Support TLS_FALLBACK_SCSV.

Reviewed-by: Rich Salz <rsalz@openssl.org>

CHANGES

@@ -4,6 +4,12 @@
 
  Changes between 1.0.0n and 1.0.0o [xx XXX xxxx]
 
+  *) Add support for TLS_FALLBACK_SCSV.
+     Client applications doing fallback retries should call
+     SSL_set_mode(s, SSL_MODE_SEND_FALLBACK_SCSV).
+     (CVE-2014-3566)
+     [Adam Langley, Bodo Moeller]
+
   *) Add additional DigestInfo checks.
  
      Reencode DigestInto in DER and check against the original when

apps/s_client.c

@@ -322,6 +322,7 @@ static void sc_usage(void)
 	BIO_printf(bio_err," -ssl3         - just use SSLv3\n");
 	BIO_printf(bio_err," -tls1         - just use TLSv1\n");
 	BIO_printf(bio_err," -dtls1        - just use DTLSv1\n");    
+	BIO_printf(bio_err," -fallback_scsv - send TLS_FALLBACK_SCSV\n");
 	BIO_printf(bio_err," -mtu          - set the link layer MTU\n");
 	BIO_printf(bio_err," -no_tls1/-no_ssl3/-no_ssl2 - turn off that protocol\n");
 	BIO_printf(bio_err," -bugs         - Switch on all SSL implementation bug workarounds\n");
@@ -436,6 +437,7 @@ int MAIN(int argc, char **argv)
 	char *sess_out = NULL;
 	struct sockaddr peer;
 	int peerlen = sizeof(peer);
+	int fallback_scsv = 0;
 	int enable_timeouts = 0 ;
 	long socket_mtu = 0;
 #ifndef OPENSSL_NO_JPAKE
@@ -602,6 +604,10 @@ int MAIN(int argc, char **argv)
 			meth=DTLSv1_client_method();
 			socket_type=SOCK_DGRAM;
 			}
+		else if (strcmp(*argv,"-fallback_scsv") == 0)
+			{
+			fallback_scsv = 1;
+			}
 		else if (strcmp(*argv,"-timeout") == 0)
 			enable_timeouts=1;
 		else if (strcmp(*argv,"-mtu") == 0)
@@ -935,6 +941,10 @@ bad:
 		SSL_set_session(con, sess);
 		SSL_SESSION_free(sess);
 		}
+
+	if (fallback_scsv)
+		SSL_set_mode(con, SSL_MODE_SEND_FALLBACK_SCSV);
+
 #ifndef OPENSSL_NO_TLSEXT
 	if (servername != NULL)
 		{

crypto/err/openssl.ec

@@ -71,6 +71,7 @@ R SSL_R_TLSV1_ALERT_EXPORT_RESTRICTION		1060
 R SSL_R_TLSV1_ALERT_PROTOCOL_VERSION		1070
 R SSL_R_TLSV1_ALERT_INSUFFICIENT_SECURITY	1071
 R SSL_R_TLSV1_ALERT_INTERNAL_ERROR		1080
+R SSL_R_SSLV3_ALERT_INAPPROPRIATE_FALLBACK	1086
 R SSL_R_TLSV1_ALERT_USER_CANCELLED		1090
 R SSL_R_TLSV1_ALERT_NO_RENEGOTIATION		1100
 R SSL_R_TLSV1_UNSUPPORTED_EXTENSION		1110

ssl/d1_lib.c

@@ -265,6 +265,16 @@ long dtls1_ctrl(SSL *s, int cmd, long larg, void *parg)
 	case DTLS_CTRL_LISTEN:
 		ret = dtls1_listen(s, parg);
 		break;
+	case SSL_CTRL_CHECK_PROTO_VERSION:
+		/* For library-internal use; checks that the current protocol
+		 * is the highest enabled version (according to s->ctx->method,
+		 * as version negotiation may have changed s->method). */
+#if DTLS_MAX_VERSION != DTLS1_VERSION
+#  error Code needs update for DTLS_method() support beyond DTLS1_VERSION.
+#endif
+		/* Just one protocol version is supported so far;
+		 * fail closed if the version is not as expected. */
+		return s->version == DTLS_MAX_VERSION;
 
