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From 431610ad7ea53d2f9f1bf926e1d353d533bdb27f Mon Sep 17 00:00:00 2001
From: Miroslav Lichvar <mlichvar@redhat.com>
Date: Mon, 1 Aug 2016 20:24:24 +0200
Subject: [PATCH 14/15] ntpd: respond only to client and symmetric active
 packets

The busybox NTP implementation doesn't check the NTP mode of packets
received on the server port and responds to any packet with the right
size. This includes responses from another NTP server. An attacker can
send a packet with a spoofed source address in order to create an
infinite loop of responses between two busybox NTP servers. Adding
more packets to the loop increases the traffic between the servers
until one of them has a fully loaded CPU and/or network.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
---
 networking/ntpd.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/networking/ntpd.c b/networking/ntpd.c
index 7f7d69e..182dd58 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -2042,6 +2042,13 @@ recv_and_process_client_pkt(void /*int fd*/)
 		goto bail;
 	}
 
+	/* Respond only to client and symmetric active packets */
+	if ((msg.m_status & MODE_MASK) != MODE_CLIENT
+	 && (msg.m_status & MODE_MASK) != MODE_SYM_ACT
+	) {
+		goto bail;
+	}
+
 	query_status = msg.m_status;
 	query_xmttime = msg.m_xmttime;
 
-- 
2.9.1