[coreboot] SPI controller and Lock bits

[Martin Kepplinger]

Am 02.10.2018 13:51 schrieb Nico Huber:
> Am 02.10.18 um 13:01 schrieb Martin Kepplinger:
>> Am 26.09.2018 01:30 schrieb Youness Alaoui:
>>> Hi,
>>> 
>>> I'm trying to add a way to lock the SPI flash to be read-only via
>>> software *after* coreboot boots. The scenario is basically with using
>>> Heads, you could authenticate to it (with a yubikey/nitrokey/librem
>>> key) then be able to flash a new rom (update your BIOS), but once you
>>> boot an OS, Heads would first lock the flash so it can't be written
>>> to.
>>> This should add some security to avoid any malware writing to the
>>> flash, or someone booting into a USB stick and using that to flash a
>>> malicious BIOS, but still gives the user the freedom of updating 
>>> their
>>> flash whenever they want to.
>>> 
>> 
>> I might be wrong, but since Heads already authenticates to you via 
>> TOTP,
>> this wouldn't really add extra security, wouldn't it?
> 
> This is the most common misunderstanding about a measured boot. If you
> don't have separate hardware that starts the measuring (usually you let
> the firmware measure itself), you need a firmware part that starts the
> measuring and is read-only for an attacker (referred to as read-only or
> static `root of trust`). Otherwise you leave the decision what to mea-
> sure to the attacker (and he can choose to measure the original 
> software
> before his tampering instead of the running program, and TOTP will 
> still
> work flawlessly).
> 
> So it's the other way around: without this, TOTP doesn't provide any
> security at all.
> 
> Nico

thanks! My high-level understanding was that Heads uses the
TPM as said "root of trust" and the TOTP shared secret will be lost
in any changed firmware.

So a changed firmware would basically need to write PCRs to the TPM
in order for the "PCR matching" to succeed and release the valid TOTP
secret. Be it by measuring and remembering the original firmware or
somehow else.

I don't know why, but I've though that the TPM would prevent a modified
firmware to do this -.-

http://osresearch.net/Keys is quite good, but maybe we'd need a wiki
with real setups compared, or something. I fear that I'm not the only 
one
who's confused with trusted boot stuff.

As a quick fix, couldn't Heads offer to create an image with the IFD 
re-locked,
in order to force (yourself) to hardware-flashing. Has this been 
discussed?

thanks

                                        martin