[CVE-2017-18017,Trusty,Zesty,1/1] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

Message ID 20180109033019.5717-2-po-hsu.lin@canonical.com
State New
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Series
  • [CVE-2017-18017,Trusty,Zesty,1/1] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff
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Commit Message

Po-Hsu Lin Jan. 9, 2018, 3:30 a.m.
From: Eric Dumazet <edumazet@google.com>

CVE-2017-18017

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@google.com>
Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit 2638fd0f92d4397884fd991d8f4925cb3f081901)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
---
 net/netfilter/xt_TCPMSS.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Patch

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index e762de5..6531d70 100644
--- 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..
 	 */