You might have to use hGetContents, then waitForProcess, and then terminateProcess -- you can then check if the process is indeed terminated using getProcessExitCode.
Er? When waitForProcess returns the process is dead; there's nothing to terminateProcess. If you do it the other way around then you may not get full output.
For starters, I would not build up the CreateProcess record directly but use the "proc" record and override it with your I/O definitions. I've been bit by this before (I think some older version of System.Process lacked useful predefined records?).
There is a severe gotcha here if you do things naively (like your example code does). If there is more output than will fit in a pipe, *no* ordering of read / wait in a single thread will work without deadlocking; you have no choice but to do the read in a separate thread. This is not a Haskell issue, it is how pipes work on POSIX systems. (I don't know if Windows has the same problem.)