linux-kernel - [PATCH 3.16 117/136] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

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Message-ID: <lsq.1518323471.281219415@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Eric Dumazet" <edumazet@...gle.com>,
        "Denys Fedoryshchenko" <nuclearcat@...learcat.com>,
        "Pablo Neira Ayuso" <pablo@...filter.org>
Subject: [PATCH 3.16 117/136] netfilter: xt_TCPMSS: add more sanity tests
 on tcph->doff

3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/netfilter/xt_TCPMSS.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -104,7 +104,7 @@ tcpmss_mangle_packet(struct sk_buff *skb
 	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
 	tcp_hdrlen = tcph->doff * 4;
 
-	if (len < tcp_hdrlen)
+	if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
 		return -1;
 
 	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -156,6 +156,10 @@ tcpmss_mangle_packet(struct sk_buff *skb
 	if (len > tcp_hdrlen)
 		return 0;
 
+	/* tcph->doff has 4 bits, do not wrap it to 0 */
+	if (tcp_hdrlen >= 15 * 4)
+		return 0;
+
 	/*
 	 * MSS Option not found ?! add it..
 	 */