From a28b1903efbfdb41d3d8437b4fe54691ced376a8 Mon Sep 17 00:00:00 2001
From: Ingela Anderton Andin <ingela@erlang.org>
Date: Wed, 15 Mar 2017 10:55:45 +0100
Subject: dtls: Handle overlapping fragments

Fragment reassembling needs to handle that a smaller
fragment then sent originally might overlap an earlier
received fragment.
---
 lib/ssl/src/dtls_handshake.erl | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

(limited to 'lib/ssl/src/dtls_handshake.erl')

diff --git a/lib/ssl/src/dtls_handshake.erl b/lib/ssl/src/dtls_handshake.erl
index fd1f9698fe..4c525fae1b 100644
--- a/lib/ssl/src/dtls_handshake.erl
+++ b/lib/ssl/src/dtls_handshake.erl
@@ -455,7 +455,7 @@ merge_fragments(#handshake_fragment{
 		   fragment_offset = PreviousOffSet,
 		   fragment_length = CurrentLen}) when CurrentLen < PreviousLen ->
     Previous;
-%% Next fragment
+%% Next fragment, might be overlapping
 merge_fragments(#handshake_fragment{
 		   fragment_offset = PreviousOffSet, 
 		   fragment_length = PreviousLen,
@@ -464,10 +464,28 @@ merge_fragments(#handshake_fragment{
 		#handshake_fragment{
 		   fragment_offset = CurrentOffSet,
 		   fragment_length = CurrentLen,
-		   fragment = CurrentData}) when PreviousOffSet + PreviousLen == CurrentOffSet->
-	    Previous#handshake_fragment{
-	      fragment_length =  PreviousLen + CurrentLen,
-	      fragment = <<PreviousData/binary, CurrentData/binary>>};
+                  fragment = CurrentData})
+  when PreviousOffSet + PreviousLen >= CurrentOffSet andalso
+       PreviousOffSet + PreviousLen < CurrentOffSet + CurrentLen ->
+    CurrentStart = PreviousOffSet + PreviousLen - CurrentOffSet,
+    <<_:CurrentStart/bytes, Data/binary>> = CurrentData,
+    Previous#handshake_fragment{
+      fragment_length =  PreviousLen + CurrentLen - CurrentStart,
+      fragment = <<PreviousData/binary, Data/binary>>};
+%% already fully contained fragment
+merge_fragments(#handshake_fragment{
+                  fragment_offset = PreviousOffSet, 
+                  fragment_length = PreviousLen,
+                  fragment = PreviousData
+                 } = Previous, 
+               #handshake_fragment{
+                  fragment_offset = CurrentOffSet,
+                  fragment_length = CurrentLen,
+                  fragment = CurrentData})
+  when PreviousOffSet + PreviousLen >= CurrentOffSet andalso
+       PreviousOffSet + PreviousLen >= CurrentOffSet + CurrentLen ->
+    Previous;
+
 %% No merge there is a gap
 merge_fragments(Previous, Current) ->
     [Previous, Current].
-- 
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