Only allow one SGC handshake restart for SSL/TLS. (CVE-2011-4619)

CHANGES

@@ -3,6 +3,9 @@
  _______________
 
  Changes between 0.9.8r and 0.9.8s [xx XXX xxxx]
+
+  *) Only allow one SGC handshake restart for SSL/TLS. (CVE-2011-4619)
+     [Adam Langley (Google)]
  
   *) Prevent malformed RFC3779 data triggering an assertion failure.
      Thanks to Andrew Chi, BBN Technologies, for discovering the flaw

ssl/s3_srvr.c

@@ -235,6 +235,7 @@ int ssl3_accept(SSL *s)
 				}
 
 			s->init_num=0;
+			s->s3->flags &= ~SSL3_FLAGS_SGC_RESTART_DONE;
 
 			if (s->state != SSL_ST_RENEGOTIATE)
 				{
@@ -697,6 +698,14 @@ int ssl3_check_client_hello(SSL *s)
 	int ok;
 	long n;
 
+	/* We only allow the client to restart the handshake once per
+	 * negotiation. */
+	if (s->s3->flags & SSL3_FLAGS_SGC_RESTART_DONE)
+		{
+		SSLerr(SSL_F_SSL3_CHECK_CLIENT_HELLO, SSL_R_MULTIPLE_SGC_RESTARTS);
+		return -1;
+		}
+
 	/* this function is called when we really expect a Certificate message,
 	 * so permit appropriate message length */
 	n=s->method->ssl_get_message(s,
@@ -725,6 +734,7 @@ int ssl3_check_client_hello(SSL *s)
 			s->s3->tmp.ecdh = NULL;
 			}
 #endif
+		s->s3->flags |= SSL3_FLAGS_SGC_RESTART_DONE;
 		return 2;
 		}
 	return 1;

ssl/ssl.h

@@ -1739,6 +1739,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_F_SSL3_CALLBACK_CTRL			 233
 #define SSL_F_SSL3_CHANGE_CIPHER_STATE			 129
 #define SSL_F_SSL3_CHECK_CERT_AND_ALGORITHM		 130
+#define SSL_F_SSL3_CHECK_CLIENT_HELLO			 292
 #define SSL_F_SSL3_CLIENT_HELLO				 131
 #define SSL_F_SSL3_CONNECT				 132
 #define SSL_F_SSL3_CTRL					 213
@@ -1974,6 +1975,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_MISSING_TMP_RSA_KEY			 172
 #define SSL_R_MISSING_TMP_RSA_PKEY			 173
 #define SSL_R_MISSING_VERIFY_MESSAGE			 174
+#define SSL_R_MULTIPLE_SGC_RESTARTS			 325
 #define SSL_R_NON_SSLV2_INITIAL_PACKET			 175
 #define SSL_R_NO_CERTIFICATES_RETURNED			 176
 #define SSL_R_NO_CERTIFICATE_ASSIGNED			 177

ssl/ssl3.h

@@ -333,6 +333,17 @@ typedef struct ssl3_buffer_st
 #define SSL3_FLAGS_DELAY_CLIENT_FINISHED	0x0002
 #define SSL3_FLAGS_POP_BUFFER			0x0004
 #define TLS1_FLAGS_TLS_PADDING_BUG		0x0008
+ 
+/* SSL3_FLAGS_SGC_RESTART_DONE is set when we
+ * restart a handshake because of MS SGC and so prevents us
+ * from restarting the handshake in a loop. It's reset on a
+ * renegotiation, so effectively limits the client to one restart
+ * per negotiation. This limits the possibility of a DDoS
+ * attack where the client handshakes in a loop using SGC to
+ * restart. Servers which permit renegotiation can still be
+ * effected, but we can't prevent that.
+ */
+#define SSL3_FLAGS_SGC_RESTART_DONE		0x0040
 
 typedef struct ssl3_state_st
 	{

ssl/ssl_err.c

@@ -1,6 +1,6 @@
 /* ssl/ssl_err.c */
 /* ====================================================================
- * Copyright (c) 1999-2008 The OpenSSL Project.  All rights reserved.
+ * Copyright (c) 1999-2011 The OpenSSL Project.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -137,6 +137,7 @@ static ERR_STRING_DATA SSL_str_functs[]=
 {ERR_FUNC(SSL_F_SSL3_CALLBACK_CTRL),	"SSL3_CALLBACK_CTRL"},
 {ERR_FUNC(SSL_F_SSL3_CHANGE_CIPHER_STATE),	"SSL3_CHANGE_CIPHER_STATE"},
 {ERR_FUNC(SSL_F_SSL3_CHECK_CERT_AND_ALGORITHM),	"SSL3_CHECK_CERT_AND_ALGORITHM"},
+{ERR_FUNC(SSL_F_SSL3_CHECK_CLIENT_HELLO),	"SSL3_CHECK_CLIENT_HELLO"},
 {ERR_FUNC(SSL_F_SSL3_CLIENT_HELLO),	"SSL3_CLIENT_HELLO"},
 {ERR_FUNC(SSL_F_SSL3_CONNECT),	"SSL3_CONNECT"},
 {ERR_FUNC(SSL_F_SSL3_CTRL),	"SSL3_CTRL"},
@@ -375,6 +376,7 @@ static ERR_STRING_DATA SSL_str_reasons[]=
 {ERR_REASON(SSL_R_MISSING_TMP_RSA_KEY)   ,"missing tmp rsa key"},
 {ERR_REASON(SSL_R_MISSING_TMP_RSA_PKEY)  ,"missing tmp rsa pkey"},
 {ERR_REASON(SSL_R_MISSING_VERIFY_MESSAGE),"missing verify message"},
+{ERR_REASON(SSL_R_MULTIPLE_SGC_RESTARTS) ,"multiple sgc restarts"},
 {ERR_REASON(SSL_R_NON_SSLV2_INITIAL_PACKET),"non sslv2 initial packet"},
 {ERR_REASON(SSL_R_NO_CERTIFICATES_RETURNED),"no certificates returned"},
 {ERR_REASON(SSL_R_NO_CERTIFICATE_ASSIGNED),"no certificate assigned"},