From 600ff0c24bb71482e7f0da948a931d5c5d72838a Mon Sep 17 00:00:00 2001
From: =?utf8?q?Ilpo=20J=C3=A4rvinen?= <ilpo.jarvinen@helsinki.fi>
Date: Tue, 13 Feb 2007 12:42:11 -0800
Subject: [PATCH] [TCP]: Prevent pseudo garbage in SYN's advertized window
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TCP may advertize up to 16-bits window in SYN packets (no window
scaling allowed). At the same time, TCP may have rcv_wnd
(32-bits) that does not fit to 16-bits without window scaling
resulting in pseudo garbage into advertized window from the
low-order bits of rcv_wnd. This can happen at least when
mss <= (1<<wscale) (see tcp_select_initial_window). This patch
fixes the handling of SYN advertized windows (compile tested
only).

In worst case (which is unlikely to occur though), the receiver
advertized window could be just couple of bytes. I'm not sure
that such situation would be handled very well at all by the
receiver!? Fortunately, the situation normalizes after the
first non-SYN ACK is received because it has the correct,
scaled window.

Alternatively, tcp_select_initial_window could be changed to
prevent too large rcv_wnd in the first place.

[ tcp_make_synack() has the same bug, and I've added a fix for
  that to this patch -DaveM ]

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/ipv4/tcp_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index cebe9aa918..dc151139b5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -481,7 +481,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 		/* RFC1323: The window in SYN & SYN/ACK segments
 		 * is never scaled.
 		 */
-		th->window	= htons(tp->rcv_wnd);
+		th->window	= htons(min(tp->rcv_wnd, 65535U));
 	} else {
 		th->window	= htons(tcp_select_window(sk));
 	}
@@ -2160,7 +2160,7 @@ struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 	}
 
 	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
-	th->window = htons(req->rcv_wnd);
+	th->window = htons(min(req->rcv_wnd, 65535U));
 
 	TCP_SKB_CB(skb)->when = tcp_time_stamp;
 	tcp_syn_build_options((__be32 *)(th + 1), dst_metric(dst, RTAX_ADVMSS), ireq->tstamp_ok,
-- 
2.39.5