make user networking hostfwd work with restrict=y

This patch allows the hostfwd option to override the restrict=y setting in
the user network stack, as explicitly stated in the documentation on the
restrict option:

     restrict=on|off
         If this option is enabled, the guest will be isolated, i.e. it
         will not be able to contact the host and no guest IP packets
         will be routed over the host to the outside. This option does
         not affect any explicitly set forwarding rules.

Qemu bug tracker:
https://bugs.launchpad.net/qemu/+bug/829455Signed-off-by: NGertjan Halkes <qemu@ghalkes.nl>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

slirp/tcp_input.c

@@ -316,16 +316,6 @@ tcp_input(struct mbuf *m, int iphlen, struct socket *inso)
 	m->m_data += sizeof(struct tcpiphdr)+off-sizeof(struct tcphdr);
 	m->m_len  -= sizeof(struct tcpiphdr)+off-sizeof(struct tcphdr);
 
-    if (slirp->restricted) {
-        for (ex_ptr = slirp->exec_list; ex_ptr; ex_ptr = ex_ptr->ex_next) {
-            if (ex_ptr->ex_fport == ti->ti_dport &&
-                ti->ti_dst.s_addr == ex_ptr->ex_addr.s_addr) {
-                break;
-            }
-        }
-        if (!ex_ptr)
-            goto drop;
-    }
 	/*
 	 * Locate pcb for segment.
 	 */
@@ -355,6 +345,22 @@ findso:
 	 * as if it was LISTENING, and continue...
 	 */
         if (so == NULL) {
+          if (slirp->restricted) {
+            /* Any hostfwds will have an existing socket, so we only get here
+             * for non-hostfwd connections. These should be dropped, unless it
+             * happens to be a guestfwd.
+             */
+            for (ex_ptr = slirp->exec_list; ex_ptr; ex_ptr = ex_ptr->ex_next) {
+                if (ex_ptr->ex_fport == ti->ti_dport &&
+                    ti->ti_dst.s_addr == ex_ptr->ex_addr.s_addr) {
+                    break;
+                }
+            }
+            if (!ex_ptr) {
+                goto dropwithreset;
+            }
+          }
+
 	  if ((tiflags & (TH_SYN|TH_FIN|TH_RST|TH_URG|TH_ACK)) != TH_SYN)
 	    goto dropwithreset;