Add verify_checksum and use it

Also fixed a YARD typo.

lib/msf/core/exploit/gdb.rb

@@ -21,6 +21,9 @@ module Exploit::Remote::Gdb
   # thrown when a response is incorrect
   class BadResponseError < RuntimeError; end
 
+  # thrown when a checksum is invalid
+  class BadChecksumError < RuntimeError; end
+
   # Default list of supported GDB features to send the to the target
   GDB_FEATURES = 'qSupported:multiprocess+;qRelocInsn+;qvCont+;'
 
@@ -58,12 +61,15 @@ module Exploit::Remote::Gdb
   # Reads (and possibly decodes) from the socket and sends an ACK to verify receipt
   # @param opts [Hash] the options hash
   # @option opts :decode [Boolean] rle decoding should be applied to the response
+  # @option opts :verify [Boolean] verify the response's checksum
   # @return [String] the response
   # @raise [BadResponseError] if the expected response is missing
+  # @raise [BadChecksumError] if the checksum is invalid
   def read_response(opts={})
-    decode = opts.fetch(:decode, false)
+    decode, verify = opts.fetch(:decode, false), opts.fetch(:verify, true)
     res = sock.get_once
     raise BadResponseError if res.nil?
+    raise BadChecksumError if (verify && !verify_checksum(res))
     res = decode_rle(res) if decode
     vprint_status('Result: '+res)
     send_ack
@@ -86,12 +92,20 @@ module Exploit::Remote::Gdb
 
   # The two-digit checksum is computed as the modulo 256 sum of all characters
   # between the leading ‘$’ and the trailing ‘#’ (an eight bit unsigned checksum).
-  # @param [String] str the string to calculate the checksum of
+  # @param str [String] the string to calculate the checksum of
   # @return [String] hex string containing checksum
   def checksum(str)
     "%02x" % str.bytes.inject(0) { |b, sum| (sum+b)%256 }
   end
 
+  # Verifies a response's checksum
+  # @param res [String] the response to check
+  # @return [Boolean] whether the checksum is valid
+  def verify_checksum(res)
+    msg, chksum = res.match(/^\$(.*)#(\h{2})$/)[1..2]
+    checksum(msg) == chksum
+  end
+
   # Writes the buffer +buf+ to the address +addr+ in the remote process's memory
   # @param buf [String] the buffer to write
   # @param addr [String] the hex-encoded address to write to