Tighten up logic around ChangeCipherSpec.

ChangeCipherSpec messages have a defined value. They also may not occur
in the middle of a handshake message. The current logic will accept a
ChangeCipherSpec with value 2. It also would accept up to three bytes of
handshake data before the ChangeCipherSpec which it would discard
(because s->init_num gets reset).

Instead, require that s->init_num is 0 when a ChangeCipherSpec comes in.

RT#4391

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

ssl/statem/statem_lib.c

diff --git a/ssl/statem/statem_lib.c b/ssl/statem/statem_lib.c
index 6ceb9ec..eb3e591 100644
--- a/ssl/statem/statem_lib.c
+++ b/ssl/statem/statem_lib.c
@@ -354,6 +354,16 @@
                 return 0;
             }
             if (recvd_type == SSL3_RT_CHANGE_CIPHER_SPEC) {
+                /*
+		 * A ChangeCipherSpec must be a single byte and may not occur
+		 * in the middle of a handshake message.
+		 */
+                if (s->init_num != 0 || i != 1 || p[0] != SSL3_MT_CCS) {
+                    al = SSL_AD_UNEXPECTED_MESSAGE;
+                    SSLerr(SSL_F_TLS_GET_MESSAGE_HEADER,
+                           SSL_R_BAD_CHANGE_CIPHER_SPEC);
+                    goto f_err;
+                }
                 s->s3->tmp.message_type = *mt = SSL3_MT_CHANGE_CIPHER_SPEC;
                 s->init_num = i - 1;
                 s->s3->tmp.message_size = i;