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

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Message-Id: <20230607145954.19324-3-fw@strlen.de>
Date: Wed,  7 Jun 2023 16:59:53 +0200
From: Florian Westphal <fw@...len.de>
To: <netdev@...r.kernel.org>
Cc: jhs@...atatu.com,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	Florian Westphal <fw@...len.de>
Subject: [PATCH net 2/3] net/sched: act_ipt: add sanity checks on skb before calling target

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@...len.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