netfilter: nf_reject: don't reply to icmp error messages

tcp reject code won't reply to a tcp reset.

But the icmp reject 'netdev' family versions will reply to icmp
dst-unreach errors, unlike icmp_send() and icmp6_send() which are used
by the inet family implementation (and internally by the REJECT target).

Check for the icmp(6) type and do not respond if its an unreachable error.

Without this, something like 'ip protocol icmp reject', when used
in a netdev chain attached to 'lo', cause a packet loop.

Same for two hosts that both use such a rule: each error packet
will be replied to.

Such situation persist until the (bogus) rule is amended to ratelimit or
checks the icmp type before the reject statement.

As the inet versions don't do this make the netdev ones follow along.

Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 25 insertions, 0 deletions

diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c
index 05631abe3f0d..fae4aa4a5f09 100644
--- a/net/ipv4/netfilter/nf_reject_ipv4.c
+++ b/net/ipv4/netfilter/nf_reject_ipv4.c
@@ -80,6 +80,27 @@ struct sk_buff *nf_reject_skb_v4_tcp_reset(struct net *net,
 }
 EXPORT_SYMBOL_GPL(nf_reject_skb_v4_tcp_reset);
 
+static bool nf_skb_is_icmp_unreach(const struct sk_buff *skb)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	u8 *tp, _type;
+	int thoff;
+
+	if (iph->protocol != IPPROTO_ICMP)
+		return false;
+
+	thoff = skb_network_offset(skb) + sizeof(*iph);
+
+	tp = skb_header_pointer(skb,
+				thoff + offsetof(struct icmphdr, type),
+				sizeof(_type), &_type);
+
+	if (!tp)
+		return false;
+
+	return *tp == ICMP_DEST_UNREACH;
+}
+
 struct sk_buff *nf_reject_skb_v4_unreach(struct net *net,
 					 struct sk_buff *oldskb,
 					 const struct net_device *dev,
@@ -100,6 +121,10 @@ struct sk_buff *nf_reject_skb_v4_unreach(struct net *net,
 	if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
 		return NULL;
 
+	/* don't reply to ICMP_DEST_UNREACH with ICMP_DEST_UNREACH. */
+	if (nf_skb_is_icmp_unreach(oldskb))
+		return NULL;
+
 	/* RFC says return as much as we can without exceeding 576 bytes. */
 	len = min_t(unsigned int, 536, oldskb->len);