[PATCH 2/2] NetworkPkg/TcpDxe: Fix unconditional window shrinking

["atepin@kraftway.ru" <atepin@kraftway.ru>]

Moving Right window edge to the left on sender side without additional
checks leads to the situation when sender assumes the receiver shrunk
its rcv buffer, when, in fact, it only reduced window size. This is a
TCP deadlock situation. Receiver ACKs proper segment, while sender
discards it for future ACK. Add check for negative usable window to
prevent erroneous window shrinking.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Andrey Tepin <atepin@kraftway.ru>
---
 NetworkPkg/TcpDxe/TcpInput.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/NetworkPkg/TcpDxe/TcpInput.c b/NetworkPkg/TcpDxe/TcpInput.c
index 04c8a82..11b3eb8 100644
--- a/NetworkPkg/TcpDxe/TcpInput.c
+++ b/NetworkPkg/TcpDxe/TcpInput.c
@@ -738,6 +738,7 @@ TcpInput (
   TCP_SEQNO   Right;
   TCP_SEQNO   Urg;
   UINT16      Checksum;
+  INT32       UsableWnd;
 
   ASSERT ((Version == IP_VERSION_4) || (Version == IP_VERSION_6));
 
@@ -1307,7 +1308,10 @@ TcpInput (
 
       if (TCP_SEQ_LT (Right, Tcb->SndNxt)) {
 
-        Tcb->SndNxt = Right;
+        UsableWnd = Tcb->SndUna + Tcb->SndWnd - Tcb->SndNxt;
+        if (UsableWnd < 0) {
+          Tcb->SndNxt = Right;
+        }
 
         if (Right == Tcb->SndUna) {
 
-- 
1.9.1