[PATCH 3.2 64/79] netfilter: xt_TCPMSS: fix handling of malformed TCP header and options

Ben Hutchings
2018-02-11T04:33:41Z

From: Pablo Neira Ayuso <pablo@netfilter.org>

commit 71ffe9c77dd7a2b62207953091efa8dafec958dd upstream.

Make sure the packet has enough room for the TCP header and
that it is not malformed.

While at it, store tcph->doff*4 in a variable, as it is used
several times.

This patch also fixes a possible off by one in case of malformed
TCP options.

Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/netfilter/xt_TCPMSS.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)

--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -51,7 +51,8 @@ tcpmss_mangle_packet(struct sk_buff *skb
{
const struct xt_tcpmss_info *info = par->targinfo;
struct tcphdr *tcph;
- unsigned int tcplen, i;
+ int len, tcp_hdrlen;
+ unsigned int i;
__be16 oldval;
u16 newmss;
u8 *opt;
@@ -63,11 +64,14 @@ tcpmss_mangle_packet(struct sk_buff *skb
if (!skb_make_writable(skb, skb->len))
return -1;

- tcplen = skb->len - tcphoff;
+ len = skb->len - tcphoff;
+ if (len < (int)sizeof(struct tcphdr))
+ return -1;
+
tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
+ tcp_hdrlen = tcph->doff * 4;

- /* Header cannot be larger than the packet */
- if (tcplen < tcph->doff*4)
+ if (len < tcp_hdrlen)
return -1;

if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
@@ -88,9 +92,8 @@ tcpmss_mangle_packet(struct sk_buff *skb
newmss = info->mss;

opt = (u_int8_t *)tcph;
- for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
- if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
- opt[i+1] == TCPOLEN_MSS) {
+ for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
+ if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
u_int16_t oldmss;

oldmss = (opt[i+2] << 8) | opt[i+3];
@@ -113,9 +116,10 @@ tcpmss_mangle_packet(struct sk_buff *skb
}

/* There is data after the header so the option can't be added
- without moving it, and doing so may make the SYN packet
- itself too large. Accept the packet unmodified instead. */
- if (tcplen > tcph->doff*4)
+ * without moving it, and doing so may make the SYN packet
+ * itself too large. Accept the packet unmodified instead.
+ */
+ if (len > tcp_hdrlen)
return 0;

/*
@@ -132,10 +136,10 @@ tcpmss_mangle_packet(struct sk_buff *skb
skb_put(skb, TCPOLEN_MSS);

opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
- memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
+ memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));

inet_proto_csum_replace2(&tcph->check, skb,
- htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
+ htons(len), htons(len + TCPOLEN_MSS), 1);
opt[0] = TCPOPT_MSS;
opt[1] = TCPOLEN_MSS;
opt[2] = (newmss & 0xff00) >> 8;