crypto: support MSFT iOS clients with faulty SNI

Microsoft iOS Remote Desktop Clients eventually send NULL-terminated
hostnames in SNI which is not allowed in the OpenSSL implementation.

Since we're not using SNI this commit adds an OpenSSL TLS extension
debug callback which modifies the SSL context in a way preventing it
from parsing this extension

diff --git a/libfreerdp/crypto/tls.c b/libfreerdp/crypto/tls.c
index fd21ede..e7e3591 100644 (file)
--- a/libfreerdp/crypto/tls.c
+++ b/libfreerdp/crypto/tls.c
@@ -759,6 +759,19 @@ int tls_connect(rdpTls* tls, BIO* underlying)
        return tls_do_handshake(tls, TRUE);
 }
 
+#ifndef OPENSSL_NO_TLSEXT
+static void tls_openssl_tlsext_debug_callback(SSL *s, int client_server,
+       int type, unsigned char *data, int len, void *arg)
+{
+       /* see code comment in tls_accept() below */
+
+       if (type == TLSEXT_TYPE_server_name) {
+               WLog_DBG(TAG, "Client uses SNI (extension disabled)");
+               s->servername_done = 2;
+       }
+}
+#endif
+
 BOOL tls_accept(rdpTls* tls, BIO* underlying, const char* cert_file, const char* privatekey_file)
 {
        long options = 0;
@@ -816,6 +829,27 @@ BOOL tls_accept(rdpTls* tls, BIO* underlying, const char* cert_file, const char*
                return FALSE;
        }
 
+#ifndef OPENSSL_NO_TLSEXT
+       /**
+        * The Microsoft iOS clients eventually send a null or even double null
+        * terminated hostname in the SNI TLS extension!
+        * If the length indicator does not equal the hostname strlen OpenSSL
+        * will abort (see openssl:ssl/t1_lib.c).
+        * Here is a tcpdump segment of Microsoft Remote Desktop Client Version
+        * 8.1.7 running on an iPhone 4 with iOS 7.1.2 showing the transmitted
+        * SNI hostname TLV blob when connection to server "abcd":
+        * 00                  name_type 0x00 (host_name)
+        * 00 06               length_in_bytes 0x0006
+        * 61 62 63 64 00 00   host_name "abcd\0\0"
+        *
+        * Currently the only (runtime) workaround is setting an openssl tls
+        * extension debug callback that sets the SSL context's servername_done
+        * to 1 which effectively disables the parsing of that extension type.
+        */
+
+       SSL_set_tlsext_debug_callback(tls->ssl, tls_openssl_tlsext_debug_callback);
+#endif
+
        return tls_do_handshake(tls, FALSE) > 0;
 }