Rework the exit path of tls13_handshake_recv_action()

If an error occurs in action->recv() for a handshake that needs to
downgrade to legacy TLS, the artistic exit path led to hiding the
error under TLS13_IO_USE_LEGACY. Rework the exit path to be easier
to follow, preserving behavior except that the error can no longer
be masked.

Detailed analysis and initial diff by Masaru Masuda.
Fixes https://github.com/libressl/openbsd/issues/146

ok beck

diff --git a/lib/libssl/tls13_handshake.c b/lib/libssl/tls13_handshake.c
index 9723edf..0dc2333 100644 (file)
--- a/lib/libssl/tls13_handshake.c
+++ b/lib/libssl/tls13_handshake.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: tls13_handshake.c,v 1.72 2022/11/26 16:08:56 tb Exp $ */
+/*     $OpenBSD: tls13_handshake.c,v 1.73 2024/02/03 19:57:14 tb Exp $ */
 /*
  * Copyright (c) 2018-2021 Theo Buehler <tb@openbsd.org>
  * Copyright (c) 2019 Joel Sing <jsing@openbsd.org>
@@ -546,22 +546,24 @@ tls13_handshake_recv_action(struct tls13_ctx *ctx,
                return TLS13_IO_FAILURE;
 
        ret = TLS13_IO_FAILURE;
-       if (action->recv(ctx, &cbs)) {
-               if (CBS_len(&cbs) != 0) {
-                       tls13_set_errorx(ctx, TLS13_ERR_TRAILING_DATA, 0,
-                           "trailing data in handshake message", NULL);
-                       ctx->alert = TLS13_ALERT_DECODE_ERROR;
-               } else {
-                       ret = TLS13_IO_SUCCESS;
-               }
+       if (!action->recv(ctx, &cbs))
+               goto err;
+
+       if (CBS_len(&cbs) != 0) {
+               tls13_set_errorx(ctx, TLS13_ERR_TRAILING_DATA, 0,
+                   "trailing data in handshake message", NULL);
+               ctx->alert = TLS13_ALERT_DECODE_ERROR;
+               goto err;
        }
 
+       ret = TLS13_IO_SUCCESS;
+       if (ctx->ssl->method->version < TLS1_3_VERSION)
+               ret = TLS13_IO_USE_LEGACY;
+
+ err:
        tls13_handshake_msg_free(ctx->hs_msg);
        ctx->hs_msg = NULL;
 
-       if (ctx->ssl->method->version < TLS1_3_VERSION)
-               return TLS13_IO_USE_LEGACY;
-
        return ret;
 }