Revert "netfilter: conntrack: fix bug in for_each_sctp_chunk"

There is no bug.  If sch->length == 0, this would result in an infinite
loop, but first caller, do_basic_checks(), errors out in this case.

After this change, packets with bogus zero-length chunks are no longer
detected as invalid, so revert & add comment wrt. 0 length check.

Fixes: 98ee0077 ("netfilter: conntrack: fix bug in for_each_sctp_chunk")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_sctp.c

@@ -142,10 +142,11 @@ static void sctp_print_conntrack(struct seq_file *s, struct nf_conn *ct)
 }
 #endif
 
+/* do_basic_checks ensures sch->length > 0, do not use before */
 #define for_each_sctp_chunk(skb, sch, _sch, offset, dataoff, count)	\
 for ((offset) = (dataoff) + sizeof(struct sctphdr), (count) = 0;	\
-	((sch) = skb_header_pointer((skb), (offset), sizeof(_sch), &(_sch))) &&	\
-	(sch)->length;	\
+	(offset) < (skb)->len &&					\
+	((sch) = skb_header_pointer((skb), (offset), sizeof(_sch), &(_sch)));	\
 	(offset) += (ntohs((sch)->length) + 3) & ~3, (count)++)
 
 /* Some validity checks to make sure the chunks are fine */