Subject: Re: [PATCH net v4] net: netfilter: Fix port selection of FTP for NF_NAT_RANGE_PROTO_SPECIFIED

From: Florian Westphal <[email-masked]>
Date: Thu, 16 Sep 2021 13:26:41 +0200

Cole Dishington <[email-masked]> wrote:
> +	/* Avoid applying nat->range to the reply direction */
> +	if (!exp->dir || !nat->range_info.min_proto.all || !nat->range_info.max_proto.all) {
> +		min = ntohs(exp->saved_proto.tcp.port);
> +		range_size = 65535 - min + 1;
> +	} else {
> +		min = ntohs(nat->range_info.min_proto.all);
> +		range_size = ntohs(nat->range_info.max_proto.all) - min + 1;
> +	}
> +
>  	/* Try to get same port: if not, try to change it. */
> -	for (port = ntohs(exp->saved_proto.tcp.port); port != 0; port++) {
> -		int ret;
> +	first_port = ntohs(exp->saved_proto.tcp.port);
> +	if (min > first_port || first_port > (min + range_size - 1))
> +		first_port = min;
>  
> +	for (i = 0, port = first_port; i < range_size; i++, port = (port - first_port + i) % range_size) {

This looks complicated.  As far as I understand, this could instead be
written like this (not even compile tested):

	/* Avoid applying nat->range to the reply direction */
	if (!exp->dir || !nat->range_info.min_proto.all || !nat->range_info.max_proto.all) {
		min = 1;
		max = 65535;
		range_size = 65535;
	} else {
		min = ntohs(nat->range_info.min_proto.all);
		max = ntohs(nat->range_info.max_proto.all);
		range_size = max - min + 1;
	}

  	/* Try to get same port: if not, try to change it. */
	port = ntohs(exp->saved_proto.tcp.port);

	if (port < min || port > max)
		port = min;

	for (i = 0; i < range_size; i++) {
  		exp->tuple.dst.u.tcp.port = htons(port);
  		ret = nf_ct_expect_related(exp, 0);
		if (ret != -EBUSY)
 			break;
		port++;
		if (port > max)
			port = min;
  	}

	if (ret != 0) {
	...

AFAICS this is the same, we loop at most range_size times,
in case range_size is 64k, we will loop through all (hmmm,
not good actually, but better make that a different change)
else through given min - max range.

If orig port was in-range, we try it first, then increment.
If port exceeds upper bound, cycle back to min.

What do you think?