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

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Date:   Sun, 02 Apr 2017 10:26:12 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Denys Fedoryshchenko <nuclearcat@...learcat.com>
Cc:     Florian Westphal <fw@...len.de>,
        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, netdev-owner@...r.kernel.org
Subject: Re: KASAN, xt_TCPMSS  finally found nasty use-after-free bug? 4.10.8

On Sun, 2017-04-02 at 10:14 -0700, Eric Dumazet wrote:

> Could that be that netfilter does not abort earlier if TCP header is
> completely wrong ?
> 

Yes, I wonder if this patch would be better, unless we replicate the
th->doff sanity check in all netfilter modules dissecting TCP frames.

diff --git a/net/netfilter/xt_tcpudp.c b/net/netfilter/xt_tcpudp.c
index ade024c90f4f129a7c384e9e1cbfdb8ffe73065f..8cb4eadd5ba1c20e74bc27ee52a0bc36a5b26725 100644
--- a/net/netfilter/xt_tcpudp.c
+++ b/net/netfilter/xt_tcpudp.c
@@ -103,11 +103,11 @@ static bool tcp_mt(const struct sk_buff *skb, struct xt_action_param *par)
 	if (!NF_INVF(tcpinfo, XT_TCP_INV_FLAGS,
 		     (((unsigned char *)th)[13] & tcpinfo->flg_mask) == tcpinfo->flg_cmp))
 		return false;
+	if (th->doff * 4 < sizeof(_tcph)) {
+		par->hotdrop = true;
+		return false;
+	}
 	if (tcpinfo->option) {
-		if (th->doff * 4 < sizeof(_tcph)) {
-			par->hotdrop = true;
-			return false;
-		}
 		if (!tcp_find_option(tcpinfo->option, skb, par->thoff,
 				     th->doff*4 - sizeof(_tcph),
 				     tcpinfo->invflags & XT_TCP_INV_OPTION,