Bug 1672703, always tolerate the first CCS in TLS 1.3, r=mt

Summary:
This flips the meaning of the flag for checking excessive CCS
messages, so it only rejects multiple CCS messages while the first CCS
message is always accepted.

Reviewers: mt

Reviewed By: mt

Bug #: 1672703

Differential Revision: https://phabricator.services.mozilla.com/D94603

--HG--
extra : rebase_source : dde5b59dff690e622f211c326dc8244ed652f764
extra : amend_source : 3f0506ed64befe5b531339101b655c622151ea47

gtests/ssl_gtest/ssl_tls13compat_unittest.cc

@@ -348,8 +348,8 @@ TEST_F(TlsConnectStreamTls13, ChangeCipherSpecBeforeClientHelloTwice) {
   client_->CheckErrorCode(SSL_ERROR_HANDSHAKE_UNEXPECTED_ALERT);
 }
 
-// The server rejects a ChangeCipherSpec if the client advertises an
-// empty session ID.
+// The server accepts a ChangeCipherSpec even if the client advertises
+// an empty session ID.
 TEST_F(TlsConnectStreamTls13, ChangeCipherSpecAfterClientHelloEmptySid) {
   EnsureTlsSetup();
   ConfigureVersion(SSL_LIBRARY_VERSION_TLS_1_3);
@@ -358,9 +358,8 @@ TEST_F(TlsConnectStreamTls13, ChangeCipherSpecAfterClientHelloEmptySid) {
   client_->Handshake();  // Send ClientHello
   client_->SendDirect(DataBuffer(kCannedCcs, sizeof(kCannedCcs)));  // Send CCS
 
-  server_->ExpectSendAlert(kTlsAlertUnexpectedMessage);
-  server_->Handshake();  // Consume ClientHello and CCS
-  server_->CheckErrorCode(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
+  Handshake();
+  CheckConnected();
 }
 
 // The server rejects multiple ChangeCipherSpec even if the client
@@ -381,7 +380,7 @@ TEST_F(Tls13CompatTest, ChangeCipherSpecAfterClientHelloTwice) {
   server_->CheckErrorCode(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
 }
 
-// The client rejects a ChangeCipherSpec if it advertises an empty
+// The client accepts a ChangeCipherSpec even if it advertises an empty
 // session ID.
 TEST_F(TlsConnectStreamTls13, ChangeCipherSpecAfterServerHelloEmptySid) {
   EnsureTlsSetup();
@@ -398,9 +397,10 @@ TEST_F(TlsConnectStreamTls13, ChangeCipherSpecAfterServerHelloEmptySid) {
                          // send ServerHello..CertificateVerify
   // Send CCS
   server_->SendDirect(DataBuffer(kCannedCcs, sizeof(kCannedCcs)));
-  client_->ExpectSendAlert(kTlsAlertUnexpectedMessage);
-  client_->Handshake();  // Consume ClientHello and CCS
-  client_->CheckErrorCode(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
+
+  // No alert is sent from the client. As Finished is dropped, we
+  // can't use Handshake() and CheckConnected().
+  client_->Handshake();
 }
 
 // The client rejects multiple ChangeCipherSpec in a row even if the

lib/ssl/ssl3con.c

@@ -6645,11 +6645,7 @@ ssl_CheckServerSessionIdCorrectness(sslSocket *ss, SECItem *sidBytes)
 
     /* TLS 1.3: We sent a session ID.  The server's should match. */
     if (!IS_DTLS(ss) && (sentRealSid || sentFakeSid)) {
-        if (sidMatch) {
-            ss->ssl3.hs.allowCcs = PR_TRUE;
-            return PR_TRUE;
-        }
-        return PR_FALSE;
+        return sidMatch;
     }
 
     /* TLS 1.3 (no SID)/DTLS 1.3: The server shouldn't send a session ID. */
@@ -8696,7 +8692,6 @@ ssl3_HandleClientHello(sslSocket *ss, PRUint8 *b, PRUint32 length)
                 errCode = PORT_GetError();
                 goto alert_loser;
             }
-            ss->ssl3.hs.allowCcs = PR_TRUE;
         }
 
         /* TLS 1.3 requires that compression include only null. */
@@ -13066,15 +13061,14 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
             ss->ssl3.hs.ws != idle_handshake &&
             cText->buf->len == 1 &&
             cText->buf->buf[0] == change_cipher_spec_choice) {
-            if (ss->ssl3.hs.allowCcs) {
-                /* Ignore the first CCS. */
-                ss->ssl3.hs.allowCcs = PR_FALSE;
+            if (!ss->ssl3.hs.rejectCcs) {
+                /* Allow only the first CCS. */
+                ss->ssl3.hs.rejectCcs = PR_TRUE;
                 return SECSuccess;
+            } else {
+                alert = unexpected_message;
+                PORT_SetError(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
             }
-
-            /* Compatibility mode is not negotiated. */
-            alert = unexpected_message;
-            PORT_SetError(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
         }
 
         if ((IS_DTLS(ss) && !dtls13_AeadLimitReached(spec)) ||

lib/ssl/sslimpl.h

@@ -710,10 +710,7 @@ typedef struct SSL3HandshakeStateStr {
                                            * or received. */
     PRBool receivedCcs;                   /* A server received ChangeCipherSpec
                                            * before the handshake started. */
-    PRBool allowCcs;                      /* A server allows ChangeCipherSpec
-                                           * as the middlebox compatibility mode
-                                           * is explicitly indicarted by
-                                           * legacy_session_id in TLS 1.3 ClientHello. */
+    PRBool rejectCcs;                     /* Excessive ChangeCipherSpecs are rejected. */
     PRBool clientCertRequested;           /* True if CertificateRequest received. */
     PRBool endOfFlight;                   /* Processed a full flight (DTLS 1.3). */
     ssl3KEADef kea_def_mutable;           /* Used to hold the writable kea_def