 	default:
 		ret = ssl3_ctrl(s, cmd, larg, parg);

ssl/dtls1.h

@@ -80,6 +80,8 @@ extern "C" {
 #endif
 
 #define DTLS1_VERSION			0xFEFF
+#define DTLS_MAX_VERSION		DTLS1_VERSION
+
 #define DTLS1_BAD_VER			0x0100
 
 #if 0
@@ -264,4 +266,3 @@ typedef struct dtls1_record_data_st
 }
 #endif
 #endif
-

ssl/s23_clnt.c

@@ -632,6 +632,9 @@ static int ssl23_get_server_hello(SSL *s)
 			goto err;
 			}
 
+		/* ensure that TLS_MAX_VERSION is up-to-date */
+		OPENSSL_assert(s->version <= TLS_MAX_VERSION);
+
 		if (p[0] == SSL3_RT_ALERT && p[5] != SSL3_AL_WARNING)
 			{
 			/* fatal alert */

ssl/s23_srvr.c

@@ -389,6 +389,9 @@ int ssl23_get_client_hello(SSL *s)
 			}
 		}
 
+	/* ensure that TLS_MAX_VERSION is up-to-date */
+	OPENSSL_assert(s->version <= TLS_MAX_VERSION);
+
 	if (s->state == SSL23_ST_SR_CLNT_HELLO_B)
 		{
 		/* we have SSLv3/TLSv1 in an SSLv2 header

ssl/s2_lib.c

@@ -391,6 +391,8 @@ long ssl2_ctrl(SSL *s, int cmd, long larg, void *parg)
 	case SSL_CTRL_GET_SESSION_REUSED:
 		ret=s->hit;
 		break;
+	case SSL_CTRL_CHECK_PROTO_VERSION:
+		return ssl3_ctrl(s, SSL_CTRL_CHECK_PROTO_VERSION, larg, parg);
 	default:
 		break;
 		}
@@ -437,7 +439,7 @@ int ssl2_put_cipher_by_char(const SSL_CIPHER *c, unsigned char *p)
 	if (p != NULL)
 		{
 		l=c->id;
-		if ((l & 0xff000000) != 0x02000000) return(0);
+		if ((l & 0xff000000) != 0x02000000 && l != SSL3_CK_FALLBACK_SCSV) return(0);
 		p[0]=((unsigned char)(l>>16L))&0xFF;
 		p[1]=((unsigned char)(l>> 8L))&0xFF;
 		p[2]=((unsigned char)(l     ))&0xFF;

ssl/s3_enc.c

@@ -888,7 +888,7 @@ int ssl3_alert_code(int code)
 	case SSL_AD_BAD_CERTIFICATE_STATUS_RESPONSE: return(SSL3_AD_HANDSHAKE_FAILURE);
 	case SSL_AD_BAD_CERTIFICATE_HASH_VALUE: return(SSL3_AD_HANDSHAKE_FAILURE);
 	case SSL_AD_UNKNOWN_PSK_IDENTITY:return(TLS1_AD_UNKNOWN_PSK_IDENTITY);
+	case SSL_AD_INAPPROPRIATE_FALLBACK:return(TLS1_AD_INAPPROPRIATE_FALLBACK);
 	default:			return(-1);
 		}
 	}
-

ssl/s3_lib.c

@@ -2492,6 +2492,29 @@ long ssl3_ctrl(SSL *s, int cmd, long larg, void *parg)
 		break;
 
