Re: [PATCH 3/7] SLIM main patch

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

Ar Mer, 2006-08-23 am 13:35 -0700, ysgrifennodd Kylene Jo Hall:
> Example: The current process is running at the USER level and writing to
> a USER file in /home/user/.  The process then attempts to read an
> UNTRUSTED file.  The current process will become UNTRUSTED and the read
> allowed to proceed but first write access to all USER files is revoked
> including the ones it has open.

Which really doesn't mean anything in many cases because there are many
ways to get data out of a file handle once you had it opened for write
including sharing via non file handle paths.

You also have to deal with existing mmap() mappings and outstanding I/O.

So here are some ways to break it

	SysV shared memory
	mmap

or just race it:

	Open the USER file
	create a new thread
	thread #1 create a pipe to a new process ("receiver")
	thread #1 fill pipe
	thread #1 issue write of buffer that will hold secret data
			[blocks after check for rights]
	
	thread #2
		wait for thread #1 to block
		read secret data into buffer
		send signal to "receiver"


	receiver now empties the pipe, the write completes and I get the
goodies.

This is why you need a proper implementation of revoke(2) in Linux. You
can't really do it any more easily.