Re: [PATCH] netfilter: xt_TCPOPTSTRIP: fix possible mangling beyond packet boundary

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, May 15, 2013 at 02:33:36PM +0200, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > This target assumes that tcph->doff is well-formed, that may be well
> > not the case. Add extra sanity checkings to avoid possible crash due
> > to read/write out of the real packet boundary.
> 
> >  static unsigned int
> >  tcpoptstrip_mangle_packet(struct sk_buff *skb,
> > -			  const struct xt_tcpoptstrip_target_info *info,
> > +			  const struct xt_action_param *par,
> >  			  unsigned int tcphoff, unsigned int minlen)
> >  {
> > +	const struct xt_tcpoptstrip_target_info *info = par->targinfo;
> >  	unsigned int optl, i, j;
> >  	struct tcphdr *tcph;
> >  	u_int16_t n, o;
> >  	u_int8_t *opt;
> > +	int len;
> > +
> [..]
> > +	len = skb->len - tcphoff;
> > +	if (len < sizeof(struct tcphdr))
> 
> I think this needs a cast 'if (len < (int) sizeof( ...?

I'm not hitting any compilation warning.

> > @@ -51,7 +67,7 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
> >  	for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
> >  		optl = optlen(opt, i);
> >  
> > -		if (i + optl > tcp_hdrlen(skb))
> > +		if (i + optl > len)
> >  			break;
> 
> I don't understand this change.  This should stop parsing if the
> option length points outside of the tcp option size.
> 
> But doesn't len include the size of the actual payload?

We already validated a bit above that there's room for the TCP
options in the packet.

        /* Truncated TCP options, skip */
        if ((tcp_hdr(skb)->doff - 5) * 4 > len - sizeof(struct tcphdr))
                return NF_DROP;

So after that point we can trust the tcph->doff field.

Therefore, you're right, I'll remove that change. Thanks.