 #endif /* !OPENSSL_NO_TLSEXT */
+
+	case SSL_CTRL_CHECK_PROTO_VERSION:
+		/* For library-internal use; checks that the current protocol
+		 * is the highest enabled version (according to s->ctx->method,
+		 * as version negotiation may have changed s->method). */
+		if (s->version == s->ctx->method->version)
+			return 1;
+		/* Apparently we're using a version-flexible SSL_METHOD
+		 * (not at its highest protocol version). */
+		if (s->ctx->method->version == SSLv23_method()->version)
+			{
+#if TLS_MAX_VERSION != TLS1_VERSION
+#  error Code needs update for SSLv23_method() support beyond TLS1_VERSION.
+#endif
+			if (!(s->options & SSL_OP_NO_TLSv1))
+				return s->version == TLS1_VERSION;
+			if (!(s->options & SSL_OP_NO_SSLv3))
+				return s->version == SSL3_VERSION;
+			if (!(s->options & SSL_OP_NO_SSLv2))
+				return s->version == SSL2_VERSION;
+			}
+		return 0; /* Unexpected state; fail closed. */
+
 	default:
 		break;
 		}
@@ -2793,6 +2816,7 @@ long ssl3_ctx_callback_ctrl(SSL_CTX *ctx, int cmd, void (*fp)(void))
 		break;
 
 #endif
+
 	default:
 		return(0);
 		}
@@ -3347,4 +3371,3 @@ need to go to SSL_ST_ACCEPT.
 		}
 	return(ret);
 	}
-

ssl/ssl.h

@@ -605,6 +605,10 @@ typedef struct ssl_session_st
  * TLS only.)  "Released" buffers are put onto a free-list in the context
  * or just freed (depending on the context's setting for freelist_max_len). */
 #define SSL_MODE_RELEASE_BUFFERS 0x00000010L
+/* Send TLS_FALLBACK_SCSV in the ClientHello.
+ * To be set by applications that reconnect with a downgraded protocol
+ * version; see draft-ietf-tls-downgrade-scsv-00 for details. */
+#define SSL_MODE_SEND_FALLBACK_SCSV 0x00000080L
 
 /* Note: SSL[_CTX]_set_{options,mode} use |= op on the previous value,
  * they cannot be used to clear bits. */
@@ -1328,6 +1332,7 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 #define SSL_AD_BAD_CERTIFICATE_STATUS_RESPONSE TLS1_AD_BAD_CERTIFICATE_STATUS_RESPONSE
 #define SSL_AD_BAD_CERTIFICATE_HASH_VALUE TLS1_AD_BAD_CERTIFICATE_HASH_VALUE
 #define SSL_AD_UNKNOWN_PSK_IDENTITY     TLS1_AD_UNKNOWN_PSK_IDENTITY /* fatal */
+#define SSL_AD_INAPPROPRIATE_FALLBACK	TLS1_AD_INAPPROPRIATE_FALLBACK /* fatal */
 
 #define SSL_ERROR_NONE			0
 #define SSL_ERROR_SSL			1
@@ -1421,6 +1426,8 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 #define SSL_CTRL_CLEAR_OPTIONS			77
 #define SSL_CTRL_CLEAR_MODE			78
 
+#define SSL_CTRL_CHECK_PROTO_VERSION		119
+
 #define DTLSv1_get_timeout(ssl, arg) \
 	SSL_ctrl(ssl,DTLS_CTRL_GET_TIMEOUT,0, (void *)arg)
 #define DTLSv1_handle_timeout(ssl) \
@@ -2106,6 +2113,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_HTTPS_PROXY_REQUEST			 155
 #define SSL_R_HTTP_REQUEST				 156
 #define SSL_R_ILLEGAL_PADDING				 283
+#define SSL_R_INAPPROPRIATE_FALLBACK			 373
 #define SSL_R_INCONSISTENT_COMPRESSION			 340
 #define SSL_R_INVALID_CHALLENGE_LENGTH			 158
 #define SSL_R_INVALID_COMMAND				 280
@@ -2244,6 +2252,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_TLSV1_ALERT_DECRYPTION_FAILED		 1021
 #define SSL_R_TLSV1_ALERT_DECRYPT_ERROR			 1051
 #define SSL_R_TLSV1_ALERT_EXPORT_RESTRICTION		 1060
+#define SSL_R_TLSV1_ALERT_INAPPROPRIATE_FALLBACK	 1086
 #define SSL_R_TLSV1_ALERT_INSUFFICIENT_SECURITY		 1071
 #define SSL_R_TLSV1_ALERT_INTERNAL_ERROR		 1080
 #define SSL_R_TLSV1_ALERT_NO_RENEGOTIATION		 1100

