[PATCH net 2/3] net/sched: act_ipt: add sanity checks on skb before calling target

[Florian Westphal <fw@strlen.de>]

Netfilter targets make assumptions on the skb state, for example
iphdr is supposed to be in the linear area.

This is normally done by IP stack, but in act_ipt case no
such checks are made.

Some targets can even assume that skb_dst will be valid.
Make a minimum effort to check for this:

- Don't call the targets eval function for non-ipv4 skbs.
- Don't call the targets eval function for POSTROUTING
  emulation when the skb has no dst set.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/sched/act_ipt.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/net/sched/act_ipt.c b/net/sched/act_ipt.c
index ea7f151e7dd2..2f0b39cc4e37 100644
--- a/net/sched/act_ipt.c
+++ b/net/sched/act_ipt.c
@@ -230,6 +230,26 @@ static int tcf_xt_init(struct net *net, struct nlattr *nla,
 			      a, &act_xt_ops, tp, flags);
 }
 
+static bool tcf_ipt_act_check(struct sk_buff *skb)
+{
+	const struct iphdr *iph;
+	unsigned int nhoff, len;
+
+	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+		return false;
+
+	nhoff = skb_network_offset(skb);
+	iph = ip_hdr(skb);
+	if (iph->ihl < 5 || iph->version != 4)
+		return false;
+
+	len = skb_ip_totlen(skb);
+	if (skb->len < nhoff + len || len < (iph->ihl * 4u))
+		return false;
+
+	return pskb_may_pull(skb, iph->ihl * 4u);
+}
+
 TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb,
 				  const struct tc_action *a,
 				  struct tcf_result *res)
@@ -244,9 +264,22 @@ TC_INDIRECT_SCOPE int tcf_ipt_act(struct sk_buff *skb,
 		.pf	= NFPROTO_IPV4,
 	};
 
+	if (skb->protocol != htons(ETH_P_IP))
+		return TC_ACT_UNSPEC;
+
 	if (skb_unclone(skb, GFP_ATOMIC))
 		return TC_ACT_UNSPEC;
 
+	if (!tcf_ipt_act_check(skb))
+		return TC_ACT_UNSPEC;
+
+	if (state.hook == NF_INET_POST_ROUTING) {
+		if (!skb_dst(skb))
+			return TC_ACT_UNSPEC;
+
+		state.out = skb->dev;
+	}
+
 	spin_lock(&ipt->tcf_lock);
 
 	tcf_lastuse_update(&ipt->tcf_tm);
-- 
2.39.3