Subject: Re: Out of tree module using LSM
From: Jon Masters <jonathan@jonmasters.org>
To: Ray Lee <ray-lk@madrabbit.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, tvrtko.ursulin@sophos.com,
       Al Viro <viro@ftp.linux.org.uk>,
       Casey Schaufler <casey@schaufler-ca.com>,
       Christoph Hellwig <hch@infradead.org>, linux-kernel@vger.kernel.org,
       Valdis.Kletnieks@vt.edu
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	 <2c0942db0711291040j4ce48acagb753b64c4b8c1357@mail.gmail.com>
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Organization: World Organi[sz]ation Of Broken Dreams
Date: Thu, 29 Nov 2007 13:56:51 -0500
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On Thu, 2007-11-29 at 10:40 -0800, Ray Lee wrote:
> On Nov 29, 2007 9:36 AM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > > closed. But more importantly further access to it can be blocked until
> > > appropriate actions are taken which also applies with your example, no? Is
> >
> > That bit is hard- very hard.
> 
> In some sense it seems like the same problem faced by dynamic
> translators such as Qemu. They really want to vet a dirtied or faulted
> page before allowing the app to run unhindered. It's be nice to have
> some way to do that without virtualizing the whole of userspace.

Like I hinted at, you can't just "vet a page". Because a page alone is
meaningless garbage, unless it happens to be an extremely small program,
with headers, all nicely aligned. Most likely you don't know if a random
page of data is code from a COFF file, ELF file, or some random crap I
typed in at a terminal after having too much coffee.

So. You'd need to scan *all the pages* of *the entire file*, every time
that you performed any type of operation. Hence, that idea is completely
out right away. The on-access scanning isn't perfect, but it's probably
about as good as you can get and still have a reasonably usable system.

Jon.


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