linux-kernel - Re: [dm-devel] [PATCH v2 0/2] Introduce the bulk IV mode for improving the crypto engine efficiency

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Message-ID: <alpine.LRH.2.02.1601122113240.24452@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Tue, 12 Jan 2016 21:18:12 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
cc:	device-mapper development <dm-devel@...hat.com>,
	Mark Brown <broonie@...nel.org>,
	Milan Broz <gmazyland@...il.com>, Jens Axboe <axboe@...nel.dk>,
	keith.busch@...el.com, linux-raid@...r.kernel.org,
	martin.petersen@...cle.com, Mike Snitzer <snitzer@...hat.com>,
	Baolin Wang <baolin.wang@...aro.org>,
	linux-block@...r.kernel.org, neilb@...e.com,
	LKML <linux-kernel@...r.kernel.org>, sagig@...lanox.com,
	tj@...nel.org, dan.j.williams@...el.com,
	Kent Overstreet <kent.overstreet@...il.com>,
	Alasdair G Kergon <agk@...hat.com>
Subject: Re: [dm-devel] [PATCH v2 0/2] Introduce the bulk IV mode for improving
 the crypto engine efficiency



On Wed, 13 Jan 2016, Arnd Bergmann wrote:

> On Tuesday 12 January 2016 18:31:19 Mikulas Patocka wrote:
> > 
> > Another possibility is to use dm-crypt block size 4k and use a filesystem 
> > with 4k blocksize on it (it will never send requests not aligned on 4k 
> > boundary, so we could reject such requests with an error).
> 
> Is there ever a reason to use something other than 4K block size on
> dm-crypt?
> 
> 	Arnd

You can't use 4k block on CBC (and most other encryption modes). If only a 
part of 4k block is written (and then system crash happens), CBC would 
corrupt the block completely.

For example, suppose that EXT2 directory block is updated, the first 
512-byte sector is written and the rest of the sectors is not written 
because of a crash. CBC would corrupt all sectors except the first one in 
this case.

You could use 4k block on XTS and ECB.

Mikulas