netfilter: ipv6: nf_defrag: fix leakage of unqueued fragments

With commit 997dd964 ("net: IP6 defrag: use rbtrees in
nf_conntrack_reasm.c"), nf_ct_frag6_reasm() is now called from
nf_ct_frag6_queue(). With this change, nf_ct_frag6_queue() can fail
after the skb has been added to the fragment queue and
nf_ct_frag6_gather() was adapted to handle this case.

But nf_ct_frag6_queue() can still fail before the fragment has been
queued. nf_ct_frag6_gather() can't handle this case anymore, because it
has no way to know if nf_ct_frag6_queue() queued the fragment before
failing. If it didn't, the skb is lost as the error code is overwritten
with -EINPROGRESS.

Fix this by setting -EINPROGRESS directly in nf_ct_frag6_queue(), so
that nf_ct_frag6_gather() can propagate the error as is.

Fixes: 997dd964 ("net: IP6 defrag: use rbtrees in nf_conntrack_reasm.c")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -293,7 +293,11 @@ static int nf_ct_frag6_queue(struct frag_queue *fq, struct sk_buff *skb,
 		skb->_skb_refdst = 0UL;
 		err = nf_ct_frag6_reasm(fq, skb, prev, dev);
 		skb->_skb_refdst = orefdst;
-		return err;
+
+		/* After queue has assumed skb ownership, only 0 or
+		 * -EINPROGRESS must be returned.
+		 */
+		return err ? -EINPROGRESS : 0;
 	}
 
 	skb_dst_drop(skb);
@@ -480,12 +484,6 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 		ret = 0;
 	}
 
-	/* after queue has assumed skb ownership, only 0 or -EINPROGRESS
-	 * must be returned.
-	 */
-	if (ret)
-		ret = -EINPROGRESS;
-
 	spin_unlock_bh(&fq->q.lock);
 	inet_frag_put(&fq->q);
 	return ret;