Re: [RFC V4 PATCH 00/15] Signature verification of hibernate snapshot

On Thu, 26 Sep 2013, James Bottomley wrote:

quoted

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

OK, so the scheme is to keep a symmetric key in BS that is passed into
the kernel each time (effectively a secret key) for signing and
validation?

Exactly.

The only two problems I see are

     1. The key isn't generational (any compromise obtains it).  This
        can be fixed by using a set of keys generated on each boot and
        passing in both K_{N-1} and K_N

I think this could be easily made optional, leaving the user with choice 
of faster or "safer" boot.

     2. No external agency other than the next kernel can do the
        validation since the validating key has to be secret

This is true, but as you said, the relevance of this seems to be rather 
questionable.

-- 
Jiri Kosina
SUSE Labs