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

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Date:   Sun, 02 Apr 2017 04:54:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Denys Fedoryshchenko <nuclearcat@...learcat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Patrick McHardy <kaber@...sh.net>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-kernel@...r.kernel.org
Subject: Re: KASAN, xt_TCPMSS  finally found nasty use-after-free bug? 4.10.8

On Sun, 2017-04-02 at 13:45 +0200, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> 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)