[PATCH 3.2 61/79] netfilter: xt_TCPOPTSTRIP: fix possible mangling beyond packet boundary

Ben Hutchings
2018-02-11T04:20:06Z

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

commit bc6bcb59dd7c184d229f9e86d08aa56059938a4c upstream.

This target assumes that tcph->doff is well-formed, that may be well
not the case. Add extra sanity checkings to avoid possible crash due
to read/write out of the real packet boundary. After this patch, the
default action on malformed TCP packets is to drop them. Moreover,
fragments are skipped.

Reported-by: Rafal Kupka <rkupka@telemetry.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/netfilter/xt_TCPOPTSTRIP.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

--- a/net/netfilter/xt_TCPOPTSTRIP.c
+++ b/net/netfilter/xt_TCPOPTSTRIP.c
@@ -30,17 +30,28 @@ static inline unsigned int optlen(const

static unsigned int
tcpoptstrip_mangle_packet(struct sk_buff *skb,
- const struct xt_tcpoptstrip_target_info *info,
+ const struct xt_action_param *par,
unsigned int tcphoff, unsigned int minlen)
{
+ const struct xt_tcpoptstrip_target_info *info = par->targinfo;
unsigned int optl, i, j;
struct tcphdr *tcph;
u_int16_t n, o;
u_int8_t *opt;
+ int len;
+
+ /* This is a fragment, no TCP header is available */
+ if (par->fragoff != 0)
+ return XT_CONTINUE;

if (!skb_make_writable(skb, skb->len))
return NF_DROP;

+ len = skb->len - tcphoff;
+ if (len < (int)sizeof(struct tcphdr) ||
+ tcp_hdr(skb)->doff * 4 > len)
+ return NF_DROP;
+
tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
opt = (u_int8_t *)tcph;

@@ -76,7 +87,7 @@ tcpoptstrip_mangle_packet(struct sk_buff
static unsigned int
tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
{
- return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
+ return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
sizeof(struct iphdr) + sizeof(struct tcphdr));
}

@@ -93,7 +104,7 @@ tcpoptstrip_tg6(struct sk_buff *skb, con
if (tcphoff < 0)
return NF_DROP;

- return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
+ return tcpoptstrip_mangle_packet(skb, par, tcphoff,
sizeof(*ipv6h) + sizeof(struct tcphdr));
}
#endif