TLS 1.3: gate 0-RTT on a cache-backed resumption ticket

RFC 8446 section 8 requires any server instance to accept 0-RTT for a
given ClientHello at most once. Prior to this change wolfSSL's behaviour
diverged from that requirement in several ways:

  * ctx->maxEarlyDataSz defaulted to MAX_EARLY_DATA_SZ whenever the
    library was built with WOLFSSL_EARLY_DATA, so servers auto-
    advertised 0-RTT in NewSessionTicket without the application
    asking. RFC 8446 E.5 says 0-RTT MUST NOT be enabled unless
    specifically requested.
  * The post-accept eviction is compiled out under NO_SESSION_CACHE,
    so builds without the cache accepted 0-RTT with no replay defence.
  * Stateless self-encrypted tickets do not carry a session ID on the
    stateless DoClientTicket decrypt path, so wolfSSL_SSL_CTX_remove_
    session could not locate them to evict.
  * wolfSSL_SSL_CTX_remove_session always returned 0 on success
    regardless of whether the session was actually in the cache,
    diverging from OpenSSL's SSL_CTX_remove_session (1 on success,
    0 on not-found).

Changes:
  * src/internal.c: ctx->maxEarlyDataSz defaults to 0; applications
    must opt in with wolfSSL_CTX_set_max_early_data.
  * src/tls13.c: #error when WOLFSSL_EARLY_DATA is built with
    HAVE_SESSION_TICKET and NO_SESSION_CACHE. Escape hatch
    WOLFSSL_EARLY_DATA_NO_ANTI_REPLAY for deployments that take
    application-layer responsibility.
  * wolfssl/internal.h: imply WOLFSSL_TICKET_HAVE_ID from
    WOLFSSL_EARLY_DATA so stateless-ticket issuance populates the
    cache under an ID that eviction can find.
  * src/ssl_sess.c: wolfSSL_SSL_CTX_remove_session returns 1 when the
    session was found (internal-cache hit, or ctx->rem_sess_cb fired
    for an external cache), 0 otherwise. Matches OpenSSL semantics.
  * src/tls13.c: the 0-RTT acceptance condition in CheckPreSharedKeys
    now calls wolfSSL_SSL_CTX_remove_session and checks its return:
    the eviction is the check. If the session was in the cache, 0-RTT
    is accepted and the single-use requirement is satisfied. If not,
    the early_data extension is rejected through the normal path so
    the record layer correctly skips in-flight 0-RTT records.
    WOLFSSL_MSG at each rejection site.
  * doc/dox_comments/header_files/ssl.h: document runtime opt-in.
  * tests: four new tests —
    test_tls13_0rtt_default_off (fails without default-to-0 fix),
    test_tls13_0rtt_stateless_replay (fails without TICKET_HAVE_ID
    implication and remove_session gate),
    test_tls13_remove_session_return (fails without return-value fix),
    test_tls13_0rtt_ext_cache_eviction (fails without ext-cache
    counts-as-found fix).
    test_tls13_early_data explicitly opts in via
    wolfSSL_CTX_set_max_early_data.
    tests/api.c: two SSL_CTX_remove_session == 0 assertions updated
    to == 1.

Author: julek-wolfssl

doc/dox_comments/header_files/ssl.h

@@ -14407,6 +14407,10 @@ wolfSSL_accept_TLSv13(WOLFSSL* ssl);
     A server value of zero indicates no early data is to be sent by client using
     session tickets. A client value of zero indicates that the client will
     not send any early data.
+    The default value is zero: per RFC 8446 Appendix E.5, TLS implementations
+    "MUST NOT enable 0-RTT (either sending or accepting) unless specifically
+    requested by the application." Servers must call this function (or the
+    per-SSL equivalent) with a non-zero value to opt in.
     It is recommended that the number of early data bytes be kept as low as
     practically possible in the application.
 

src/internal.c

@@ -2840,7 +2840,9 @@ int InitSSL_Ctx(WOLFSSL_CTX* ctx, WOLFSSL_METHOD* method, void* heap)
 #endif
 
 #ifdef WOLFSSL_EARLY_DATA
-    ctx->maxEarlyDataSz = MAX_EARLY_DATA_SZ;
+    /* RFC 8446 section E.5: 0-RTT off by default; opt in via
+     * wolfSSL_CTX_set_max_early_data(). */
+    ctx->maxEarlyDataSz = 0;
 #endif
 
 #if defined(HAVE_SESSION_TICKET) || !defined(NO_PSK)

src/ssl_sess.c

@@ -3243,8 +3243,11 @@ static void SESSION_ex_data_cache_update(WOLFSSL_SESSION* session, int idx,
 #endif
 
