[dm-crypt] [RFC PATCH 0/4] Allow file systems to selectively bypass dm-crypt

Arno Wagner arno at wagner.name
Fri Jun 16 16:31:36 CEST 2017


On Fri, Jun 16, 2017 at 14:55:11 CEST, Michael Kjörling wrote:
> On 15 Jun 2017 10:24 -0700, from mhalcrow at google.com (Michael Halcrow):
> >> If this is accepted, we basically allow attacker to trick system to
> >> write plaintext to media just by setting this flag. This must never
> >> ever happen with FDE - BY DESIGN.
> > 
> > That's an important point.  This expands the attack surface to include
> > the file system, so if an adversary can provide a bad encryption key
> > or policy at the file system layer or if there's a bug in the file
> > system that an adversary can exploit, then users setting the
> > allow_encrypt_override option on dmcrypt would be vulnerable.
> 
> No; it would seem to expand the attack surface to _anything_ that can
> set this flag on write. That implies that at the very least _anything_
> that runs as root can now plant _plain text_ on storage media which is
> intended to be fully encrypted. 

On the surfacte, root can do that anyways. But this would allow
to do that without a kernel compromise or direct write to disk.
And it would make detection of a related attack much harder,
hence violating KISS rather badly.


[...]
> If double encryption is too expensive, particularly in the product
> space where one device is typically controlled by a single individual
> or entity, why do file system layer encryption at all? Just offload
> that to the device layer; in this case, LUKS and dm-crypt. File system
> layer encryption makes more sense where a large number of users all
> have deep level access to a system, possibly being able to read disks
> directly, and want to keep data secure from other users of the same
> system. I fail to see how that threat model is particularly relevant
> in the mobile space.

Same here. And I do not see why there would be significant 
performance or energy impact either. AES hardware is fast 
and does not consume a lot of energy. 

Regards,
Arno

-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
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