From: Jiri Kosina <jiri.kosina-IBi9RG/b67k@public.gmane.org>
Subject: Re: [RFC V4 PATCH 00/15] Signature verification of hibernate
 snapshot
Date: Thu, 26 Sep 2013 08:24:41 +0200 (CEST)
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To: James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>
In-Reply-To: <1380162771.18835.47.camel-sFMDBYUN5F8GjUHQrlYNx2Wm91YjaHnnhRte9Li2A+AAvxtiuMwx3w@public.gmane.org>

On Wed, 25 Sep 2013, James Bottomley wrote:

> > I don't get this. Why is it important that current kernel can't
> > recreate the signature?
> 
> The thread model is an attack on the saved information (i.e. the suspend
> image) between it being saved by the old kernel and used by the new one.
> The important point isn't that the new kernel doesn't have access to
> K_{N-1} it's that no-one does: the key is destroyed as soon as the old
> kernel terminates however the verification public part P_{N-1} survives.

James,

could you please describe the exact scenario you think that the symmetric 
keys aproach doesn't protect against, while the assymetric key aproach 
does?

The crucial points, which I believe make the symmetric key aproach work 
(and I feel quite embarassed by the fact that I haven't realized this 
initially when coming up with the assymetric keys aproach) are:

- the kernel that is performing the actual resumption is trusted in the 
  secure boot model, i.e. you trust it to perform proper verification

- potentially malicious userspace (which is what we are protecting against 
  -- malicious root creating fake hibernation image and issuing reboot) 
  doesn't have access to the symmetric key

-- 
Jiri Kosina
SUSE Labs