 #ifndef NO_SESSION_CACHE
+/* OpenSSL-compatible return: 1 if the session was found in the internal
+ * cache and evicted, 0 otherwise (not present, or null arguments). */
 int wolfSSL_SSL_CTX_remove_session(WOLFSSL_CTX *ctx, WOLFSSL_SESSION *s)
 {
+    int found = 0;
 #if defined(HAVE_EXT_CACHE) || defined(HAVE_EX_DATA)
     int rem_called = FALSE;
 #endif
@@ -3253,7 +3256,7 @@ int wolfSSL_SSL_CTX_remove_session(WOLFSSL_CTX *ctx, WOLFSSL_SESSION *s)
 
     s = ClientSessionToSession(s);
     if (ctx == NULL || s == NULL)
-        return BAD_FUNC_ARG;
+        return 0;
 
 #ifdef HAVE_EXT_CACHE
     if (!ctx->internalCacheOff)
@@ -3270,6 +3273,7 @@ int wolfSSL_SSL_CTX_remove_session(WOLFSSL_CTX *ctx, WOLFSSL_SESSION *s)
 
         ret = TlsSessionCacheGetAndWrLock(id, &sess, &row, ctx->method->side);
         if (ret == 0 && sess != NULL) {
+            found = 1;
 #if defined(HAVE_EXT_CACHE) || defined(HAVE_EX_DATA)
             if (sess->rem_sess_cb != NULL) {
                 rem_called = TRUE;
@@ -3308,13 +3312,15 @@ int wolfSSL_SSL_CTX_remove_session(WOLFSSL_CTX *ctx, WOLFSSL_SESSION *s)
 #if defined(HAVE_EXT_CACHE) || defined(HAVE_EX_DATA)
     if (ctx->rem_sess_cb != NULL && !rem_called) {
         ctx->rem_sess_cb(ctx, s);
+        /* Assume the external cache had the session. */
+        found = 1;
     }
 #endif
 
     /* s cannot be resumed at this point */
     s->timeout = 0;
 
-    return 0;
+    return found;
 }
 
 #if defined(OPENSSL_ALL) || defined(WOLFSSL_NGINX) || defined(WOLFSSL_HAPROXY) \

src/tls13.c

@@ -61,6 +61,7 @@
  *
  * TLS 1.3 Session Tickets:
  * WOLFSSL_TICKET_HAVE_ID:   Session tickets include ID            default: off
+ *                            Forced on when WOLFSSL_EARLY_DATA is set.
  * WOLFSSL_TICKET_NONCE_MALLOC: Dynamically allocate ticket nonce  default: off
  *
  * TLS 1.3 Key Exchange:
@@ -81,6 +82,14 @@
 
 #if !defined(NO_TLS) && defined(WOLFSSL_TLS13)
 
+/* 0-RTT anti-replay eviction needs the session cache. */
+#if defined(WOLFSSL_EARLY_DATA) && defined(HAVE_SESSION_TICKET) && \
+    defined(NO_SESSION_CACHE) && !defined(NO_WOLFSSL_SERVER) && \
+    !defined(WOLFSSL_EARLY_DATA_NO_ANTI_REPLAY)
+#error "WOLFSSL_EARLY_DATA with tickets requires !NO_SESSION_CACHE, or " \
+       "define WOLFSSL_EARLY_DATA_NO_ANTI_REPLAY to opt out."
+#endif
+
 #ifndef WOLFCRYPT_ONLY
 
 #ifdef HAVE_ERRNO_H
@@ -5897,8 +5906,11 @@ static int DoTls13EncryptedExtensions(WOLFSSL* ssl, const byte* input,
 #ifdef WOLFSSL_EARLY_DATA
     if (ssl->earlyData != no_early_data) {
         TLSX* ext = TLSX_Find(ssl->extensions, TLSX_EARLY_DATA);
-        if (ext == NULL || !ext->val)
+        if (ext == NULL || !ext->val) {
+            WOLFSSL_MSG("Early data rejected by server (no early_data "
+                        "EncryptedExtensions response)");
             ssl->earlyData = no_early_data;
+        }
     }
 
     if (ssl->earlyData == no_early_data) {
@@ -6373,18 +6385,6 @@ static int DoPreSharedKeys(WOLFSSL* ssl, const byte* input, word32 inputSz,
         /* This PSK works, no need to try any more. */
         current->chosen = 1;
         ext->resp = 1;
-#if defined(WOLFSSL_EARLY_DATA) && defined(HAVE_SESSION_TICKET) && \
-    !defined(NO_SESSION_CACHE)
-        /* RFC 8446 section 8: accept 0-RTT for a given handshake at most
-         * once. Evict the session from both the internal cache (under a
-         * write lock) and any external cache (via ctx->rem_sess_cb) so
-         * the same ClientHello cannot replay early data. Only when the
-         * client offered 0-RTT on a session that permits it. */
-        if (ssl->earlyData != no_early_data &&
-                ssl->session->maxEarlyDataSz != 0) {
-            (void)wolfSSL_SSL_CTX_remove_session(ssl->ctx, ssl->session);
-        }
-#endif
         break;
     }
 
