Re: KASAN, xt_TCPMSS finally found nasty use-after-free bug? 4.10.8

[Denys Fedoryshchenko]

On 2017-04-02 15:19, Eric Dumazet wrote:
> On Sun, 2017-04-02 at 04:54 -0700, Eric Dumazet wrote:
> > On Sun, 2017-04-02 at 13:45 +0200, Florian Westphal wrote:
> > > Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> > > > -	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
> > > > +	for (i = sizeof(struct tcphdr); i < tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
> > > >  		if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
> > > >  			u_int16_t oldmss;
> > >
> > > maybe I am low on caffeeine but this looks fine, for tcp header with
> > > only tcpmss this boils down to "20 <= 24 - 4" so we acccess offsets 20-23 which seems ok.
> >
> > I am definitely low on caffeine ;)
> >
> > An issue in this function is that we might add the missing MSS option,
> > without checking that TCP options are already full.
> >
> > But this should not cause a KASAN splat, only some malformed TCP 
> > packet
> >
> > (tcph->doff would wrap)
>
> Something like that maybe.
>
> diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
> index
> 27241a767f17b4b27d24095a31e5e9a2d3e29ce4..1465aaf0e3a15d69d105d0a50b0429b11b6439d3
> 100644
> --- a/net/netfilter/xt_TCPMSS.c
> +++ b/net/netfilter/xt_TCPMSS.c
> @@ -151,7 +151,9 @@ tcpmss_mangle_packet(struct sk_buff *skb,
>  	 */
>  	if (len > tcp_hdrlen)
>  		return 0;
> -
> +	/* tcph->doff is 4 bits wide, do not wrap its value to 0 */
> +	if (tcp_hdrlen >= 15 * 4)
> +		return 0;
>  	/*
>  	 * MSS Option not found ?! add it..
>  	 */
I will add also WARN_ON_ONCE(tcp_hdrlen >= 15 * 4) before, for 
curiosity, if this condition are triggered. Is it fine like that?