From ae25a485094adc3e022df92d1ac10aa07b15c376 Mon Sep 17 00:00:00 2001
From: David Garske <david@wolfssl.com>
Date: Thu, 17 Mar 2022 14:01:57 -0700
Subject: [PATCH] Improve the build message to not always allocate the IV (16
 byte) (use fixed buffer if <= 16 bytes).

---
 src/internal.c     | 17 +++++++++++++----
 wolfssl/internal.h |  1 +
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/internal.c b/src/internal.c
index 109b256a9..8658c0d27 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -18116,8 +18116,10 @@ int BuildCertHashes(WOLFSSL* ssl, Hashes* hashes)
 void FreeBuildMsgArgs(WOLFSSL* ssl, BuildMsgArgs* args)
 {
     if (args) {
-        if (ssl && args->iv)
+        /* only free the IV if it was dynamically allocated */
+        if (ssl && args->iv && (args->iv != args->staticIvBuffer)) {
             XFREE(args->iv, ssl->heap, DYNAMIC_TYPE_SALT);
+        }
         XMEMSET(args, 0, sizeof(BuildMsgArgs));
     }
 }
@@ -18312,9 +18314,16 @@ int BuildMessage(WOLFSSL* ssl, byte* output, int outSz, const byte* input,
             }
 
             if (args->ivSz > 0) {
-                args->iv = (byte*)XMALLOC(args->ivSz, ssl->heap, DYNAMIC_TYPE_SALT);
-                if (args->iv == NULL)
-                    ERROR_OUT(MEMORY_E, exit_buildmsg);
+                if (args->ivSz > sizeof(args->staticIvBuffer)) {
+                    args->iv = (byte*)XMALLOC(args->ivSz, ssl->heap,
+                                              DYNAMIC_TYPE_SALT);
+                    if (args->iv == NULL) {
+                        ERROR_OUT(MEMORY_E, exit_buildmsg);
+                    }
+                }
+                else {
+                    args->iv = args->staticIvBuffer;
+                }
 
                 ret = wc_RNG_GenerateBlock(ssl->rng, args->iv, args->ivSz);
                 if (ret != 0)
diff --git a/wolfssl/internal.h b/wolfssl/internal.h
index 3832f3aa9..496a9b682 100644
--- a/wolfssl/internal.h
+++ b/wolfssl/internal.h
@@ -4183,6 +4183,7 @@ typedef struct BuildMsgArgs {
     word16 size;
     word32 ivSz;      /* TLSv1.1  IV */
     byte*  iv;
+    ALIGN16 byte staticIvBuffer[MAX_IV_SZ];
 } BuildMsgArgs;
 #endif