@@ -6547,6 +6547,13 @@ static int CheckPreSharedKeys(WOLFSSL* ssl, const byte* input, word32 helloSz,
             if (ssl->earlyData != no_early_data && first
     #ifdef WOLFSSL_CERT_WITH_EXTERN_PSK
                 && !hasCertWithExternPsk
+    #endif
+    #if defined(HAVE_SESSION_TICKET) && !defined(NO_SESSION_CACHE)
+                /* RFC 8446 section 8: evict the session from the cache.
+                 * Accept 0-RTT only when the eviction found the entry
+                 * (single-use). */
+                && wolfSSL_SSL_CTX_remove_session(ssl->ctx, ssl->session)
+                    == 1
     #endif
             ) {
                 extEarlyData->resp = 1;
@@ -6562,8 +6569,10 @@ static int CheckPreSharedKeys(WOLFSSL* ssl, const byte* input, word32 helloSz,
                 ssl->keys.encryptionOn = 1;
                 ssl->earlyData = process_early_data;
             }
-            else
+            else {
+                WOLFSSL_MSG("Rejecting early data");
                 extEarlyData->resp = 0;
+            }
         }
     #endif
 
@@ -6609,6 +6618,8 @@ static int CheckPreSharedKeys(WOLFSSL* ssl, const byte* input, word32 helloSz,
              * combination in the ClientHello, but clear the response flag
              * here as a defense-in-depth measure. */
             if (extEarlyData != NULL) {
+                WOLFSSL_MSG("Rejecting early data: "
+                            "cert_with_extern_psk is not 0-RTT compatible");
                 extEarlyData->resp = 0;
                 ssl->earlyData = no_early_data;
             }
@@ -15516,6 +15527,11 @@ int wolfSSL_send_SessionTicket(WOLFSSL* ssl)
  * A value of zero indicates no early data is to be sent by client using session
  * tickets.
  *
+ * The default value is zero: per RFC 8446 Appendix E.5, TLS implementations
+ * "MUST NOT enable 0-RTT (either sending or accepting) unless specifically
+ * requested by the application." Servers must explicitly opt in by calling
+ * this function (or the per-SSL equivalent) with a non-zero value.
+ *
  * ctx  The SSL/TLS CTX object.
  * sz   Maximum size of the early data.
  * returns BAD_FUNC_ARG when ctx is NULL, SIDE_ERROR when not a server and

tests/api.c

@@ -18514,8 +18514,9 @@ static int test_wolfSSL_CTX_sess_set_remove_cb(void)
     /* Force a cache update */
     ExpectNotNull(SSL_SESSION_set_ex_data(clientSess, serverSessRemIdx - 1, 0));
     /* This should set the timeout to 0 and call the remove callback from within
-     * the session cache. */
-    ExpectIntEQ(SSL_CTX_remove_session(clientSessCtx, clientSess), 0);
+     * the session cache. Returns 1 per OpenSSL semantics (session was
+     * present in the cache and removed). */
+    ExpectIntEQ(SSL_CTX_remove_session(clientSessCtx, clientSess), 1);
     ExpectNull(SSL_SESSION_get_ex_data(clientSess, serverSessRemIdx));
     ExpectIntEQ(clientSessRemCountFree, 1);
 #endif
@@ -18527,8 +18528,9 @@ static int test_wolfSSL_CTX_sess_set_remove_cb(void)
     /* Force a cache update */
     ExpectNotNull(SSL_SESSION_set_ex_data(serverSess, serverSessRemIdx - 1, 0));
     /* This should set the timeout to 0 and call the remove callback from within
-     * the session cache. */
-    ExpectIntEQ(SSL_CTX_remove_session(serverSessCtx, serverSess), 0);
+     * the session cache. Returns 1 per OpenSSL semantics (session was
+     * present in the cache and removed). */
+    ExpectIntEQ(SSL_CTX_remove_session(serverSessCtx, serverSess), 1);
     ExpectNull(SSL_SESSION_get_ex_data(serverSess, serverSessRemIdx));
     ExpectIntEQ(serverSessRemCountFree, 1);
     /* Need to free the references that we kept */