Re: [gentoo-dev] UEFI secure boot and Gentoo

[Rich Freeman <rich0@gentoo.org>]

On Fri, Jun 15, 2012 at 7:32 AM, Walter Dnes <waltdnes@waltdnes.org> wrote:
>  Question... how would "blacklisting" work on linux machines?  Let's
> say Joe Blow gets a signing key and then passes it around.  I can see
> that if you want to build an executable (*.exe) to run under Windows,
> you'll run into problems if the monthly MS Windows Update kills that
> specific key.

I took the time to read the MS Hardware Certification guide.  I
haven't read the full UEFI spec though it is referenced to it.  It
sounds like UEFI has a provision for revocation, and that includes an
area of flash to store revoked keys.  So, if you booted the device on
Windows, then Windows could download a list of keys MS doesn't like,
and then since Windows is trusted by the firmware it could add those
keys to the flash.  Then on a reboot the firmware would no longer boot
those keys in secure mode.

So, the revocation is non-volatile, and doesn't require a firmware
update.  Of course, if you never run Windows on the device then it
probably won't get the update.  It wasn't 100% clear, but it sounds
like a full factory reset of the firmware might clear these revoked
keys out (it definitely is supposed to clear out any custom keys you
load).

After reading up it seems to me that the best bet for somebody who
wants free as in freedom is to just run in custom mode and load their
own keys.

The MS document leaves a lot of policy questions unanswered though.
The vendor has to trust the MS key, and has to secure their root keys.
 However, they can trust any number of keys, and nothing is said about
those keys having to be secure.  It seems like that is a loophole that
would be rapidly closed in practice if a vendor got "out of line."

For ARM users are up the creek unless they can get the vendor to
include their keys, or get a signature from somebody whose keys are
included.  ARM lacks the ability to use custom mode, which means you
can't load your own keys, and it can't disable secure boot.

Then again, all of this is only as good as the implementation.  My
current Android phone used just about all the tricks in the spec
(flash is locked by bootloader, no downgrading, and so on).  However,
in the case of my phone messing with the power supply can reset the
flash controller before it resets the CPU, unlocking it and allowing a
rooted device to flash the bootloader.  However, the UEFI specs
themselves seem secure, and you can't count on every piece of hardware
having an exploit.

I think that anybody that really cares about security should be
running in custom mode anyway, and should just re-sign anything they
want to run.  Custom mode lets you clear every single key in the
system from the vendor on down, and gives you the ability to ensure
the system only boots stuff you want it to.  The MS spec does require
a full factory reset to restore the original keys, though if you're
using secure boot and TPM you could ensure that your hard drive
becomes unreadable if this is done (unless the TPM has some backdoor
and your vendor is complicit in accessing it).  I don't have a problem
with secure boot, and obviously to use it any pre-loaded OS has to
have pre-loaded keys.  What I don't like is the way certain companies
are getting privileged in the process.  If a full factory reset
unlocked the machine, letting the first CD you boot from restore that
OS vendor's keys or whatever, then then that would be more neutral.
The whole bit about not allowing users to load their keys on ARM is
obviously objectionable - I'm fine with ensuring security by making
the user go into the pre-boot firmware, but the computer owner should
have the final say.

Rich