From 10666f5c1ef69e50a3a250281c9484b2728e3e35 Mon Sep 17 00:00:00 2001
From: Jonathan Hui <jonhui@nestlabs.com>
Date: Mon, 2 Jul 2018 10:44:28 -0500
Subject: [PATCH] [mac] ensure frame validation checks occur before reading FCF
 (#2845)

Credit to OSS-Fuzz.
---
 src/core/mac/mac_frame.cpp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/core/mac/mac_frame.cpp b/src/core/mac/mac_frame.cpp
index f945e529b..7d30aa186 100644
--- a/src/core/mac/mac_frame.cpp
+++ b/src/core/mac/mac_frame.cpp
@@ -891,8 +891,7 @@ exit:
 
 uint8_t Frame::FindPayloadIndex(void) const
 {
-    uint16_t fcf   = GetFrameControlField();
-    uint8_t  index = SkipSecurityHeaderIndex();
+    uint8_t index = SkipSecurityHeaderIndex();
 #if OPENTHREAD_CONFIG_ENABLE_TIME_SYNC
     uint8_t *cur    = NULL;
     uint8_t *footer = GetFooter();
@@ -929,7 +928,7 @@ uint8_t Frame::FindPayloadIndex(void) const
 #endif
 
     // Command ID
-    if ((fcf & kFcfFrameTypeMask) == kFcfFrameMacCmd)
+    if ((GetFrameControlField() & kFcfFrameTypeMask) == kFcfFrameMacCmd)
     {
         index += kCommandIdSize;
     }