netfilter: reject: don't send icmp error if csum is invalid

tcp resets are never emitted if the packet that triggers the
reject/reset has an invalid checksum.

For icmp error responses there was no such check.
It allows to distinguish icmp response generated via

iptables -I INPUT -p udp --dport 42 -j REJECT

and those emitted by network stack (won't respond if csum is invalid,
REJECT does).

Arguably its possible to avoid this by using conntrack and only
using REJECT with -m conntrack NEW/RELATED.

However, this doesn't work when connection tracking is not in use
or when using nf_conntrack_checksum=0.

Furthermore, sending errors in response to invalid csums doesn't make
much sense so just add similar test as in nf_send_reset.

Validate csum if needed and only send the response if it is ok.

Reference: http://bugzilla.redhat.com/show_bug.cgi?id=1169829
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/net/ipv6/netfilter/nf_reject_ipv6.c b/net/ipv6/netfilter/nf_reject_ipv6.c
index d05b36440e8b..68e0bb4db1bf 100644
--- a/net/ipv6/netfilter/nf_reject_ipv6.c
+++ b/net/ipv6/netfilter/nf_reject_ipv6.c
@@ -208,4 +208,39 @@ void nf_send_reset6(struct net *net, struct sk_buff *oldskb, int hook)
 }
 EXPORT_SYMBOL_GPL(nf_send_reset6);
 
+static bool reject6_csum_ok(struct sk_buff *skb, int hook)
+{
+	const struct ipv6hdr *ip6h = ipv6_hdr(skb);
+	int thoff;
+	__be16 fo;
+	u8 proto;
+
+	if (skb->csum_bad)
+		return false;
+
+	if (skb_csum_unnecessary(skb))
+		return true;
+
+	proto = ip6h->nexthdr;
+	thoff = ipv6_skip_exthdr(skb, ((u8*)(ip6h+1) - skb->data), &proto, &fo);
+
+	if (thoff < 0 || thoff >= skb->len || (fo & htons(~0x7)) != 0)
+		return false;
+
+	return nf_ip6_checksum(skb, hook, thoff, proto) == 0;
+}
+
+void nf_send_unreach6(struct net *net, struct sk_buff *skb_in,
+		      unsigned char code, unsigned int hooknum)
+{
+	if (!reject6_csum_ok(skb_in, hooknum))
+		return;
+
+	if (hooknum == NF_INET_LOCAL_OUT && skb_in->dev == NULL)
+		skb_in->dev = net->loopback_dev;
+
+	icmpv6_send(skb_in, ICMPV6_DEST_UNREACH, code, 0);
+}
+EXPORT_SYMBOL_GPL(nf_send_unreach6);
+
 MODULE_LICENSE("GPL");