On Thu, 2013-09-26 at 08:24 +0200, Jiri Kosina wrote:
> 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
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?
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
2. No external agency other than the next kernel can do the
validation since the validating key has to be secret
The importance of 2 is just tripwire like detection ... perhaps it
doesn't really matter in a personal computer situation. It would matter
in an enterprise where images are stored and re-used but until servers
have UEFI secure boot, that's not going to be an issue.
James
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