[PATCH v2 net 1/5] icmp: introduce helper for NAT'd source address in network device context

["Jason A. Donenfeld" <Jason@zx2c4.com>]

This introduces a helper function to be called only by network drivers
that wraps calls to icmp[v6]_send in a conntrack transformation, in case
NAT has been used. The transformation happens only on a non-shared skb,
and the skb is fixed back up to its original state after, in case the
calling code continues to use it.

We don't want to pollute the non-driver path, though, so we introduce
this as a helper to be called by places that actually make use of
this, as suggested by Florian.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Florian Westphal <fw@strlen.de>
---
 include/linux/icmpv6.h |  6 ++++++
 include/net/icmp.h     |  6 ++++++
 net/ipv4/icmp.c        | 29 +++++++++++++++++++++++++++++
 net/ipv6/ip6_icmp.c    | 30 ++++++++++++++++++++++++++++++
 4 files changed, 71 insertions(+)

diff --git a/include/linux/icmpv6.h b/include/linux/icmpv6.h
index ef1cbb5f454f..93338fd54af8 100644
--- a/include/linux/icmpv6.h
+++ b/include/linux/icmpv6.h
@@ -31,6 +31,12 @@ static inline void icmpv6_send(struct sk_buff *skb,
 }
 #endif
 
+#if IS_ENABLED(CONFIG_NF_NAT)
+void icmpv6_ndo_send(struct sk_buff *skb_in, u8 type, u8 code, __u32 info);
+#else
+#define icmpv6_ndo_send icmpv6_send
+#endif
+
 extern int				icmpv6_init(void);
 extern int				icmpv6_err_convert(u8 type, u8 code,
 							   int *err);
diff --git a/include/net/icmp.h b/include/net/icmp.h
index 5d4bfdba9adf..9ac2d2672a93 100644
--- a/include/net/icmp.h
+++ b/include/net/icmp.h
@@ -43,6 +43,12 @@ static inline void icmp_send(struct sk_buff *skb_in, int type, int code, __be32
 	__icmp_send(skb_in, type, code, info, &IPCB(skb_in)->opt);
 }
 
+#if IS_ENABLED(CONFIG_NF_NAT)
+void icmp_ndo_send(struct sk_buff *skb_in, int type, int code, __be32 info);
+#else
+#define icmp_ndo_send icmp_send
+#endif
+
 int icmp_rcv(struct sk_buff *skb);
 int icmp_err(struct sk_buff *skb, u32 info);
 int icmp_init(void);
diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 18068ed42f25..5ca36181d4f4 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -748,6 +748,35 @@ out:;
 }
 EXPORT_SYMBOL(__icmp_send);
 
+#if IS_ENABLED(CONFIG_NF_NAT)
+#include <net/netfilter/nf_conntrack.h>
+void icmp_ndo_send(struct sk_buff *skb_in, int type, int code, __be32 info)
+{
+	struct sk_buff *cloned_skb = NULL;
+	enum ip_conntrack_info ctinfo;
+	struct nf_conn *ct;
+	__be32 orig_ip;
+
+	ct = nf_ct_get(skb_in, &ctinfo);
+	if (ct) {
+		if (skb_shared(skb_in)) {
+			skb_in = cloned_skb = skb_clone(skb_in, GFP_ATOMIC);
+			if (unlikely(!skb_in))
+				return;
+		}
+		orig_ip = ip_hdr(skb_in)->saddr;
+		ip_hdr(skb_in)->saddr = ct->tuplehash[0].tuple.src.u3.ip;
+	}
+	icmp_send(skb_in, type, code, info);
+	if (ct) {
+		if (cloned_skb)
+			consume_skb(cloned_skb);
+		else
+			ip_hdr(skb_in)->saddr = orig_ip;
+	}
+}
+EXPORT_SYMBOL(icmp_ndo_send);
+#endif
 
 static void icmp_socket_deliver(struct sk_buff *skb, u32 info)
 {
diff --git a/net/ipv6/ip6_icmp.c b/net/ipv6/ip6_icmp.c
index 02045494c24c..ee364d61b789 100644
--- a/net/ipv6/ip6_icmp.c
+++ b/net/ipv6/ip6_icmp.c
@@ -45,4 +45,34 @@ void icmpv6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info)
 	rcu_read_unlock();
 }
 EXPORT_SYMBOL(icmpv6_send);
+
+#if IS_ENABLED(CONFIG_NF_NAT)
+#include <net/netfilter/nf_conntrack.h>
+void icmpv6_ndo_send(struct sk_buff *skb_in, u8 type, u8 code, __u32 info)
+{
+	struct sk_buff *cloned_skb = NULL;
+	enum ip_conntrack_info ctinfo;
+	struct in6_addr orig_ip;
+	struct nf_conn *ct;
+
+	ct = nf_ct_get(skb_in, &ctinfo);
+	if (ct) {
+		if (skb_shared(skb_in)) {
+			skb_in = cloned_skb = skb_clone(skb_in, GFP_ATOMIC);
+			if (unlikely(!skb_in))
+				return;
+		}
+		orig_ip = ipv6_hdr(skb_in)->saddr;
+		ipv6_hdr(skb_in)->saddr = ct->tuplehash[0].tuple.src.u3.in6;
+	}
+	icmpv6_send(skb_in, type, code, info);
+	if (ct) {
+		if (cloned_skb)
+			consume_skb(cloned_skb);
+		else
+			ipv6_hdr(skb_in)->saddr = orig_ip;
+	}
+}
+EXPORT_SYMBOL(icmpv6_ndo_send);
+#endif
 #endif
-- 
2.25.0