ipv6: Check ip6_find_1stfragopt() return value properly.

Do not use unsigned variables to see if it returns a negative
error or not.

Fixes: 2423496a ("ipv6: Prevent overrun when parsing v6 header options")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

CVE-2017-9074

(backported from commit 7dd7eb95)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

net/ipv6/ip6_offload.c

@@ -62,7 +62,7 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 	const struct net_offload *ops;
 	int proto;
 	struct frag_hdr *fptr;
-	unsigned int unfrag_ip6hlen;
+	unsigned int payload_len;
 	u8 *prevhdr;
 	int offset = 0;
 	bool encap, udpfrag;
@@ -121,10 +121,10 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 		skb->network_header = (u8 *)ipv6h - skb->head;
 
 		if (udpfrag) {
-			unfrag_ip6hlen = ip6_find_1stfragopt(skb, &prevhdr);
-			if (unfrag_ip6hlen < 0)
-				return ERR_PTR(unfrag_ip6hlen);
-			fptr = (struct frag_hdr *)((u8 *)ipv6h + unfrag_ip6hlen);
+			int err = ip6_find_1stfragopt(skb, &prevhdr);
+			if (err < 0)
+				return ERR_PTR(err);
+			fptr = (struct frag_hdr *)((u8 *)ipv6h + err);
 			fptr->frag_off = htons(offset);
 			if (skb->next)
 				fptr->frag_off |= htons(IP6_MF);

net/ipv6/ip6_output.c

@@ -571,11 +571,10 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	int ptr, offset = 0, err = 0;
 	u8 *prevhdr, nexthdr = 0;
 
-	hlen = ip6_find_1stfragopt(skb, &prevhdr);
-	if (hlen < 0) {
-		err = hlen;
+	err = ip6_find_1stfragopt(skb, &prevhdr);
+	if (err < 0)
 		goto fail;
-	}
+	hlen = err;
 	nexthdr = *prevhdr;
 
 	mtu = ip6_skb_dst_mtu(skb);

net/ipv6/udp_offload.c

@@ -29,6 +29,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 	u8 frag_hdr_sz = sizeof(struct frag_hdr);
 	__wsum csum;
 	int tnl_hlen;
+	int err;
 
 	mss = skb_shinfo(skb)->gso_size;
 	if (unlikely(skb->len <= mss))
@@ -97,9 +98,10 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 		/* Find the unfragmentable header and shift it left by frag_hdr_sz
 		 * bytes to insert fragment header.
 		 */
-		unfrag_ip6hlen = ip6_find_1stfragopt(skb, &prevhdr);
-		if (unfrag_ip6hlen < 0)
-			return ERR_PTR(unfrag_ip6hlen);
+		err = ip6_find_1stfragopt(skb, &prevhdr);
+		if (err < 0)
+			return ERR_PTR(err);
+		unfrag_ip6hlen = err;
 		nexthdr = *prevhdr;
 		*prevhdr = NEXTHDR_FRAGMENT;
 		unfrag_len = (skb_network_header(skb) - skb_mac_header(skb)) +