From 2638fd0f92d4397884fd991d8f4925cb3f081901 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 3 Apr 2017 10:55:11 -0700 Subject: [PATCH] netfilter: xt_TCPMSS: add more sanity tests on tcph->doff 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 Reported-by: Denys Fedoryshchenko Signed-off-by: Pablo Neira Ayuso --- 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 27241a767f17b4..c64aca611ac5c5 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) { @@ -152,6 +152,10 @@ 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.. */