From e10a362f69191506e73bfa31778da45f4c5df482 Mon Sep 17 00:00:00 2001
From: Daiki Ueno <dueno@redhat.com>
Date: Mon, 12 Oct 2020 17:42:01 +0000
Subject: [PATCH] Bug 1641480, TLS 1.3: tighten CCS handling in compatibility
 mode, r=mt

This makes the server reject CCS when the client doesn't indicate the
use of the middlebox compatibility mode with a non-empty
ClientHello.legacy_session_id, or it sends multiple CCS in a row.

Differential Revision: https://phabricator.services.mozilla.com/D79994

--HG--
extra : moz-landing-system : lando
---
 gtests/ssl_gtest/ssl_tls13compat_unittest.cc | 33 ++++++++++++++++++++
 lib/ssl/ssl3con.c                            | 18 ++++-
 lib/ssl/sslimpl.h                            |  4 +
 3 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/nss/gtests/ssl_gtest/ssl_tls13compat_unittest.cc b/nss/gtests/ssl_gtest/ssl_tls13compat_unittest.cc
index 6905ed0c0..dcede798c 100644
--- a/nss/gtests/ssl_gtest/ssl_tls13compat_unittest.cc
+++ b/nss/gtests/ssl_gtest/ssl_tls13compat_unittest.cc
@@ -348,6 +348,39 @@ TEST_F(TlsConnectStreamTls13, ChangeCipherSpecBeforeClientHelloTwice) {
   client_->CheckErrorCode(SSL_ERROR_HANDSHAKE_UNEXPECTED_ALERT);
 }
 
+// The server rejects a ChangeCipherSpec if the client advertises an
+// empty session ID.
+TEST_F(TlsConnectStreamTls13, ChangeCipherSpecAfterClientHelloEmptySid) {
+  EnsureTlsSetup();
+  ConfigureVersion(SSL_LIBRARY_VERSION_TLS_1_3);
+
+  StartConnect();
+  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);
+}
+
+// The server rejects multiple ChangeCipherSpec even if the client
+// indicates compatibility mode with non-empty session ID.
+TEST_F(Tls13CompatTest, ChangeCipherSpecAfterClientHelloTwice) {
+  EnsureTlsSetup();
+  ConfigureVersion(SSL_LIBRARY_VERSION_TLS_1_3);
+  EnableCompatMode();
+
+  StartConnect();
+  client_->Handshake();  // Send ClientHello
+  // Send CCS twice in a row
+  client_->SendDirect(DataBuffer(kCannedCcs, sizeof(kCannedCcs)));
+  client_->SendDirect(DataBuffer(kCannedCcs, sizeof(kCannedCcs)));
+
+  server_->ExpectSendAlert(kTlsAlertUnexpectedMessage);
+  server_->Handshake();  // Consume ClientHello and CCS.
+  server_->CheckErrorCode(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
+}
+
 // If we negotiate 1.2, we abort.
 TEST_F(TlsConnectStreamTls13, ChangeCipherSpecBeforeClientHello12) {
   EnsureTlsSetup();
diff --git a/nss/lib/ssl/ssl3con.c b/nss/lib/ssl/ssl3con.c
index 8be53d357..5f22872f8 100644
--- a/nss/lib/ssl/ssl3con.c
+++ b/nss/lib/ssl/ssl3con.c
@@ -6645,7 +6645,11 @@ 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)) {
-        return sidMatch;
+        if (sidMatch) {
+            ss->ssl3.hs.allowCcs = PR_TRUE;
+            return PR_TRUE;
+        }
+        return PR_FALSE;
     }
 
     /* TLS 1.3 (no SID)/DTLS 1.3: The server shouldn't send a session ID. */
@@ -8692,6 +8696,7 @@ 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. */
@@ -13061,8 +13066,15 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
             ss->ssl3.hs.ws != idle_handshake &&
             cText->buf->len == 1 &&
             cText->buf->buf[0] == change_cipher_spec_choice) {
-            /* Ignore the CCS. */
-            return SECSuccess;
+            if (ss->ssl3.hs.allowCcs) {
+                /* Ignore the first CCS. */
+                ss->ssl3.hs.allowCcs = PR_FALSE;
+                return SECSuccess;
+            }
+
+            /* Compatibility mode is not negotiated. */
+            alert = unexpected_message;
+            PORT_SetError(SSL_ERROR_RX_MALFORMED_CHANGE_CIPHER);
         }
 
         if (IS_DTLS(ss) ||
diff --git a/nss/lib/ssl/sslimpl.h b/nss/lib/ssl/sslimpl.h
index b0e44a088..44c43a0e6 100644
--- a/nss/lib/ssl/sslimpl.h
+++ b/nss/lib/ssl/sslimpl.h
@@ -710,6 +710,10 @@ 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 clientCertRequested;           /* True if CertificateRequest received. */
     ssl3KEADef kea_def_mutable;           /* Used to hold the writable kea_def
                                            * we use for TLS 1.3 */