linux-kernel - Re: [PATCH v2] modsign: use shred to overwrite the private key before deleting it

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Date:	Sun, 25 Jan 2015 12:42:50 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Pádraig Brady <P@...igBrady.com>,
	linux-kernel@...r.kernel.org
CC:	linux-kbuild@...r.kernel.org, Michal Marek <mmarek@...e.cz>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] modsign: use shred to overwrite the private key before
 deleting it

Now, after I ended up into flaming a lot (sorry again, but this topic 
made me angry for so long and I had to spent too much time to get rid of 
unwanted content and answering other peoples question in regard to that 
topic), I should offer something more useful.

So I've written down in some short words, how I think it could be done:

First offer a syscall named sunlink() (or whatever name) which fails if 
it can't overwrite or securely trim the contents of a file before 
deleting it.

That could be done like this:

(1) If it's a SSD or MMC without offering "Secure Trim" fail.
(2) If it's a plain FLASH or conventional harddisk where writing a block 
means that block will be overwritten or if it's a SSD or MMC with 
"Secure Trim) go on with
(3) Identify the blocks which contain the file contents (should be 
doable by using the same mechanisms used to read and write a file)
(4) Mark the file as deleted
(5) Overwrite or securely trim blocks which can be deleted completely
(6) Build new blocks for blocks which can only partly deleted because 
they contain information still used by the FS or other files
(7) Instruct the FS to us the new blocks instead of the old ones
(8) Overwrite or securely trim the old blocks which previously contained 
partly information of other stuff.

Afterwards use that new syscall in shred.

Of course, this is just a totally simplified instruction in regard to 
how complicated filesystems have become, but I think there isn't any 
black magic involved in offering the user a simple way to really delete 
files.

Regards,

Alexander Holler
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