FreeRDP/libfreerdp-crypto/tls.c

/**
 * FreeRDP: A Remote Desktop Protocol Implementation
 * Transport Layer Security
 *
 * Copyright 2011-2012 Marc-Andre Moreau <marcandre.moreau@gmail.com>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *		 http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <freerdp/utils/stream.h>
#include <freerdp/utils/memory.h>

#include <freerdp/crypto/tls.h>

static CryptoCert tls_get_certificate(rdpTls* tls, boolean peer)
{
	CryptoCert cert;
	X509* server_cert;

	if (peer)
		server_cert = SSL_get_peer_certificate(tls->ssl);
	else
		server_cert = SSL_get_certificate(tls->ssl);

	if (!server_cert)
	{
		printf("tls_get_certificate: failed to get the server TLS certificate\n");
		cert = NULL;
	}
	else
	{
		cert = xmalloc(sizeof(*cert));
		cert->px509 = server_cert;
	}

	return cert;
}

static void tls_free_certificate(CryptoCert cert)
{

	X509_free(cert->px509);
	xfree(cert);
}

boolean tls_connect(rdpTls* tls)
{
	CryptoCert cert;
	int connection_status;

	tls->ctx = SSL_CTX_new(TLSv1_client_method());

	if (tls->ctx == NULL)
	{
		printf("SSL_CTX_new failed\n");
		return false;
	}

	/*
	 * This is necessary, because the Microsoft TLS implementation is not perfect.
	 * SSL_OP_ALL enables a couple of workarounds for buggy TLS implementations,
	 * but the most important workaround being SSL_OP_TLS_BLOCK_PADDING_BUG.
	 * As the size of the encrypted payload may give hints about its contents,
	 * block padding is normally used, but the Microsoft TLS implementation
	 * won't recognize it and will disconnect you after sending a TLS alert.
	 */
	SSL_CTX_set_options(tls->ctx, SSL_OP_ALL);

	tls->ssl = SSL_new(tls->ctx);

	if (tls->ssl == NULL)
	{
		SSL_CTX_free(tls->ctx);
		printf("SSL_new failed\n");
		return false;
	}

	if (SSL_set_fd(tls->ssl, tls->sockfd) < 1)
	{
		SSL_free(tls->ssl);
		SSL_CTX_free(tls->ctx);
		printf("SSL_set_fd failed\n");
		return false;
	}

	connection_status = SSL_connect(tls->ssl);

	if (connection_status <= 0)
	{
		if (tls_print_error("SSL_connect", tls->ssl, connection_status))
		{
			SSL_free(tls->ssl);
			SSL_CTX_free(tls->ctx);
			return false;
		}
	}

	cert = tls_get_certificate(tls, true);

	if (cert == NULL)
	{
		printf("tls_connect: tls_get_certificate failed to return the server certificate.\n");
		return false;
	}

	if (!crypto_cert_get_public_key(cert, &tls->public_key))
	{
		printf("tls_connect: crypto_cert_get_public_key failed to return the server public key.\n");
		return false;
	}

	if (!tls_verify_certificate(tls, cert, tls->settings->hostname))
		tls_disconnect(tls);

	tls_free_certificate(cert);

	return true;
}

boolean tls_accept(rdpTls* tls, const char* cert_file, const char* privatekey_file)
{
	CryptoCert cert;
	int connection_status;

	tls->ctx = SSL_CTX_new(SSLv23_server_method());

	if (tls->ctx == NULL)
	{
		printf("SSL_CTX_new failed\n");
		return false;
	}

	/*
	 * We only want SSLv3 and TLSv1, so disable SSLv2.
	 * SSLv3 is used by, eg. Microsoft RDC for Mac OS X.
	 */
	SSL_CTX_set_options(tls->ctx, SSL_OP_NO_SSLv2);

	if (SSL_CTX_use_RSAPrivateKey_file(tls->ctx, privatekey_file, SSL_FILETYPE_PEM) <= 0)
	{
		printf("SSL_CTX_use_RSAPrivateKey_file failed\n");
		return false;
	}

	tls->ssl = SSL_new(tls->ctx);

	if (tls->ssl == NULL)
	{
		printf("SSL_new failed\n");
		return false;
	}

	if (SSL_use_certificate_file(tls->ssl, cert_file, SSL_FILETYPE_PEM) <= 0)
	{
		printf("SSL_use_certificate_file failed\n");
		return false;
	}

	cert = tls_get_certificate(tls, false);

	if (cert == NULL)
	{
		printf("tls_connect: tls_get_certificate failed to return the server certificate.\n");
		return false;
	}

