linux-ext4 - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Message-ID: <20101119161600.GH10039@thunk.org>
Date:	Fri, 19 Nov 2010 11:16:00 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Mark Lord <kernel@...savvy.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate
 super_operation

On Fri, Nov 19, 2010 at 10:50:03AM -0500, Christoph Hellwig wrote:
> 
> It's a huge issue for virtualization, where naive TRIM implementations
> can expose data deleted in one VM to others.  It's also a huge issues
> for RAIDs as mentioned by you.

Fair enough, I'll buy that.  If you are sharing an SSD using
virtualization across two VM's with different trust bounaries,
non-deterministic TRIM could very well be an issue, depending on how
it the "non-deterministic" bit was implemented.

(I can think of one PCIe-attached flash implementation I know of where
the trim is simply not persistent across a power failure, such that if
Alice trims a block, it will return 0, but if no one rewrites the
block before a power failure, then reading that some block will return
Alice's original data, and not data belonging to Bob.  This would be
an example of a non-deterministic TRIM that would be problematic for
RAID, but not from a security perspective.  OTOH, the trim command is
blazingly fast on this implementation, since there is absolutely no
flash write or erase operation associated with the TRIM, and so mount
-o discard makes a lot of sense for this particular storage device.)

						- Ted
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