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

1 files changed, 14 insertions, 12 deletions

diff --git a/src/lib/libssl/tls13_handshake.c b/src/lib/libssl/tls13_handshake.c
index 9723edfea4..0dc2333708 100644
--- a/src/lib/libssl/tls13_handshake.c
+++ b/src/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;
 }