	if (!crypto_cert_get_public_key(cert, &tls->public_key))
	{
		printf("tls_connect: crypto_cert_get_public_key failed to return the server public key.\n");
		return false;
	}

	if (SSL_set_fd(tls->ssl, tls->sockfd) < 1)
	{
		printf("SSL_set_fd failed\n");
		return false;
	}

	connection_status = SSL_accept(tls->ssl);

	if (connection_status <= 0)
	{
		if (tls_print_error("SSL_accept", tls->ssl, connection_status))
			return false;
	}

	printf("TLS connection accepted\n");

	return true;
}

boolean tls_disconnect(rdpTls* tls)
{
	SSL_shutdown(tls->ssl);
	return true;
}

int tls_read(rdpTls* tls, uint8* data, int length)
{
	int status;

	status = SSL_read(tls->ssl, data, length);

	switch (SSL_get_error(tls->ssl, status))
	{
		case SSL_ERROR_NONE:
			break;

		case SSL_ERROR_WANT_READ:
		case SSL_ERROR_WANT_WRITE:
			status = 0;
			break;

		default:
			tls_print_error("SSL_read", tls->ssl, status);
			status = -1;
			break;
	}

	return status;
}

int tls_write(rdpTls* tls, uint8* data, int length)
{
	int status;

	status = SSL_write(tls->ssl, data, length);

	switch (SSL_get_error(tls->ssl, status))
	{
		case SSL_ERROR_NONE:
			break;

		case SSL_ERROR_WANT_READ:
		case SSL_ERROR_WANT_WRITE:
			status = 0;
			break;

		default:
			tls_print_error("SSL_write", tls->ssl, status);
			status = -1;
			break;
	}

	return status;
}

static void tls_errors(const char *prefix)
{
	unsigned long error;

	while ((error = ERR_get_error()) != 0)
		printf("%s: %s\n", prefix, ERR_error_string(error, NULL));
}

boolean tls_print_error(char* func, SSL* connection, int value)
{
	switch (SSL_get_error(connection, value))
	{
		case SSL_ERROR_ZERO_RETURN:
			printf("%s: Server closed TLS connection\n", func);
			return true;

		case SSL_ERROR_WANT_READ:
			printf("SSL_ERROR_WANT_READ\n");
			return false;

		case SSL_ERROR_WANT_WRITE:
			printf("SSL_ERROR_WANT_WRITE\n");
			return false;

		case SSL_ERROR_SYSCALL:
			printf("%s: I/O error\n", func);
			tls_errors(func);
			return true;

		case SSL_ERROR_SSL:
			printf("%s: Failure in SSL library (protocol error?)\n", func);
			tls_errors(func);
			return true;

		default:
			printf("%s: Unknown error\n", func);
			tls_errors(func);
			return true;
	}
}

boolean tls_verify_certificate(rdpTls* tls, CryptoCert cert, char* hostname)
{
	int match;
	int index;
	char* common_name = NULL;
	int common_name_length = 0;
	char** alt_names = NULL;
	int alt_names_count = 0;
	int* alt_names_lengths = NULL;
	boolean certificate_status;
	boolean hostname_match = false;
	boolean verification_status = false;
	rdpCertificateData* certificate_data;

	/* ignore certificate verification if user explicitly required it (discouraged) */
	if (tls->settings->ignore_certificate)
		return true;  /* success! */

	/* if user explicitly specified a certificate name, use it instead of the hostname */
	if (tls->settings->certificate_name)
		hostname = tls->settings->certificate_name;

	/* attempt verification using OpenSSL and the ~/.freerdp/certs certificate store */
	certificate_status = x509_verify_certificate(cert, tls->certificate_store->path);

	/* verify certificate name match */
	certificate_data = crypto_get_certificate_data(cert->px509, hostname);

	/* extra common name and alternative names */
	common_name = crypto_cert_subject_common_name(cert->px509, &common_name_length);
	alt_names = crypto_cert_subject_alt_name(cert->px509, &alt_names_count, &alt_names_lengths);

	/* compare against common name */

	if (common_name != NULL)
	{
		if (strlen(hostname) == common_name_length)
		{
			if (memcmp((void*) hostname, (void*) common_name, common_name_length) == 0)
				hostname_match = true;
		}
	}

	/* compare against alternative names */

	if (alt_names != NULL)
	{
		for (index = 0; index < alt_names_count; index++)
		{
			if (strlen(hostname) == alt_names_lengths[index])
			{
				if (memcmp((void*) hostname, (void*) alt_names[index], alt_names_lengths[index]) == 0)
					hostname_match = true;
			}
		}
	}

