[FixRM #8749] - Make spawn_meterpreter respect lport/lhost options

[FixRM #8749] Basically the spawn_meterpreter script doesn't
actually allow the user to set their own LHOST/LPORT datastore
options, because they come from the session object, not from the
active module or the framework object.

The fix is to allow the user to config them from framework. But
if they forget to do this (because naturally people probably
assume that active module datastore options are the same as the
ones set in framework), then for LHOST, we default whatever we get
from Rex::Socket.source_address. As for LPORT, we'll pick a one
that's not used by any of the sessions.

scripts/shell/spawn_meterpreter.rb

@@ -18,17 +18,39 @@ def progress(total, sent)
 end
 
 
-raise RuntimeError, "You must select a session." if (not session)
-raise RuntimeError, "Selected session is not a command shell session!" if (session.type != "shell")
+#
+# Returns if a port is used by a session
+#
+def is_port_used?(port)
+  framework.sessions.each do |sid, obj|
+    local_info = obj.instance_variable_get(:@local_info)
+    return true if local_info =~ /:#{port}$/
+  end
 
-# Check for required datastore options
-if (not session.exploit_datastore['LHOST'] or not session.exploit_datastore['LPORT'])
-  raise RuntimeError, "You must set LPORT and LHOST for this script to work."
+  false
 end
 
+#
+# Mimics what MSF alreayd does if the user doesn't manually select a payload and lhost
+#
+lhost = framework.datastore['LHOST']
+unless lhost
+  lhost = Rex::Socket.source_address('50.50.50.50')
+end
 
-lhost = session.exploit_datastore['LHOST']
-lport = session.exploit_datastore['LPORT']
+#
+# If there is no LPORT defined in framework, then pick a random one that's not used
+# by current sessions. This is possible if the user assumes module datastore options
+# are the same as framework datastore options.
+#
+lport = framework.datastore['LPORT']
+unless lport
+  lport = 4444 # Default meterpreter port
+  while is_port_used?(lport)
+    # Pick a port that's not used
+    lport = [*49152..65535].sample
+  end
+end
 
 # maybe we want our sessions going to another instance?
 use_handler = true