linux-ext4 - Re: [PATCH] ext4: Add support for SFITRIM, an ioctl for secure FITRIM.

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Date:	Fri, 13 Jun 2014 10:31:57 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Dave Chinner <david@...morbit.com>
Cc:	JP Abgrall <jpa@...gle.com>, Eric Sandeen <sandeen@...hat.com>,
	linux-ext4@...r.kernel.org, Geremy Condra <gcondra@...gle.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ext4: Add support for SFITRIM, an ioctl for secure
 FITRIM.

On Fri, Jun 13, 2014 at 10:20:54AM -0400, Theodore Ts'o wrote:
> 
> If you really want this to work, and be 100% secure, you really need
> to do the secure discard at the file system layer.  The file system
> could make sure that every single block gets a secure discard before
> it gets reused.

BTW, one major downside of doing a secure trim after every time that a
block has been released is that it will massively increase the flash
wear, since if you do a secure trim on a single 4k block in 512k erase
block, assuming that secure trim has been implemented properly from a
security perspective, it will need to copy out all of the used portion
of the 512k erase block, and then erase it.

This is one of the reasons why I asked if you really need to worry
about securely discarding all of the blocks on the file system, or
just blocks containing specific really security-sensitive information
(i.e., for Google Wallet, etc.)

If so, you might be better off either doing per-file encryption, or
per-file secure discard.

Cheers,

					- Ted
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