Re: [PATCH 0/6][RFC] Add EFI secure key to key retention service

[joeyli]

Hi James,

On Sun, Aug 05, 2018 at 10:47:26AM -0700, James Bottomley wrote:
> On Sun, 2018-08-05 at 09:25 +0200, Ard Biesheuvel wrote:
> > Hello Chun,yi,
> > 
> > On 5 August 2018 at 05:21, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx>
> > wrote:
> > > When secure boot is enabled, only signed EFI binary can access
> > > EFI boot service variable before ExitBootService. Which means that
> > > the EFI boot service variable is secure.
> > > 
> > 
> > No it, isn't, and this is a very dangerous assumption to make.
> > 
> > 'Secure' means different things to different people. 'Secure boot' is
> > a misnomer, since it is too vague: it should be called 'authenticated
> > boot', and the catch is that authentication using public-key crypto
> > does not involve secrets at all.
> 
> Hang on, let's not throw the baby out with the bathwater here.
> 
> The design of "secure boot" is to create a boot time environment where
> only trusted code may execute.  We rely on this trust guarantee when we
> pivot from the EFI to the MoK root of trust in shim.
> 
> The reason we in Linux trust this guarantee is that it pertains to the
> boot environment only, so any violation would allow Windows boot to be
> compromised as well and we trust Microsoft's Business interests in
> securing windows far enough to think this would be dealt with very
> severely and it's an outcome the ODMs (who also add secure boot keys)
> are worried enough about to be very careful.
> 
> The rub (and this is where I'm agreeing with Ard) is that any use case
> we come up with where a violation wouldn't cause a problem in windows
> is a use case where we cannot rely on the guarantee because Microsoft
> no longer has a strong business interest in policing it.  This, for
> instance, is why we don't populate the Linux trusted keyrings with the
> secure boot keys (we may trust them in the boot environment where
> compromise would be shared with windows but we can't trust them in the
> Linux OS environment where it wouldn't).  So this means we have to be
> very careful coming up with uses for secure boot that aren't strictly
> rooted in the guarantee as enforced by the business interests of
> Microsoft and the ODMs.
>

Thank you for providing the view point from Microsoft bussiness ineterests.
I agreed with you. Honestly I didn't think this point before.
 
> >  The UEFI variable store was not designed with confidentiality in
> > mind, and assuming [given the reputation of EFI on the implementation
> > side] that you can use it to keep secrets is rather unwise imho.
> 
> Agree completely here: Microsoft doesn't use UEFI variables for
> confidentiality, so we shouldn't either.  If you want confidentiality,
> use a TPM (like Microsoft does for the bitlocker key).
>

OK~~ Then I will use TPM trusted key + encrypted key in hibernation
encryption/authentication.

Thanks for James and Ard's comments.

Joey Lee