netfilter: ipv6: nf_defrag: accept duplicate fragments again

When fixing the skb leak introduced by the conversion to rbtree, I
forgot about the special case of duplicate fragments. The condition
under the 'insert_error' label isn't effective anymore as
nf_ct_frg6_gather() doesn't override the returned value anymore. So
duplicate fragments now get NF_DROP verdict.

To accept duplicate fragments again, handle them specially as soon as
inet_frag_queue_insert() reports them. Return -EINPROGRESS which will
translate to NF_STOLEN verdict, like any accepted fragment. However,
such packets don't carry any new information and aren't queued, so we
just drop them immediately.

Fixes: a0d56cb ("netfilter: ipv6: nf_defrag: fix leakage of unqueued fragments")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -265,8 +265,14 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 
 	prev = fq->q.fragments_tail;
 	err = inet_frag_queue_insert(&fq->q, skb, offset, end);
-	if (err)
+	if (err) {
+		if (err == IPFRAG_DUP) {
+			/* No error for duplicates, pretend they got queued. */
+			kfree_skb(skb);
+			return -EINPROGRESS;
+		}
 		goto insert_error;
+	}
 
 	if (dev)
 		fq->iif = dev->ifindex;
@@ -304,8 +310,6 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 	return -EINPROGRESS;
 
 insert_error:
-	if (err == IPFRAG_DUP)
-		goto err;
 	inet_frag_kill(&fq->q);
 err:
 	skb_dst_drop(skb);