ssl/ssl3.h

@@ -128,9 +128,14 @@
 extern "C" {
 #endif
 
-/* Signalling cipher suite value: from draft-ietf-tls-renegotiation-03.txt */
+/* Signalling cipher suite value from RFC 5746
+ * (TLS_EMPTY_RENEGOTIATION_INFO_SCSV) */
 #define SSL3_CK_SCSV				0x030000FF
 
+/* Signalling cipher suite value from draft-ietf-tls-downgrade-scsv-00
+ * (TLS_FALLBACK_SCSV) */
+#define SSL3_CK_FALLBACK_SCSV			0x03005600
+
 #define SSL3_CK_RSA_NULL_MD5			0x03000001
 #define SSL3_CK_RSA_NULL_SHA			0x03000002
 #define SSL3_CK_RSA_RC4_40_MD5 			0x03000003

ssl/ssl_err.c

@@ -361,6 +361,7 @@ static ERR_STRING_DATA SSL_str_reasons[]=
 {ERR_REASON(SSL_R_HTTPS_PROXY_REQUEST)   ,"https proxy request"},
 {ERR_REASON(SSL_R_HTTP_REQUEST)          ,"http request"},
 {ERR_REASON(SSL_R_ILLEGAL_PADDING)       ,"illegal padding"},
+{ERR_REASON(SSL_R_INAPPROPRIATE_FALLBACK),"inappropriate fallback"},
 {ERR_REASON(SSL_R_INCONSISTENT_COMPRESSION),"inconsistent compression"},
 {ERR_REASON(SSL_R_INVALID_CHALLENGE_LENGTH),"invalid challenge length"},
 {ERR_REASON(SSL_R_INVALID_COMMAND)       ,"invalid command"},
@@ -499,6 +500,7 @@ static ERR_STRING_DATA SSL_str_reasons[]=
 {ERR_REASON(SSL_R_TLSV1_ALERT_DECRYPTION_FAILED),"tlsv1 alert decryption failed"},
 {ERR_REASON(SSL_R_TLSV1_ALERT_DECRYPT_ERROR),"tlsv1 alert decrypt error"},
 {ERR_REASON(SSL_R_TLSV1_ALERT_EXPORT_RESTRICTION),"tlsv1 alert export restriction"},
+{ERR_REASON(SSL_R_TLSV1_ALERT_INAPPROPRIATE_FALLBACK),"tlsv1 alert inappropriate fallback"},
 {ERR_REASON(SSL_R_TLSV1_ALERT_INSUFFICIENT_SECURITY),"tlsv1 alert insufficient security"},
 {ERR_REASON(SSL_R_TLSV1_ALERT_INTERNAL_ERROR),"tlsv1 alert internal error"},
 {ERR_REASON(SSL_R_TLSV1_ALERT_NO_RENEGOTIATION),"tlsv1 alert no renegotiation"},

ssl/ssl_lib.c

@@ -1360,6 +1360,8 @@ int ssl_cipher_list_to_bytes(SSL *s,STACK_OF(SSL_CIPHER) *sk,unsigned char *p,
 
 	if (sk == NULL) return(0);
 	q=p;
+	if (put_cb == NULL)
+		put_cb = s->method->put_cipher_by_char;
 
