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, 23 insertions, 0 deletions

diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c
index 536da7bc598a..b7405eb7f1ef 100644
--- a/net/ipv4/netfilter/nf_reject_ipv4.c
+++ b/net/ipv4/netfilter/nf_reject_ipv4.c
@@ -164,4 +164,27 @@ void nf_send_reset(struct sk_buff *oldskb, int hook)
 }
 EXPORT_SYMBOL_GPL(nf_send_reset);
 
+void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
+{
+	struct iphdr *iph = ip_hdr(skb_in);
+	u8 proto;
+
+	if (skb_in->csum_bad || iph->frag_off & htons(IP_OFFSET))
+		return;
+
+	if (skb_csum_unnecessary(skb_in)) {
+		icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
+		return;
+	}
+
+	if (iph->protocol == IPPROTO_TCP || iph->protocol == IPPROTO_UDP)
+		proto = iph->protocol;
+	else
+		proto = 0;
+
+	if (nf_ip_checksum(skb_in, hook, ip_hdrlen(skb_in), proto) == 0)
+		icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
+}
+EXPORT_SYMBOL_GPL(nf_send_unreach);
+
 MODULE_LICENSE("GPL");