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

[Cole Dishington <Cole.Dishington@alliedtelesis.co.nz>]

On Thu, 2021-09-16 at 13:26 +0200, Florian Westphal wrote:
> Cole Dishington <Cole.Dishington@alliedtelesis.co.nz> 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;
> 	}

The original code defined the range as [ntohs(exp->saved_proto.tcp.port), 65535]. The above would
cause a change in behaviour, should we try to avoid it?

> 
>   	/* 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?
Looks good, just the one question above.

Thanks for your time reviewing!