Subject: Re: [patch] netfilter: prevent harmless integer overflow

From: Dan Carpenter <[email-masked]>
Date: Thu, 20 Jun 2013 14:09:44 +0300

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 <[email-masked]>
> > 
> > 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