Re: [RFC PATCH 0/3] Add additional MOK vars

[Eric Snowberg]

> On May 21, 2021, at 5:44 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> On Thu, 2021-05-20 at 14:37 -0600, Eric Snowberg wrote:
>>> On May 20, 2021, at 6:22 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
>>> I really do understand the need for extending the root of trust beyond
>>> the builtin keys and allowing end user keys to be loaded onto a kernel
>>> keyring, but it needs to be done safely.  The first step might include
>>> locally signing the MOK keys being loaded onto the secondary keyring
>>> and then somehow safely providing the local-CA key id to the kernel.
>> 
>> If the machine owner and Linux distributor are independent of one another,
>> I don’t see how MOK key signing could work.  There wouldn’t be a way for
>> the kernel to verify the end-user supplied signed MOK key.  An end-user 
>> choosing a Linux distro is trusting the company/organization building the 
>> kernel, but the trust doesn’t go the other way.  Do you have a solution 
>> in mind on how this would be possible? If you do, I’m happy to move in
>> a different direction to solve this problem.
> 
> We are working with the distros to address this problem.  The first
> attempt at extending the secondary keyring's root of trust relied on a
> TPM2 NV Index[1].

Yes, I saw that patch.  I view (which could be a mistake on my part) 
digital signature based IMA appraisal as an extension of a verified boot. 
Once a TPM is introduced, it is an extension of a measured boot. It seems 
like this patch is using measured boot to solve a problem that exists on 
the verified boot side. While it may be a good solution for someone using 
both measured boot and verified boot at the same time, not all machines or 
VMs contain a TPM. 

> Using MOK is a possible alternative, if it can be done safely.

I do want to point out, in case this was missed, when the new MOK variable 
is set to trust the platform keyring, PCR14 gets extended [1]. The UEFI BS 
var MokTPKState is mirrored to a freshly created UEFI RT var called 
MokTrustPlatform.   The contents are extended into PCR14. This happens every 
time the system boots. The UEFI RT var does not persist across reboots, it 
is alway recreated by shim.  The same thing happens with keys in the MOKList.
Since the contents are mirrored, a key change can be detected on each boot. 
This makes it possible to use attestation to see if the system was booted 
with the proper variables set. For someone using measured boot, would this 
satisfy the requirement of safely protecting the system from a MOK update?

> For example, if the boot command line could be protected from modification,
> the end-user could enroll a key in MOK and identify the specific MOK
> key on the boot command line[2].  The boot command line would then
> become an additional root of trust source.
> 
> The root of trust for loading keys on the different trusted keyrings
> are self documenting -  restrict_link_by_builtin_trusted,
> restrict_link_by_builtin_and_secondary_trusted().  A new function would
> need to be defined to include the boot command line as a new or
> additional root of trust source.


[1] https://github.com/esnowberg/shim/commit/ee3961b503e7d39cae7412ddf4e8d6709d998e87#diff-b1dd148baf92edaddd15cc8cd768201114ed86d991502bc492a827c66bbffb69R259