 	for (i=0; i<sk_SSL_CIPHER_num(sk); i++)
 		{
@@ -1375,24 +1377,36 @@ int ssl_cipher_list_to_bytes(SSL *s,STACK_OF(SSL_CIPHER) *sk,unsigned char *p,
 		    s->psk_client_callback == NULL)
 			continue;
 #endif /* OPENSSL_NO_PSK */
-		j = put_cb ? put_cb(c,p) : ssl_put_cipher_by_char(s,c,p);
+		j = put_cb(c,p);
 		p+=j;
 		}
-	/* If p == q, no ciphers and caller indicates an error. Otherwise
+	/* If p == q, no ciphers; caller indicates an error.
-	 * add SCSV if not renegotiating.
+	 * Otherwise, add applicable SCSVs. */
-	 */
+	if (p != q)
-	if (p != q && !s->new_session)
 		{
-		static SSL_CIPHER scsv =
+		if (!s->new_session)
 			{
-			0, NULL, SSL3_CK_SCSV, 0, 0, 0, 0, 0, 0, 0, 0, 0
+			static SSL_CIPHER scsv =
-			};
+				{
-		j = put_cb ? put_cb(&scsv,p) : ssl_put_cipher_by_char(s,&scsv,p);
+				0, NULL, SSL3_CK_SCSV, 0, 0, 0, 0, 0, 0, 0, 0, 0
-		p+=j;
+				};
+			j = put_cb(&scsv,p);
+			p+=j;
 #ifdef OPENSSL_RI_DEBUG
-		fprintf(stderr, "SCSV sent by client\n");
+			fprintf(stderr, "TLS_EMPTY_RENEGOTIATION_INFO_SCSV sent by client\n");
 #endif
-		}
+			}
+
+		if (s->mode & SSL_MODE_SEND_FALLBACK_SCSV)
+			{
+			static SSL_CIPHER scsv =
+				{
+				0, NULL, SSL3_CK_FALLBACK_SCSV, 0, 0, 0, 0, 0, 0, 0, 0, 0
+				};
+			j = put_cb(&scsv,p);
+			p+=j;
+			}
+ 		}
 
 	return(p-q);
 	}
@@ -1403,11 +1417,12 @@ STACK_OF(SSL_CIPHER) *ssl_bytes_to_cipher_list(SSL *s,unsigned char *p,int num,
 	const SSL_CIPHER *c;
 	STACK_OF(SSL_CIPHER) *sk;
 	int i,n;
+
 	if (s->s3)
 		s->s3->send_connection_binding = 0;
 
 	n=ssl_put_cipher_by_char(s,NULL,NULL);
-	if ((num%n) != 0)
+	if (n == 0 || (num%n) != 0)
 		{
 		SSLerr(SSL_F_SSL_BYTES_TO_CIPHER_LIST,SSL_R_ERROR_IN_RECEIVED_CIPHER_LIST);
 		return(NULL);
@@ -1422,7 +1437,7 @@ STACK_OF(SSL_CIPHER) *ssl_bytes_to_cipher_list(SSL *s,unsigned char *p,int num,
 
 	for (i=0; i<num; i+=n)
 		{
-		/* Check for SCSV */
+		/* Check for TLS_EMPTY_RENEGOTIATION_INFO_SCSV */
 		if (s->s3 && (n != 3 || !p[0]) &&
 			(p[n-2] == ((SSL3_CK_SCSV >> 8) & 0xff)) &&
 			(p[n-1] == (SSL3_CK_SCSV & 0xff)))
@@ -1442,6 +1457,23 @@ STACK_OF(SSL_CIPHER) *ssl_bytes_to_cipher_list(SSL *s,unsigned char *p,int num,
 			continue;
 			}
 
