Re: xt_recent fails with kernel 3.19.0

[Chris Vine <chris@cvine.freeserve.co.uk>]

On Thu, 12 Feb 2015 10:25:53 +0000
Chris Vine <chris@cvine.freeserve.co.uk> wrote:
> Loading 'recent' xtables match support for iptables fails where the
> following sample rule is appended with SSH_TRIES set to 4:
> 
>   iptables -A SSH_CHAIN -m conntrack --ctstate NEW \
>     -m recent --update --seconds $SSH_LOGIN_PERIOD --hitcount
> $SSH_TRIES -j DROP
> 
> It fails with this message:
> 
>   kernel: xt_recent: hitcount (4) is larger than packets to be
>   remembered (4) for table DEFAULT
> 
> This appears to be due to an off-by-one error in testing the hit
> count in recent_mt_check().  This occurs because nstamp_mask is set
> to one less than the value of ip_pkt_list_tot (if any) or of
> hit_count rounded up to a power of two value.  When that hit count
> boundary is actually reached nstamp_mask is therefore exceeded by one.
> 
> I can't say I fully understand the heuristics of nstamp_mask, but the
> patch below deals with this and works for me(TM).
> 
> Signed-of-by: Chris Vine <vine.chris@gmail.com>
> 
> --- linux-3.19.0/net/netfilter/xt_recent.c~	2015-02-10
> 09:18:44.657376355 +0000 +++
> linux-3.19.0/net/netfilter/xt_recent.c	2015-02-11
> 17:58:33.311608835 +0000 @@ -378,7 +378,7 @@
> mutex_lock(&recent_mutex); t = recent_table_lookup(recent_net,
> info->name); if (t != NULL) {
> -		if (info->hit_count > t->nstamps_max_mask) {
> +		if (info->hit_count > t->nstamps_max_mask + 1) {
>  			pr_info("hitcount (%u) is larger than
> packets to be remembered (%u) for table %s\n", info->hit_count,
> t->nstamps_max_mask + 1, info->name);

Scrub that.  This now fails when SSH_TRIES is set to other than a power
of two boundary.  There seems to be something fundamentally wrong with
the heuristic employed here.

Chris
the