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

[Eric Dumazet]

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,