Change StageEncoder behaviour

Following https://github.com/rapid7/metasploit-framework/pull/3770, some
change have been done as multiple encoders is managed by encoded_payload
to be used by Encoder and StageEncoder in the same time.

lib/msf/core/payload/stager.rb

@@ -17,7 +17,7 @@ module Msf::Payload::Stager
         Msf::OptBool.new("EnableStageEncoding", [ false, "Encode the second stage payload", false ]),
         Msf::OptString.new("StageEncoder", [ false, "Encoder to use if EnableStageEncoding is set", nil ]),
         Msf::OptString.new("StageEncoderSaveRegisters", [ false, "Additional registers to preserve in the staged payload if EnableStageEncoding is set", "" ]),
-        Msf::OptBool.new("StageEncodingFallback", [ false, "Fallback to default encoders or no encoding if the selected StageEncoder is not compatible", true ])
+        Msf::OptBool.new("StageEncodingFallback", [ false, "Fallback to no encoding if the selected StageEncoder is not compatible", true ])
       ], Msf::Payload::Stager)
 
   end
@@ -221,22 +221,11 @@ module Msf::Payload::Stager
   # @return [String] Encoded version of +stg+
   def encode_stage(stg)
     return stg unless encode_stage?
-    stage_enc_mod = []
+    stage_enc_mod = nil
 
     # Handle StageEncoder if specified by the user
     if datastore['StageEncoder'].to_s.length > 0
-      # Allow multiple encoders separated by commas
+      stage_enc_mod = datastore["StageEncoder"]
-      stage_enc_mod = datastore["StageEncoder"].split(',').map(&:strip).select{|x| x.to_s.length > 0}.uniq
-    end
-
-    # Add automatic encoding as a fallback if needed
-    if datastore['StageEncodingFallback']
-      stage_enc_mod << nil
-    end
-
-    # If fallback has been disabled and no encoder was parsed, exit early and rop the session
-    if stage_enc_mod.length == 0
-      raise RuntimeError, "StageEncoder is invalid and StageEncodingFallback is disabled"
     end
 
     # Allow the user to specify additional registers to preserve
@@ -247,35 +236,33 @@ module Msf::Payload::Stager
     saved_registers.strip!
 
     estg = nil
-
+    begin
-    stage_enc_mod.each do |encoder_refname_from_user|
-
       # Generate an encoded version of the stage.  We tell the encoding system
       # to save certain registers to ensure that it does not get clobbered.
       encp = Msf::EncodedPayload.create(
         self,
         'Raw'                => stg,
-        'Encoder'            => encoder_refname_from_user,
+        'Encoder'            => stage_enc_mod,
         'EncoderOptions'     => { 'SaveRegisters' => saved_registers },
         'ForceSaveRegisters' => true,
         'ForceEncode'        => true)
 
       if encp.encoder
+        if stage_enc_mod
+          print_status("Encoded stage with #{stage_enc_mod}")
+        else
           print_status("Encoded stage with #{encp.encoder.refname}")
+        end
         estg = encp.encoded
-
-        break
       end
-    end
+    rescue
-
       if datastore['StageEncodingFallback'] && estg.nil?
         print_warning("StageEncoder failed, falling back to no encoding")
         estg = stg
-    end
+      else
-
-    unless estg
         raise RuntimeError, "Stage encoding failed and StageEncodingFallback is disabled"
       end
+    end
 
     estg
   end