[dm-crypt] Hybrid drives

Arno Wagner arno at wagner.name
Sun Jul 18 19:01:50 CEST 2010

On Sat, Jul 17, 2010 at 09:13:13AM +0200, Heinz Diehl wrote:
> Hi,
> I just took a closer look at one of the (new?) hybrid harddisk drives,
> such as the Seagate Momentus XT, and now I'm wondering if this 
> is a top level security problem. If I understand it correctly, the 4 GB
> SSD memory on the harddisk operates independendly of the drive itself,
> acting as a buffer/cache for virtually all the data stored to the drive. Some
> logic in the harddrives controller is going to serve the 4 GB SSD memory
> part, which is not available/accessable to the OS.
> In other words: such hybrid drives can not safely be encrypted with
> LUKS/dmcrypt (or any other WDE software), because the controller 
> randomly swaps out 4 GB data to the SSD area, and even after shutting 
> down the machine, parts of/the whole LUKS header/key could possibly 
> be left on the SSD part?

There is only one risk, namely a keyslot with an old, later changed
key still being on the SSD part. That is a real risk.
However, it is only a risk if you change a key and the old one is
a security problem before the old keyslot in the SSD cache
is at least partially overwritten.

Keys never get written to disk, whether hybrid or not. Doing
that would be a massive risk in the first place, whether
hybrid or not so it os never done.

For plain dm-crypt there is absolutely no risk at all.

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
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