linux-kernel - Re: [PATCH 3/7] SLIM main patch

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Date:	Thu, 24 Aug 2006 12:26:55 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Kylene Jo Hall <kjhall@...ibm.com>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	LSM ML <linux-security-module@...r.kernel.org>,
	Dave Safford <safford@...ibm.com>,
	Mimi Zohar <zohar@...ibm.com>, Serge Hallyn <sergeh@...ibm.com>
Subject: Re: [PATCH 3/7] SLIM main patch

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.


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