+		/* Check for TLS_FALLBACK_SCSV */
+		if ((n != 3 || !p[0]) &&
+			(p[n-2] == ((SSL3_CK_FALLBACK_SCSV >> 8) & 0xff)) &&
+			(p[n-1] == (SSL3_CK_FALLBACK_SCSV & 0xff)))
+			{
+			/* The SCSV indicates that the client previously tried a higher version.
+			 * Fail if the current version is an unexpected downgrade. */
+			if (!SSL_ctrl(s, SSL_CTRL_CHECK_PROTO_VERSION, 0, NULL))
+				{
+				SSLerr(SSL_F_SSL_BYTES_TO_CIPHER_LIST,SSL_R_INAPPROPRIATE_FALLBACK);
+				if (s->s3)
+					ssl3_send_alert(s,SSL3_AL_FATAL,SSL_AD_INAPPROPRIATE_FALLBACK);
+				goto err;
+				}
+			continue;
+			}
+
 		c=ssl_get_cipher_by_char(s,p);
 		p+=n;
 		if (c != NULL)

ssl/t1_enc.c

@@ -1035,6 +1035,7 @@ int tls1_alert_code(int code)
 	case SSL_AD_BAD_CERTIFICATE_STATUS_RESPONSE: return(TLS1_AD_BAD_CERTIFICATE_STATUS_RESPONSE);
 	case SSL_AD_BAD_CERTIFICATE_HASH_VALUE: return(TLS1_AD_BAD_CERTIFICATE_HASH_VALUE);
 	case SSL_AD_UNKNOWN_PSK_IDENTITY:return(TLS1_AD_UNKNOWN_PSK_IDENTITY);
+	case SSL_AD_INAPPROPRIATE_FALLBACK:return(TLS1_AD_INAPPROPRIATE_FALLBACK);
 #if 0 /* not appropriate for TLS, not used for DTLS */
 	case DTLS1_AD_MISSING_HANDSHAKE_MESSAGE: return 
 					  (DTLS1_AD_MISSING_HANDSHAKE_MESSAGE);

ssl/tls1.h

@@ -159,17 +159,24 @@ extern "C" {
 
 #define TLS1_ALLOW_EXPERIMENTAL_CIPHERSUITES	0
 
-#define TLS1_2_VERSION			0x0303
+#define TLS1_VERSION			0x0301
-#define TLS1_2_VERSION_MAJOR		0x03
-#define TLS1_2_VERSION_MINOR		0x03
-
 #define TLS1_1_VERSION			0x0302
+#define TLS1_2_VERSION			0x0303
+/* TLS 1.1 and 1.2 are not supported by this version of TLS, so
+ * TLS_MAX_VERSION indicates TLS 1.0 regardless of the above
+ * definitions. (s23_clnt.c and s23_srvr.c have an OPENSSL_assert()
+ * check that would catch the error if TLS_MAX_VERSION was too low.)
+ */
+#define TLS_MAX_VERSION			TLS1_VERSION
+
+#define TLS1_VERSION_MAJOR		0x03
+#define TLS1_VERSION_MINOR		0x01
+
 #define TLS1_1_VERSION_MAJOR		0x03
 #define TLS1_1_VERSION_MINOR		0x02
 
-#define TLS1_VERSION			0x0301
+#define TLS1_2_VERSION_MAJOR		0x03
-#define TLS1_VERSION_MAJOR		0x03
+#define TLS1_2_VERSION_MINOR		0x03
-#define TLS1_VERSION_MINOR		0x01
 
 #define TLS1_get_version(s) \
 		((s->version >> 8) == TLS1_VERSION_MAJOR ? s->version : 0)
@@ -187,6 +194,7 @@ extern "C" {
 #define TLS1_AD_PROTOCOL_VERSION	70	/* fatal */
 #define TLS1_AD_INSUFFICIENT_SECURITY	71	/* fatal */
 #define TLS1_AD_INTERNAL_ERROR		80	/* fatal */
+#define TLS1_AD_INAPPROPRIATE_FALLBACK	86	/* fatal */
 #define TLS1_AD_USER_CANCELLED		90
 #define TLS1_AD_NO_RENEGOTIATION	100
 /* codes 110-114 are from RFC3546 */