On Thu, Sep 26, 2013 at 04:48:00PM +0200, Jiri Kosina wrote:
> > 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.
Ideally, the key should be regenerated on each true reboot and kept the
same if it is just a resume. Unfortunately, I don't see a way to
distinguish those before we call ExitBootServices().
The reasoning behind that is that in the case of a kernel compromise, a
suspended-and-resumed kernel will still be compromised, so there is no
value in passing it a new key. A freshly booted kernel, though, should
get a new key, exactly because the attacker could have obtained a key
from the previous, compromised one.
This speeds up the ususal suspend-and-resume cycle, but provides full
security once the user performs a full reboot.
The question that remains is how to tell in advance.
> > 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.
Indeed, it's hard to imagine a scenario that is also valid within the
secure boot threat model.
--
Vojtech Pavlik
Director SUSE Labs
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