Fix bug #6815: A race condition that results in an invalid handle.

Under certain conditions, the module may run into an "The handle
is invalid" while obtaining registry keys and values from the
victim machine.  The fix is to retry a couple of times, and hope
we don't hit the race condition again.

modules/post/windows/gather/hashdump.rb

@@ -58,6 +58,8 @@ class Metasploit3 < Msf::Post
 	end
 
 	def run
+		tries = 0
+
 		begin
 
 			print_status("Obtaining the boot key...")
@@ -93,8 +95,18 @@ class Metasploit3 < Msf::Post
 		rescue ::Interrupt
 			raise $!
 		rescue ::Rex::Post::Meterpreter::RequestError => e
-			print_error("Meterpreter Exception: #{e.class} #{e}")
-			print_error("This script requires the use of a SYSTEM user context (hint: migrate into service process)")
+			# Sometimes we get this invalid handle race condition.
+			# So let's retry a couple of times before giving up.
+			# See bug #6815
+			if tries < 5 and e.to_s =~ /The handle is invalid/
+				print_status("Handle is invalid, retrying...")
+				tries += 1
+				retry
+
+			else
+				print_error("Meterpreter Exception: #{e.class} #{e}")
+				print_error("This script requires the use of a SYSTEM user context (hint: migrate into service process)")
+			end
 		#rescue ::Exception => e
 		#	print_error("Error: #{e.class} #{e} #{e.backtrace}")
 		end