crypto: Allow gracefully ending the TLS session

QEMU's TLS session code provides no way to call gnutls_bye() to
terminate a TLS session. Callers of qcrypto_tls_session_read() can
choose to ignore a GNUTLS_E_PREMATURE_TERMINATION error by setting the
gracefulTermination argument.

The QIOChannelTLS ignores the premature termination error whenever
shutdown() has already been issued. This was found to be not enough for
the migration code because shutdown() might not have been issued before
the connection is terminated.

Add support for calling gnutls_bye() in the tlssession layer so users
of QIOChannelTLS can clearly identify the end of a TLS session.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

crypto/tlssession.c

@@ -585,6 +585,40 @@ qcrypto_tls_session_get_handshake_status(QCryptoTLSSession *session)
     }
 }
 
+int
+qcrypto_tls_session_bye(QCryptoTLSSession *session, Error **errp)
+{
+    int ret;
+
+    if (!session->handshakeComplete) {
+        return 0;
+    }
+
+    ret = gnutls_bye(session->handle, GNUTLS_SHUT_WR);
+
+    if (!ret) {
+        return QCRYPTO_TLS_BYE_COMPLETE;
+    }
+
+    if (ret == GNUTLS_E_INTERRUPTED || ret == GNUTLS_E_AGAIN) {
+        int direction = gnutls_record_get_direction(session->handle);
+        return direction ? QCRYPTO_TLS_BYE_SENDING : QCRYPTO_TLS_BYE_RECVING;
+    }
+
+    if (session->rerr || session->werr) {
+        error_setg(errp, "TLS termination failed: %s: %s", gnutls_strerror(ret),
+                   error_get_pretty(session->rerr ?
+                                    session->rerr : session->werr));
+    } else {
+        error_setg(errp, "TLS termination failed: %s", gnutls_strerror(ret));
+    }
+
+    error_free(session->rerr);
+    error_free(session->werr);
+    session->rerr = session->werr = NULL;
+
+    return -1;
+}
 
 int
 qcrypto_tls_session_get_key_size(QCryptoTLSSession *session,
@@ -699,6 +733,13 @@ qcrypto_tls_session_get_handshake_status(QCryptoTLSSession *sess)
 }
 
 
+int
+qcrypto_tls_session_bye(QCryptoTLSSession *session, Error **errp)
+{
+    return QCRYPTO_TLS_BYE_COMPLETE;
+}
+
+
 int
 qcrypto_tls_session_get_key_size(QCryptoTLSSession *sess,
                                  Error **errp)

include/crypto/tlssession.h

@@ -323,6 +323,28 @@ typedef enum {
 QCryptoTLSSessionHandshakeStatus
 qcrypto_tls_session_get_handshake_status(QCryptoTLSSession *sess);
 
+typedef enum {
+    QCRYPTO_TLS_BYE_COMPLETE,
+    QCRYPTO_TLS_BYE_SENDING,
+    QCRYPTO_TLS_BYE_RECVING,
+} QCryptoTLSSessionByeStatus;
+
+/**
+ * qcrypto_tls_session_bye:
+ * @session: the TLS session object
+ * @errp: pointer to a NULL-initialized error object
+ *
+ * Start, or continue, a TLS termination sequence. If the underlying
+ * data channel is non-blocking, then this method may return control
+ * before the termination is complete. The return value will indicate
+ * whether the termination has completed, or is waiting to send or
+ * receive data. In the latter cases, the caller should setup an event
+ * loop watch and call this method again once the underlying data
+ * channel is ready to read or write again.
+ */
+int
+qcrypto_tls_session_bye(QCryptoTLSSession *session, Error **errp);
+
 /**
  * qcrypto_tls_session_get_key_size:
  * @sess: the TLS session object