On Tue, 2017-11-14 at 15:55 -0500, Matthew Garrett wrote:

On Tue, Nov 14, 2017 at 3:50 PM, Luis R. Rodriguez <mcgrof@kernel.org

quoted

wrote:

On Tue, Nov 14, 2017 at 12:18:54PM -0800, Linus Torvalds wrote:

quoted

This is all theoretical security masturbation. The _real_ attacks
have been elsewhere.

In my research on this front I'll have to agree with this, in terms
of justification and there are only *two* arguments which I've so?
far have found to justify firmware signing:

a) If you want signed modules, you therefore should want signed
firmware.
???This however seems to be solved by using trusted boot thing,
given it
???seems trusted boot requires having firmware be signed as well.
(Docs
???would be useful to get about where in the specs this is
mandated,
???anyone?). Are there platforms that don't have trusted boot or
for which
???they don't enforce hardware checking for signed firmware for
which
???we still want to support firmware signing for? Are there
platforms
???that require and use module signing but don't and won't have a
trusted
???boot of some sort? Do we care?

TPM-backed Trusted Boot means you don't /need/ to sign anything,
since the measurements of what you loaded will end up in the TPM. But
signatures make it a lot easier, since you can just assert that only
signed material will be loaded and so you only need to measure the
kernel and the trusted keys.

Actually, I'd disagree with that quite a lot: measured boot only works
if you're attesting to something outside of your system that has the
capability for doing something about a wrong measurement. ?Absent that,
measured boot has no safety whatsoever. ?Secure boot, on the other
hand, can enforce not booting with elements that fail the signature
check.

The question, really, in any system, is how you want to prove security.
?In a standalone server system, measured boot is pretty useless because
you don't have an external entity to attest to, so signatures and
secure boot are really the bulwark against breaches. ?In a properly
attested server cluster whose attestation controller has the ability to
reboot you, perhaps signatures and secure boot don't add that much more
value.

James

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