netdev - Re: [patch] netfilter: prevent harmless integer overflow

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Message-ID: <20130620110944.GS5008@mwanda>
Date:	Thu, 20 Jun 2013 14:09:44 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	Bart De Schuymer <bart.de.schuymer@...dora.be>,
	Patrick McHardy <kaber@...sh.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, netfilter@...r.kernel.org,
	coreteam@...filter.org, bridge@...ts.linux-foundation.org,
	netdev@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [patch] netfilter: prevent harmless integer overflow

On Thu, Jun 20, 2013 at 12:23:52PM +0200, Pablo Neira Ayuso wrote:
> On Tue, Jun 18, 2013 at 10:46:03AM +0300, Dan Carpenter wrote:
> > This overflow is harmless because a few lines later we check:
> > 
> > 	if (num_counters != t->private->nentries) {
> > 
> > But it still upsets the static checkers.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > 
> > diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
> > index 3d110c4..141350e 100644
> > --- a/net/bridge/netfilter/ebtables.c
> > +++ b/net/bridge/netfilter/ebtables.c
> > @@ -1278,6 +1278,8 @@ static int do_update_counters(struct net *net, const char *name,
> >  
> >  	if (num_counters == 0)
> >  		return -EINVAL;
> > +	if (num_counters > INT_MAX / sizeof(*tmp))
> > +		return -ENOMEM;
> 
> This is artificially limiting to INT_MAX / sizeof(struct counters).
> Before this patch, the limit is UINT_MAX / sizeof(struct counters). I
> think it's very unlikely to hit such a limit though, but as you
> mentioned we cover the overflow already.  Adding it to calm down a
> static checker sound a bit too much for me.

I think we may be talking about different things?

"num_counters" comes from the user in update_counters() and we can
definitely overflow.  I just copied the checks from do_replace() so
that's why it uses INT_MAX instead of UINT_MAX.

Like I said, the overflow is not harmful because later in the
function we check "(num_counters != t->private->nentries)" and
return an error before using "tmp".  So I don't feel strongly about
this patch either way.

regards,
dan carpenter

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