linux-kernel - [PATCH 3.10 080/139] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

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Date:   Wed,  1 Nov 2017 22:26:21 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 080/139] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

From: Eric Dumazet <edumazet@...gle.com>

commit 2638fd0f92d4397884fd991d8f4925cb3f081901 upstream.

Denys provided an awesome KASAN report pointing to an use
after free in xt_TCPMSS

I have provided three patches to fix this issue, either in xt_TCPMSS or
in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
impact.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Denys Fedoryshchenko <nuclearcat@...learcat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
[wt: adjust context]

Signed-off-by: Willy Tarreau <w@....eu>
---
 net/netfilter/xt_TCPMSS.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 7011c71..c656269 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -68,7 +68,7 @@ tcpmss_mangle_packet(struct sk_buff *skb,
 	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 
 	/* Header cannot be larger than the packet */
-	if (tcplen < tcph->doff*4)
+	if (tcplen < tcph->doff*4 || tcph->doff*4 < sizeof(struct tcphdr))
 		return -1;
 
 	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -117,6 +117,10 @@ tcpmss_mangle_packet(struct sk_buff *skb,
 	if (tcplen > tcph->doff*4)
 		return 0;
 
+	/* tcph->doff has 4 bits, do not wrap it to 0 */
+	if (tcph->doff >= 15)
+		return 0;
+
 	/*
 	 * MSS Option not found ?! add it..
 	 */
-- 
2.8.0.rc2.1.gbe9624a