net: support marking accepting TCP sockets

When using mark-based routing, sockets returned from accept()
may need to be marked differently depending on the incoming
connection request.

This is the case, for example, if different socket marks identify
different networks: a listening socket may want to accept
connections from all networks, but each connection should be
marked with the network that the request came in on, so that
subsequent packets are sent on the correct network.

This patch adds a sysctl to mark TCP sockets based on the fwmark
of the incoming SYN packet. If enabled, and an unmarked socket
receives a SYN, then the SYN packet's fwmark is written to the
connection's inet_request_sock, and later written back to the
accepted socket when the connection is established.  If the
socket already has a nonzero mark, then the behaviour is the same
as it is today, i.e., the listening socket's fwmark is used.

Black-box tested using user-mode linux:

- IPv4/IPv6 SYN+ACK, FIN, etc. packets are routed based on the
  mark of the incoming SYN packet.
- The socket returned by accept() is marked with the mark of the
  incoming SYN packet.
- Tested with syncookies=1 and syncookies=2.

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 11 insertions, 0 deletions

diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 1833c3f389ee..b1edf17bec01 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -90,6 +90,7 @@ struct inet_request_sock {
         kmemcheck_bitfield_end(flags);
         struct ip_options_rcu   *opt;
         struct sk_buff          *pktopts;
+        u32                     ir_mark;
 };
 
 static inline struct inet_request_sock *inet_rsk(const struct request_sock *sk)
@@ -97,6 +98,15 @@ static inline struct inet_request_sock *inet_rsk(const struct request_sock *sk)
         return (struct inet_request_sock *)sk;
 }
 
+static inline u32 inet_request_mark(struct sock *sk, struct sk_buff *skb)
+{
+        if (!sk->sk_mark && sock_net(sk)->ipv4.sysctl_tcp_fwmark_accept) {
+                return skb->mark;
+        } else {
+                return sk->sk_mark;
+        }
+}
+
 struct inet_cork {
         unsigned int            flags;
         __be32                  addr;
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index a32fc4d705da..2f0cfad66666 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -78,6 +78,7 @@ struct netns_ipv4 {
         int sysctl_ip_fwd_use_pmtu;
 
         int sysctl_fwmark_reflect;
+        int sysctl_tcp_fwmark_accept;
 
         struct ping_group_range ping_group_range;