openvswitch: Fix IPv6 exthdr handling with ct helpers.

Static code analysis reveals the following bug:

        net/openvswitch/conntrack.c:281 ovs_ct_helper()
        warn: unsigned 'protoff' is never less than zero.

This signedness bug breaks error handling for IPv6 extension headers when
using conntrack helpers. Fix the error by using a local signed variable.

Fixes:  cae3a2627520: "openvswitch: Allow attaching helpers to ct
action"
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/openvswitch/conntrack.c b/net/openvswitch/conntrack.c
index e8e524ad8a01..002a755fa07e 100644
--- a/net/openvswitch/conntrack.c
+++ b/net/openvswitch/conntrack.c
@@ -275,13 +275,15 @@ static int ovs_ct_helper(struct sk_buff *skb, u16 proto)
         case NFPROTO_IPV6: {
                 u8 nexthdr = ipv6_hdr(skb)->nexthdr;
                 __be16 frag_off;
+                int ofs;
 
-                protoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr),
-                                           &nexthdr, &frag_off);
-                if (protoff < 0 || (frag_off & htons(~0x7)) != 0) {
+                ofs = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
+                                       &frag_off);
+                if (ofs < 0 || (frag_off & htons(~0x7)) != 0) {
                         pr_debug("proto header not found\n");
                         return NF_ACCEPT;
                 }
+                protoff = ofs;
                 break;
         }
         default: