) debug_printf("initialising GnuTLS as a client on fd %d\n", fd);

rc = tls_init(host, certificate, privatekey,
    sni, verify_certs, verify_crl, require_ciphers, &state);
if (rc != OK) return rc;

gnutls_dh_set_prime_bits(state->session, EXIM_CLIENT_DH_MIN_BITS);

if (verify_certs == NULL)
  {
  DEBUG(D_tls) debug_printf("TLS: server certificate verification not required\n");
  state->verify_requirement = VERIFY_NONE;
  /* we still ask for it, to log it, etc */
  gnutls_certificate_server_set_request(state->session, GNUTLS_CERT_REQUEST);
  }
else
  {
  DEBUG(D_tls) debug_printf("TLS: server certificate verification required\n");
  state->verify_requirement = VERIFY_REQUIRED;
  gnutls_certificate_server_set_request(state->session, GNUTLS_CERT_REQUIRE);
  }

gnutls_transport_set_ptr(state->session, (gnutls_transport_ptr)fd);
state->fd_in = fd;
state->fd_out = fd;

/* There doesn't seem to be a built-in timeout on connection. */

sigalrm_seen = FALSE;
alarm(timeout);
do
  {
  rc = gnutls_handshake(state->session);
  } while ((rc == GNUTLS_E_AGAIN) || (rc == GNUTLS_E_INTERRUPTED));
alarm(0);

DEBUG(D_tls) debug_printf("gnutls_handshake was successful\n");

/* Verify late */

if (state->verify_requirement != VERIFY_NONE &&
    !verify_certificate(state, &error))
  return tls_error(US"certificate verification failed", error, state->host);

/* Figure out peer DN, and if authenticated, etc. */

rc = peer_status(state);
if (rc != OK) return rc;

/* Sets various Exim expansion variables; always safe within server */

extract_exim_vars_from_tls_state(state);

return OK;
}




/*************************************************
*         Close down a TLS session               *
*************************************************/

/* This is also called from within a delivery subprocess forked from the
daemon, to shut down the TLS library, without actually doing a shutdown (which
would tamper with the TLS session in the parent process).

Arguments:   TRUE if gnutls_bye is to be called
Returns:     nothing
*/

void
tls_close(BOOL shutdown)
{
exim_gnutls_state_st *state = current_global_tls_state;

if (tls_active < 0) return;  /* TLS was not active */

if (shutdown)
  {
  DEBUG(D_tls) debug_printf("tls_close(): shutting down TLS\n");
  gnutls_bye(state->session, GNUTLS_SHUT_WR);
  }

gnutls_deinit(state->session);

memcpy(state, &exim_gnutls_state_init, sizeof(exim_gnutls_state_init));

if ((state_server.session == NULL) && (state_client.session == NULL))
  {
  gnutls_global_deinit();
  exim_gnutls_base_init_done = FALSE;
  }

tls_active = -1;
}




/*************************************************
*            TLS version of getc                 *
*************************************************/

/* This gets the next byte from the TLS input buffer. If the buffer is empty,
it refills the buffer via the GnuTLS reading function.

This feeds DKIM and should be used for all message-body reads.

Arguments:  none
Returns:    the next character or EOF
*/

int
tls_getc(void)
{
exim_gnutls_state_st *state = current_global_tls_state;
if (state->xfer_buffer_lwm >= state->xfer_buffer_hwm)
  {
  ssize_t inbytes;

  DEBUG(D_tls) debug_printf("Calling gnutls_record_recv(%p, %p, %u)\n",
    state->session, state->xfer_buffer, ssl_xfer_buffer_size);

  if (smtp_receive_timeout > 0) alarm(smtp_receive_timeout);
  inbytes = gnutls_record_recv(state->session, state->xfer_buffer,
    ssl_xfer_buffer_size);
  alarm(0);

  /* A zero-byte return appears to mean that the TLS session has been
     closed down, not that the socket itself has been closed down. Revert to
     non-TLS handling. */

  if (inbytes == 0)
    {
    DEBUG(D_tls) debug_printf("Got TLS_EOF\n");

    receive_getc = smtp_getc;
    receive_ungetc = smtp_ungetc;
    receive_feof = smtp_feof;
    receive_ferror = smtp_ferror;
    receive_smtp_buffered = smtp_buffered;

    gnutls_deinit(state->session);
    state->session = NULL;
    tls_active = -1;
    tls_bits = 0;
    tls_certificate_verified = FALSE;
    tls_channelbinding_b64 = NULL;
    tls_cipher = NULL;
    tls_peerdn = NULL;

    return smtp_getc();
    }

  /* Handle genuine errors */

  else if (inbytes < 0)
    {
    record_io_error(state, (int) inbytes, US"recv", NULL);
    state->xfer_error = 1;
    return EOF;
    }
#ifndef DISABLE_DKIM
  dkim_exim_verify_feed(state->xfer_buffer, inbytes);
#endif
  state->xfer_buffer_hwm = (int) inbytes;
  state->xfer_buffer_lwm = 0;
  }

