[clug] Scrubbing non-allocated blocks on ext2

Kim Holburn kim.holburn at anu.edu.au
Wed Jul 20 10:15:25 GMT 2005

There's a paper by the guy http://www.cl.cam.ac.uk/~mgk25/ who  
invented the crt optical tempest <http://www.cl.cam.ac.uk/~mgk25/ 
on stegfs <http://www.cl.cam.ac.uk/~mgk25/ih99-stegfs.pdf> an  
encrypted file system with "plausible deniability" which stores files  
in free space and writes random data in general to free space.

On 2005 Jul 20 at 4:24 PM, Paul Wayper wrote:
> Hi all,
> So far my fairly extensive researches have shown that
> there's no program that will go through an ext2 filesystem
> and basically remove evidence of deleted files.  AFAICS,
> this would require three stages:
> 1) Go through the directory structure and reorganise
> directories.
> 2) Construct a list of all blocks, and partial blocks, not
> in use.
> 3) Scrub those blocks.
> Step 1 is important because, from what I read, when you
> delete a file its space is reallocated to the previous file
> as room to grow into.  Or maybe not - I haven't absorbed the
> entire "Design and Implementation of the Second Extended
> File System" yet.  Either way, standard file system design
> means that you don't immediately try to reallocate those
> blocks to the next file to be written, you try and localise
> reference and/or choose a chunk of free space that suits
> your new file.  Deleted files still remain in the directory
> but are marked.
> Yes, I know about the 's' attribute for file systems, that's
> supposed to force the kernel to overwrite the space with
> random data, or zeroes, or smiley faces.  It doesn't work;
> it hasn't since about kernel 2.2.something, when Linus found
> that it caused a bit of a slowdown and didn't work in all
> cases.  No-one shows any sign of trying to reimplement it.
> There used to be a defragmenter for ext2, but it's out of
> date as it assumes that blocks are 1K in size.  Yes, ext2
> does actually suffer from fragmentation, but magic pixies
> somewhere in the OS make it less of an issue.  I'll
> understand how those magic pixies do their work when I've
> finished reading the above document.  A defragmenter would
> do a third of what I want; it would be a good starting
> point.
> I know about wipe, shred and similar programs that will
> erase a file and overwrite it.  If I'd thought about using
> them, then I would have.  It's too late now.  I know about
> The Defiler's Toolkit, and it seems to sort of work except
> for the fact that even the most elementary probing with
> debugfs shows the files that necrofile is supposed to have
> removed all trace of, and klismafile doesn't seem to work in
> the way the example shows.  Maybe it's the fact that the
> disk I'm testing it on is in a USB case.  It certainly seems
> to be synced, anyway.
> So I thought I'd call upon the wisdom of the CLUG.  Is there
> something that will defragment a disk?  Is it just going to
> be down to doing a dump backup, wiping the disk, and then
> restoring it?  Is this, indeed, enough?  Is there a better
> way?
> I'm assuming that actually trying to learn how write a
> program to do this will result in extreme concussion from
> hitting my head against the brick wall too much.
> Have fun,
> Paul
> -- 
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> linux at lists.samba.org
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Kim Holburn
Network and Security Manager, National ICT Australia Ltd.
Ph: +61 2 61258620 M: +61 417820641  F: +61 2 6230 6121 aim://kimholburn
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