Ensure SSL3_FLAGS_CCS_OK (or d1->change_cipher_spec_ok for DTLS) is reset

once the ChangeCipherSpec message is received. Previously, the server would
set the flag once at SSL3_ST_SR_CERT_VRFY and again at SSL3_ST_SR_FINISHED.
This would allow a second CCS to arrive and would corrupt the server state.

(Because the first CCS would latch the correct keys and subsequent CCS
messages would have to be encrypted, a MitM attacker cannot exploit this,
though.)

Thanks to Joeri de Ruiter for reporting this issue.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit e94a6c0ede)

Conflicts:
	CHANGES
	ssl/s3_srvr.c

ssl/ssl3.h

@@ -393,8 +393,12 @@ typedef struct ssl3_buffer_st
 #define TLS1_FLAGS_TLS_PADDING_BUG		0x0008
 #define TLS1_FLAGS_SKIP_CERT_VERIFY		0x0010
 #define TLS1_FLAGS_KEEP_HANDSHAKE		0x0020
+/*
+ * Set when the handshake is ready to process peer's ChangeCipherSpec message.
+ * Cleared after the message has been processed.
+ */
 #define SSL3_FLAGS_CCS_OK			0x0080
- 
+
 /* 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
@@ -456,8 +460,11 @@ typedef struct ssl3_state_st
 	 * and freed and MD_CTX-es for all required digests are stored in
 	 * this array */
 	EVP_MD_CTX **handshake_dgst;
-	/* this is set whenerver we see a change_cipher_spec message
-	 * come in when we are not looking for one */
+	/*
+	 * Set whenever an expected ChangeCipherSpec message is processed.
+	 * Unset when the peer's Finished message is received.
+	 * Unexpected ChangeCipherSpec messages trigger a fatal alert.
+	 */
 	int change_cipher_spec;
 
 	int warn_alert;