[dm-crypt] (More) Questions about LUKS / LVM

Arno Wagner arno at wagner.name
Tue Sep 20 13:47:24 CEST 2011


On Tue, Sep 20, 2011 at 08:36:58PM +1000, Robbie Smith wrote:
[...]
> What are some potential worst-case scenarios? i.e. the system had a hard
> reset, either because the power got cut or (somehow) an application
> brought the system to a complete halt? How would this affect the
> encryption, and could it result in total data loss? 

The encryption can be established as long as the header and
at least on ekeyslot are intact. If you cut the power just in the
microsecond while a keyslot is written you would damage that
keyslot. If it was your only one and you do not have a header backup,
then you would have total data loss. That is the only scenario
I can think of. In normal operation, the header is not written.

With an SSD, things are a bit different. Due to the large
internal sector size, the header can be in a sector that
also has data that gets rewritten in it. As sectors are
always written completely, the header then is at risk whenever
that data gets rewritten.

Also keep in mind that HDDs have about a 5%/year rate of failure
(even if some vendors claim 0.5%, but that is under perfect
conditions). A backup of the data is non-optional. The
header-backup is just an additional precaution.

> The FAQ makes mention that the most frequent cause of data loss is
> either losing access to the keys or somehow corrupting the LUKS header.
> The former I can understand, and "common" sense would dictate to have a
> couple of backup keys in secure locations. I am at a loss though as to
> how someone could unintentionally corrupt the header though. 

This is from observations on the mailing-list. It happens pretty
often. And most people are surprised by it. 
 
> I'm inclined to set up my system with /boot and a LUKS partition, and
> then use LVM inside that, so if I decide to rearrange virtual partitions
> I won't run the risk of messing up the LUKS header. This also seems like
> the simplest setup.

Keep in mind that LVM adds to the complexity when you have to do
data recovery when something went wrong. Other that that it sounds
like a good approach.

> (I keep daily backups of $HOME and of essential system settings, the
> rest can be reinstalled if needed, but I'd prefer not to have to spend a
> few days recovering everything if I had a hard reset or something like
> that.)

You will not damage the encrypted data in normal operation.
All the header-damages reported were done during installation,
repartitioning, moving partitions, etc. 

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
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