From: Roy Marples <roy@marples.name>
Date: Wed, 16 Oct 2019 13:56:21 +0000 (+0100)
Subject: DHCP: Fix strict aliasing of checksumming the pseudo header
X-Git-Tag: v8.1.1~2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ec9fe8039fedacd6eaa7cbdfebab7838826ef030;p=thirdparty%2Fdhcpcd.git

DHCP: Fix strict aliasing of checksumming the pseudo header

in_cksum relies on accessing the data by uint16_t blocks.

Because the underlying object we send to in_cksum is struct ip,
we need to use union to allow access by uint16_t so strict aliasing
rules are not broken.
---

diff --git a/src/dhcp.c b/src/dhcp.c
index c989843b..356b7708 100644
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -3313,13 +3313,15 @@ checksums_valid(void *packet,
     struct in_addr *from, unsigned int flags)
 {
 	struct ip *ip = packet;
-	struct ip pseudo_ip = {
-		.ip_p = IPPROTO_UDP,
-		.ip_src = ip->ip_src,
-		.ip_dst = ip->ip_dst
+	union pip {
+		struct ip ip;
+		uint16_t w[sizeof(struct ip)];
+	} pip = {
+		.ip.ip_p = IPPROTO_UDP,
+		.ip.ip_src = ip->ip_src,
+		.ip.ip_dst = ip->ip_dst,
 	};
 	size_t ip_hlen;
-	uint16_t udp_len, uh_sum;
 	struct udphdr udp;
 	char *udpp, *uh_sump;
 	uint32_t csum;
@@ -3341,22 +3343,23 @@ checksums_valid(void *packet,
 
 	/* UDP checksum is based on a pseudo IP header alongside
 	 * the UDP header and payload. */
-	udp_len = ntohs(udp.uh_ulen);
-	uh_sum = udp.uh_sum;
+	pip.ip.ip_len = udp.uh_ulen;
+	csum = 0;
 
 	/* Need to zero the UDP sum in the packet for the checksum to work. */
 	uh_sump = udpp + offsetof(struct udphdr, uh_sum);
 	memset(uh_sump, 0, sizeof(udp.uh_sum));
 
-	pseudo_ip.ip_len = udp.uh_ulen;
-	csum = 0;
-	in_cksum(&pseudo_ip, sizeof(pseudo_ip), &csum);
-	csum = in_cksum(udpp, udp_len, &csum);
+	/* Checksum psuedo header and then UDP + payload. */
+	in_cksum(pip.w, sizeof(pip.w), &csum);
+	csum = in_cksum(udpp, ntohs(udp.uh_ulen), &csum);
 
+#if 0	/* Not needed, just here for completeness. */
 	/* Put the checksum back. */
-	memcpy(uh_sump, &uh_sum, sizeof(udp.uh_sum));
+	memcpy(uh_sump, &udp.uh_sum, sizeof(udp.uh_sum));
+#endif
 
-	return csum == uh_sum;
+	return csum == udp.uh_sum;
 }
 
 static void