	/* if the certificate is valid and the certificate name matches, verification succeeds */
	if (certificate_status && hostname_match)
	{
		if (common_name)
			xfree(common_name);

		verification_status = true; /* success! */
	}

	/* if the certificate is valid but the certificate name does not match, warn user, do not accept */
	if (certificate_status && !hostname_match)
		tls_print_certificate_name_mismatch_error(hostname, common_name, alt_names, alt_names_count);

	/* verification could not succeed with OpenSSL, use known_hosts file and prompt user for manual verification */

	if (!certificate_status)
	{
		char* issuer;
		char* subject;
		char* fingerprint;
		boolean accept_certificate = false;

		issuer = crypto_cert_issuer(cert->px509);
		subject = crypto_cert_subject(cert->px509);
		fingerprint = crypto_cert_fingerprint(cert->px509);

		/* search for matching entry in known_hosts file */
		match = certificate_data_match(tls->certificate_store, certificate_data);

		if (match == 1)
		{
			/* no entry was found in known_hosts file, prompt user for manual verification */

			freerdp* instance = (freerdp*) tls->settings->instance;

			if (!hostname_match)
				tls_print_certificate_name_mismatch_error(hostname, common_name, alt_names, alt_names_count);

			if (instance->VerifyCertificate)
				accept_certificate = instance->VerifyCertificate(instance, subject, issuer, fingerprint);

			if (!accept_certificate)
			{
				/* user did not accept, abort and do not add entry in known_hosts file */
				verification_status = false;  /* failure! */
			}
			else
			{
				/* user accepted certificate, add entry in known_hosts file */
				certificate_data_print(tls->certificate_store, certificate_data);
				verification_status = true; /* success! */
			}
		}
		else if (match == -1)
		{
			/* entry was found in known_hosts file, but fingerprint does not match */
			tls_print_certificate_error(hostname, fingerprint);
			verification_status = false; /* failure! */
		}
		else if (match == 0)
		{
			verification_status = true; /* success! */
		}

		xfree(issuer);
		xfree(subject);
		xfree(fingerprint);
	}

#ifndef _WIN32
	if (common_name)
		xfree(common_name);

	if (alt_names)
	{
		for (index = 0; index < alt_names_count; index++)
			xfree(alt_names[index]);

		xfree(alt_names);
	}
#endif

	if (certificate_data)
	{
		xfree(certificate_data->fingerprint);
		xfree(certificate_data->hostname);
		xfree(certificate_data);
	}

	return verification_status;
}

void tls_print_certificate_error(char* hostname, char* fingerprint)
{
	printf("The host key for %s has changed\n", hostname);
	printf("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n");
	printf("@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @\n");
	printf("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n");
	printf("IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!\n");
	printf("Someone could be eavesdropping on you right now (man-in-the-middle attack)!\n");
	printf("It is also possible that a host key has just been changed.\n");
	printf("The fingerprint for the host key sent by the remote host is\n%s\n", fingerprint);
	printf("Please contact your system administrator.\n");
	printf("Add correct host key in ~/.freerdp/known_hosts to get rid of this message.\n");
	printf("Host key for %s has changed and you have requested strict checking.\n", hostname);
	printf("Host key verification failed.\n");
}

void tls_print_certificate_name_mismatch_error(char* hostname, char* common_name, char** alt_names, int alt_names_count)
{
	int index;

	printf("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n");
	printf("@           WARNING: CERTIFICATE NAME MISMATCH!           @\n");
	printf("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n");
	printf("The hostname used for this connection (%s) \n", hostname);

	if (alt_names_count < 1)
	{
		printf("does not match the name given in the certificate:\n");
		printf("%s\n", common_name);
	}
	else
	{
		printf("does not match the names given in the certificate:\n");
		printf("%s", common_name);

		for (index = 0; index < alt_names_count; index++)
		{
			printf(", %s", alt_names[index]);
		}

		printf("\n");
	}

	printf("A valid certificate for the wrong name should NOT be trusted!\n");
}

rdpTls* tls_new(rdpSettings* settings)
{
	rdpTls* tls;

	tls = (rdpTls*) xzalloc(sizeof(rdpTls));

	if (tls != NULL)
	{
		SSL_load_error_strings();
		SSL_library_init();

		tls->settings = settings;
		tls->certificate_store = certificate_store_new(settings);
	}

	return tls;
}

void tls_free(rdpTls* tls)
{
	if (tls != NULL)
	{
		if (tls->ssl)
			SSL_free(tls->ssl);

		if (tls->ctx)
			SSL_CTX_free(tls->ctx);

		freerdp_blob_free(&tls->public_key);

		certificate_store_free(tls->certificate_store);

		xfree(tls);
	}
}