Re: [dm-crypt] LUKS - SSD trim

[Mikko Rauhala <mjrauhal@cc.helsinki.fi>]

pe, 2010-04-23 kello 10:49 +0200, Milan Broz kirjoitti:
> On 04/23/2010 12:22 AM, Richard Zidlicky wrote:
> > isn't TRIM considered information leakage in the case of dm-crypt?

It is leakage indeed, but for many purposes acceptable at least as such.

I personally would want TRIM support on my dm-crypt-on-lvm SSD to
improve lifetime and performance; I'm not too worried about leaking
empty block information. (I would be somewhat interested though if
someone knows of particular ways that empty block data could leak other
information; known-plaintext attacks against filesystem data structures
and through them against the encryption key springs to mind, but I
recognize I'm just talking out of my ass here, grasping at worst-case
straws.)

I do think that dm-crypt, as a security solution, should probably offer
optional dropping of TRIM commands when dm gains support for it; some
uses may be more sensitive (and/or some users may wish to wear thicker
tinfoil than myself). This is probably not critical, though:

> If it is problem, you should not use FS with TRIM support in the first place.
> dm-crypt basically should support TRIM if the request comes, it is just block
> device.

This would detract from the usefulness of dm-crypt, as you would be
artificially limited in what you can run on top of it to meet your
paranoia quota. However, considering in addition that many (most/all? -
at least ext4, btrfs, gfs2) filesystems that make use of TRIM on Linux
do appear on a quick glance to make this optional through the
(no)discard mount options, it wouldn't indeed be a very bad thing for
dm-crypt to Just Do It anyway.

> dm-crypt is just transparent layer, it is configured some way and
> configuration depends on your requirements and previous analysis.

A "Drop TRIMs" flag could still be useful as just such a configuration
option that you could set accordingly after said analysis, the ability
to _often_ block TRIMs higher up on the stack notwithstanding.

-- 
Mikko Rauhala <mjr@iki.fi>       - http://www.iki.fi/mjr/blog/  
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