dnsmasq: forward.c: fix CVE-2017-13704

Fix SIGSEGV in rfc1035.c answer_request() line 1228 where memset()
is called with header & limit pointing at the same address and thus
tries to clear memory from before the buffer begins.

answer_request() is called with an invalid edns packet size provided by
the client.  Ensure the udp_size provided by the client is bounded by
512 and configured maximum as per RFC 6891 6.2.3 "Values lower than 512
MUST be treated as equal to 512"

The client that exposed the problem provided a payload udp size of 0.

Signed-off-by: Kevin Darbyshire-Bryant <[email protected]>
Acked-by: Hans Dedecker <[email protected]>

package/network/services/dnsmasq/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=dnsmasq
 PKG_VERSION:=2.77
-PKG_RELEASE:=9
+PKG_RELEASE:=10
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=http://thekelleys.org.uk/dnsmasq/

package/network/services/dnsmasq/patches/025-fix-CVE-2017-13704.patch

@@ -0,0 +1,37 @@
+From 38af9b1ac3242a4128e88069c495024caa565f0e Mon Sep 17 00:00:00 2001
+From: Kevin Darbyshire-Bryant <[email protected]>
+Date: Tue, 29 Aug 2017 12:35:40 +0100
+Subject: [PATCH] forward.c: fix CVE-2017-13704
+
+Fix SIGSEGV in rfc1035.c answer_request() line 1228 where memset()
+is called with header & limit pointing at the same address and thus
+tries to clear memory from before the buffer begins.
+
+answer_request() is called with an invalid edns packet size provided by
+the client.  Ensure the udp_size provided by the client is bounded by
+512 and configured maximum as per RFC 6891 6.2.3 "Values lower than 512
+MUST be treated as equal to 512"
+
+The client that exposed the problem provided a payload udp size of 0.
+
+Signed-off-by: Kevin Darbyshire-Bryant <[email protected]>
+---
+ src/forward.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/src/forward.c b/src/forward.c
+index f22556a..62c5a5a 100644
+--- a/src/forward.c
++++ b/src/forward.c
+@@ -1408,6 +1408,8 @@ void receive_query(struct listener *listen, time_t now)
+ 	 defaults to 512 */
+       if (udp_size > daemon->edns_pktsz)
+ 	udp_size = daemon->edns_pktsz;
++      if (udp_size < 512)
++	udp_size = 512; /* RFC 6891 6.2.3 */
+     }
+ 
+ #ifdef HAVE_AUTH
+-- 
+2.7.4
+