ipv6: Check ip6_find_1stfragopt() return value properly.

commit 7dd7eb95 upstream.

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>
[bwh: Backported to 3.2: adjust filenames, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv6/af_inet6.c

@@ -785,7 +785,6 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb, u32 features)
 	const struct inet6_protocol *ops;
 	int proto;
 	struct frag_hdr *fptr;
-	unsigned int unfrag_ip6hlen;
 	u8 *prevhdr;
 	int offset = 0;
 
@@ -824,11 +823,11 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb, u32 features)
 		ipv6h->payload_len = htons(skb->len - skb->mac_len -
 					   sizeof(*ipv6h));
 		if (proto == IPPROTO_UDP) {
-			unfrag_ip6hlen = ip6_find_1stfragopt(skb, &prevhdr);
-			if (unfrag_ip6hlen < 0)
-				return ERR_PTR(unfrag_ip6hlen);
+			int err = ip6_find_1stfragopt(skb, &prevhdr);
+			if (err < 0)
+				return ERR_PTR(err);
 			fptr = (struct frag_hdr *)(skb_network_header(skb) +
-				unfrag_ip6hlen);
+				err);
 			fptr->frag_off = htons(offset);
 			if (skb->next != NULL)
 				fptr->frag_off |= htons(IP6_MF);

net/ipv6/ip6_output.c

@@ -631,11 +631,10 @@ int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	u8 *prevhdr, nexthdr = 0;
 	struct net *net = dev_net(skb_dst(skb)->dev);
 
-	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.c

@@ -1316,6 +1316,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb, u32 features)
 	u8 frag_hdr_sz = sizeof(struct frag_hdr);
 	int offset;
 	__wsum csum;
+	int err;
 
 	mss = skb_shinfo(skb)->gso_size;
 	if (unlikely(skb->len <= mss))
@@ -1352,9 +1353,10 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb, u32 features)
 	/* 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) +