If a ticket key callback returns 0 in TLSv1.3 don't send a ticket

If we can't construct the ticket don't send one. This requires a change
to the TLS state machine to be able to a handle a construction function
deciding not to send a message after all.

Fixes #18977

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18990)

2 files changed, 64 insertions, 22 deletions

diff --git a/ssl/statem/statem.c b/ssl/statem/statem.c
index 7daa220443..ad227d2a9f 100644
--- a/ssl/statem/statem.c
+++ b/ssl/statem/statem.c
@@ -888,10 +888,24 @@ static SUB_STATE_RETURN write_state_machine(SSL_CONNECTION *s)
                 SSLfatal(s, SSL_AD_INTERNAL_ERROR, ERR_R_INTERNAL_ERROR);
                 return SUB_STATE_ERROR;
             }
-            if (confunc != NULL && !confunc(s, &pkt)) {
-                WPACKET_cleanup(&pkt);
-                check_fatal(s);
-                return SUB_STATE_ERROR;
+            if (confunc != NULL) {
+                int tmpret;
+
+                tmpret = confunc(s, &pkt);
+                if (tmpret <= 0) {
+                    WPACKET_cleanup(&pkt);
+                    check_fatal(s);
+                    return SUB_STATE_ERROR;
+                } else if (tmpret == 2) {
+                    /*
+                     * The construction function decided not to construct the
+                     * message after all and continue. Skip sending.
+                     */
+                    WPACKET_cleanup(&pkt);
+                    st->write_state = WRITE_STATE_POST_WORK;
+                    st->write_state_work = WORK_MORE_A;
+                    break;
+                } /* else success */
             }
             if (!ssl_close_construct_packet(s, &pkt, mt)
                     || !WPACKET_finish(&pkt)) {
diff --git a/ssl/statem/statem_srvr.c b/ssl/statem/statem_srvr.c
index ac7cdda9da..cafc18ef54 100644
--- a/ssl/statem/statem_srvr.c
+++ b/ssl/statem/statem_srvr.c
@@ -3696,6 +3696,10 @@ static int create_ticket_prequel(SSL_CONNECTION *s, WPACKET *pkt,
     return 1;
 }
 
+/*
+ * Returns 1 on success, 0 to abort construction of the ticket (non-fatal), or
+ * -1 on fatal error
+ */
 static int construct_stateless_ticket(SSL_CONNECTION *s, WPACKET *pkt,
                                       uint32_t age_add,
                                       unsigned char *tick_nonce)
@@ -3711,7 +3715,7 @@ static int construct_stateless_ticket(SSL_CONNECTION *s, WPACKET *pkt,
     SSL_CTX *tctx = s->session_ctx;
     unsigned char iv[EVP_MAX_IV_LENGTH];
     unsigned char key_name[TLSEXT_KEYNAME_LENGTH];
-    int iv_len, ok = 0;
+    int iv_len, ok = -1;
     size_t macoffset, macendoffset;
     SSL *ssl = SSL_CONNECTION_GET_SSL(s);
     SSL_CTX *sctx = SSL_CONNECTION_GET_CTX(s);
@@ -3794,7 +3798,15 @@ static int construct_stateless_ticket(SSL_CONNECTION *s, WPACKET *pkt,
 #endif
 
         if (ret == 0) {
-
+            /*
+             * In TLSv1.2 we construct a 0 length ticket. In TLSv1.3 a 0
+             * length ticket is not allowed so we abort construction of the
+             * ticket
+             */
+            if (SSL_CONNECTION_IS_TLS13(s)) {
+                ok = 0;
+                goto err;
+            }
             /* Put timeout and length */
             if (!WPACKET_put_bytes_u32(pkt, 0)
                     || !WPACKET_put_bytes_u16(pkt, 0)) {
@@ -3908,6 +3920,20 @@ static int construct_stateful_ticket(SSL_CONNECTION *s, WPACKET *pkt,
     return 1;
 }
 
+static void tls_update_ticket_counts(SSL_CONNECTION *s)
+{
+    /*
+     * Increment both |sent_tickets| and |next_ticket_nonce|. |sent_tickets|
+     * gets reset to 0 if we send more tickets following a post-handshake
+     * auth, but |next_ticket_nonce| does not.  If we're sending extra
+     * tickets, decrement the count of pending extra tickets.
+     */
+    s->sent_tickets++;
+    s->next_ticket_nonce++;
+    if (s->ext.extra_tickets_expected > 0)
+        s->ext.extra_tickets_expected--;
+}
+
 int tls_construct_new_session_ticket(SSL_CONNECTION *s, WPACKET *pkt)
 {
     SSL_CTX *tctx = s->session_ctx;
@@ -3916,6 +3942,7 @@ int tls_construct_new_session_ticket(SSL_CONNECTION *s, WPACKET *pkt)
         unsigned char age_add_c[sizeof(uint32_t)];
         uint32_t age_add;
     } age_add_u;
+    int ret = 0;
 
     age_add_u.age_add = 0;
 
@@ -4014,10 +4041,20 @@ int tls_construct_new_session_ticket(SSL_CONNECTION *s, WPACKET *pkt)
             /* SSLfatal() already called */
             goto err;
         }
-    } else if (!construct_stateless_ticket(s, pkt, age_add_u.age_add,
-                                           tick_nonce)) {
-        /* SSLfatal() already called */
-        goto err;
+    } else {
+        int tmpret;
+
+        tmpret = construct_stateless_ticket(s, pkt, age_add_u.age_add,
+                                            tick_nonce);
+        if (tmpret != 1) {
+            if (tmpret == 0) {
+                ret = 2; /* Non-fatal. Abort construction but continue */
+                /* We count this as a success so update the counts anwyay */
+                tls_update_ticket_counts(s);
+            }
+            /* else SSLfatal() already called */
+            goto err;
+        }
     }
 
     if (SSL_CONNECTION_IS_TLS13(s)) {
@@ -4027,22 +4064,13 @@ int tls_construct_new_session_ticket(SSL_CONNECTION *s, WPACKET *pkt)
             /* SSLfatal() already called */
             goto err;
         }
-        /*
-         * Increment both |sent_tickets| and |next_ticket_nonce|. |sent_tickets|
-         * gets reset to 0 if we send more tickets following a post-handshake
-         * auth, but |next_ticket_nonce| does not.  If we're sending extra
-         * tickets, decrement the count of pending extra tickets.
-         */
-        s->sent_tickets++;
-        s->next_ticket_nonce++;
-        if (s->ext.extra_tickets_expected > 0)
-            s->ext.extra_tickets_expected--;
+        tls_update_ticket_counts(s);
         ssl_update_cache(s, SSL_SESS_CACHE_SERVER);
     }
 
-    return 1;
+    ret = 1;
  err:
-    return 0;
+    return ret;
 }
 
 /*