/* Something in the buffer; return next uschar */

return state->xfer_buffer[state->xfer_buffer_lwm++];
}




/*************************************************
*          Read bytes from TLS channel           *
*************************************************/

/* This does not feed DKIM, so if the caller uses this for reading message body,
then the caller must feed DKIM.
Arguments:
  buff      buffer of data
  len       size of buffer

Returns:    the number of bytes read
            -1 after a failed read
*/

int
tls_read(uschar *buff, size_t len)
{
exim_gnutls_state_st *state = current_global_tls_state;
ssize_t inbytes;

if (len > INT_MAX)
  len = INT_MAX;

if (state->xfer_buffer_lwm < state->xfer_buffer_hwm)
  DEBUG(D_tls)
    debug_printf("*** PROBABLY A BUG *** " \
        "tls_read() called with data in the tls_getc() buffer, %d ignored\n",
        state->xfer_buffer_hwm - state->xfer_buffer_lwm);

DEBUG(D_tls)
  debug_printf("Calling gnutls_record_recv(%p, %p, " SIZE_T_FMT ")\n",
      state->session, buff, len);

inbytes = gnutls_record_recv(state->session, buff, len);
if (inbytes > 0) return inbytes;
if (inbytes == 0)
  {
  DEBUG(D_tls) debug_printf("Got TLS_EOF\n");
  }
else record_io_error(state, (int)inbytes, US"recv", NULL);

return -1;
}




/*************************************************
*         Write bytes down TLS channel           *
*************************************************/

/*
Arguments:
  buff      buffer of data
  len       number of bytes

Returns:    the number of bytes after a successful write,
            -1 after a failed write
*/

int
tls_write(const uschar *buff, size_t len)
{
ssize_t outbytes;
size_t left = len;
exim_gnutls_state_st *state = current_global_tls_state;

DEBUG(D_tls) debug_printf("tls_do_write(%p, " SIZE_T_FMT ")\n", buff, left);
while (left > 0)
  {
  DEBUG(D_tls) debug_printf("gnutls_record_send(SSL, %p, " SIZE_T_FMT ")\n",
      buff, left);
  outbytes = gnutls_record_send(state->session, buff, left);

  DEBUG(D_tls) debug_printf("outbytes=" SSIZE_T_FMT "\n", outbytes);
  if (outbytes < 0)
    {
    record_io_error(state, outbytes, US"send", NULL);
    return -1;
    }
  if (outbytes == 0)
    {
    record_io_error(state, 0, US"send", US"TLS channel closed on write");
    return -1;
    }

  left -= outbytes;
  buff += outbytes;
  }

if (len > INT_MAX)
  {
  DEBUG(D_tls)
    debug_printf("Whoops!  Wrote more bytes (" SIZE_T_FMT ") than INT_MAX\n",
        len);
  len = INT_MAX;
  }

return (int) len;
}




/*************************************************
*            Random number generation            *
*************************************************/

/* Pseudo-random number generation.  The result is not expected to be
cryptographically strong but not so weak that someone will shoot themselves
in the foot using it as a nonce in input in some email header scheme or
whatever weirdness they'll twist this into.  The result should handle fork()
and avoid repeating sequences.  OpenSSL handles that for us.

Arguments:
  max       range maximum
Returns     a random number in range [0, max-1]
*/

#ifdef HAVE_GNUTLS_RND
int
vaguely_random_number(int max)
{
unsigned int r;
int i, needed_len;
uschar *p;
uschar smallbuf[sizeof(r)];

if (max <= 1)
  return 0;

needed_len = sizeof(r);
/* Don't take 8 times more entropy than needed if int is 8 octets and we were
 * asked for a number less than 10. */
for (r = max, i = 0; r; ++i)
  r >>= 1;
i = (i + 7) / 8;
if (i < needed_len)
  needed_len = i;

i = gnutls_rnd(GNUTLS_RND_NONCE, smallbuf, needed_len);
if (i < 0)
  {
  DEBUG(D_all) debug_printf("gnutls_rnd() failed, using fallback.\n");
  return vaguely_random_number_fallback(max);
  }
r = 0;
for (p = smallbuf; needed_len; --needed_len, ++p)
  {
  r *= 256;
  r += *p;
  }

/* We don't particularly care about weighted results; if someone wants
 * smooth distribution and cares enough then they should submit a patch then. */
return r % max;
}
#else /* HAVE_GNUTLS_RND */
int
vaguely_random_number(int max)
{
  return vaguely_random_number_fallback(max);
}
#endif /* HAVE_GNUTLS_RND */




/*************************************************
*         Report the library versions.           *
*************************************************/

/* See a description in tls-openssl.c for an explanation of why this exists.

Arguments:   a FILE* to print the results to
Returns:     nothing
*/

void
tls_version_report(FILE *f)
{
fprintf(f, "Library version: GnuTLS: Compile: %s\n"
           "                         Runtime: %s\n",
           LIBGNUTLS_VERSION,
           gnutls_check_version(NULL));
}

/* End of tls-gnu.c */