metasploit-framework/lib/net/ssh/transport/packet_stream.rb

require 'net/ssh/buffered_io'
require 'net/ssh/errors'
require 'net/ssh/packet'
require 'net/ssh/transport/cipher_factory'
require 'net/ssh/transport/hmac'
require 'net/ssh/transport/state'

module Net; module SSH; module Transport

  # A module that builds additional functionality onto the Net::SSH::BufferedIo
  # module. It adds SSH encryption, compression, and packet validation, as
  # per the SSH2 protocol. It also adds an abstraction for polling packets,
  # to allow for both blocking and non-blocking reads.
  module PacketStream
    include BufferedIo

    def self.extended(object)
      object.__send__(:initialize_ssh)
    end

    # The map of "hints" that can be used to modify the behavior of the packet
    # stream. For instance, when authentication succeeds, an "authenticated"
    # hint is set, which is used to determine whether or not to compress the
    # data when using the "delayed" compression algorithm.
    attr_reader :hints

    # The server state object, which encapsulates the algorithms used to interpret
    # packets coming from the server.
    attr_reader :server

    # The client state object, which encapsulates the algorithms used to build
    # packets to send to the server.
    attr_reader :client

    # The name of the client (local) end of the socket, as reported by the
    # socket.
    def client_name
      @client_name ||= begin
        sockaddr = getsockname
        begin
          Socket.getnameinfo(sockaddr, Socket::NI_NAMEREQD).first
        rescue
          begin
            Socket.getnameinfo(sockaddr).first
          rescue
            begin
              Socket.gethostbyname(Socket.gethostname).first
            rescue
              lwarn { "the client ipaddr/name could not be determined" }
              "unknown"
            end
          end
        end
      end
    end

    # The IP address of the peer (remote) end of the socket, as reported by
    # the Rex socket.
    def peer_ip
		@peer_ip ||= getpeername[1]
	end

    # Returns true if the IO is available for reading, and false otherwise.
    def available_for_read?
      result = IO.select([self], nil, nil, 0)
      result && result.first.any?
    end

    # Returns the next full packet. If the mode parameter is :nonblock (the
    # default), then this will return immediately, whether a packet is
    # available or not, and will return nil if there is no packet ready to be
    # returned. If the mode parameter is :block, then this method will block
    # until a packet is available.
    def next_packet(mode=:nonblock)
      case mode
      when :nonblock then
        fill if available_for_read?
        poll_next_packet

      when :block then
        loop do
          packet = poll_next_packet
          return packet if packet

          loop do
            result = IO.select([self]) or next
            break if result.first.any?
          end

          if fill <= 0
            raise Net::SSH::Disconnect, "connection closed by remote host"
          end
        end

      else
        raise ArgumentError, "expected :block or :nonblock, got #{mode.inspect}"
      end
    end

    # Enqueues a packet to be sent, and blocks until the entire packet is
    # sent.
    def send_packet(payload)
      enqueue_packet(payload)
      wait_for_pending_sends
    end

    # Enqueues a packet to be sent, but does not immediately send the packet.
    # The given payload is pre-processed according to the algorithms specified
    # in the client state (compression, cipher, and hmac).
    def enqueue_packet(payload)
      # try to compress the packet
      payload = client.compress(payload)

      # the length of the packet, minus the padding
      actual_length = 4 + payload.length + 1

      # compute the padding length
      padding_length = client.cipher.block_size - (actual_length % client.cipher.block_size)
      padding_length += client.cipher.block_size if padding_length < 4

      # compute the packet length (sans the length field itself)
      packet_length = payload.length + padding_length + 1

      padding = Array.new(padding_length) { rand(256) }.pack("C*")

      unencrypted_data = [packet_length, padding_length, payload, padding].pack("NCA*A*")
      mac = client.hmac.digest([client.sequence_number, unencrypted_data].pack("NA*"))

      encrypted_data = client.update_cipher(unencrypted_data) << client.final_cipher
      message = encrypted_data + mac

      debug { "queueing packet nr #{client.sequence_number} type #{payload.getbyte(0)} len #{packet_length}" }
      enqueue(message)

      client.increment(packet_length)

      self
    end

    # Performs any pending cleanup necessary on the IO and its associated
    # state objects. (See State#cleanup).
    def cleanup
      client.cleanup
      server.cleanup
    end

    # If the IO object requires a rekey operation (as indicated by either its
    # client or server state objects, see State#needs_rekey?), this will
    # yield. Otherwise, this does nothing.
    def if_needs_rekey?
      if client.needs_rekey? || server.needs_rekey?
        yield
        client.reset! if client.needs_rekey?
        server.reset! if server.needs_rekey?
      end
    end

    protected

      # Called when this module is used to extend an object. It initializes
      # the states and generally prepares the object for use as a packet stream.
      def initialize_ssh
        @hints  = {}
        @server = State.new(self, :server)
        @client = State.new(self, :client)
        @packet = nil
        initialize_buffered_io
      end

      # Tries to read the next packet. If there is insufficient data to read
      # an entire packet, this returns immediately, otherwise the packet is
      # read, post-processed according to the cipher, hmac, and compression
      # algorithms specified in the server state object, and returned as a
      # new Packet object.
      def poll_next_packet
        if @packet.nil?
          minimum = server.cipher.block_size < 4 ? 4 : server.cipher.block_size
          return nil if available < minimum
          data = read_available(minimum)

          # decipher it
          @packet = Net::SSH::Buffer.new(server.update_cipher(data))
          @packet_length = @packet.read_long
        end

        need = @packet_length + 4 - server.cipher.block_size
        raise Net::SSH::Exception, "padding error, need #{need} block #{server.cipher.block_size}" if need % server.cipher.block_size != 0

        return nil if available < need + server.hmac.mac_length

        if need > 0
          # read the remainder of the packet and decrypt it.
          data = read_available(need)
          @packet.append(server.update_cipher(data))
        end

        # get the hmac from the tail of the packet (if one exists), and
        # then validate it.
        real_hmac = read_available(server.hmac.mac_length) || ""

        @packet.append(server.final_cipher)
        padding_length = @packet.read_byte

        payload = @packet.read(@packet_length - padding_length - 1)
        padding = @packet.read(padding_length) if padding_length > 0

        my_computed_hmac = server.hmac.digest([server.sequence_number, @packet.content].pack("NA*"))
        raise Net::SSH::Exception, "corrupted mac detected" if real_hmac != my_computed_hmac

        # try to decompress the payload, in case compression is active
        payload = server.decompress(payload)

        debug { "received packet nr #{server.sequence_number} type #{payload.getbyte(0)} len #{@packet_length}" }

        server.increment(@packet_length)
        @packet = nil

        return Packet.new(payload)
      end
  end

end; end; end