Re: [RFC PATCH 00/12] x86: Trenchboot secure late launch Linux kernel support

[Daniel P. Smith]

On 3/26/20 6:41 PM, Matthew Garrett wrote:
> On Thu, Mar 26, 2020 at 3:37 PM Daniel P. Smith
> <dpsmith@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On 3/26/20 4:54 PM, Matthew Garrett wrote:
>>> PCs depend on the availability of EFI runtime services - it's not
>>> possible to just assert that they're untrusted and so unsupported. The
>>> TPM code is part of boot services which (based on your design) are
>>> unavailable at this point, so I agree that you need your own
>>> implementation.
>>>
>>
>> I appreciate this has been a heated area of debate, but with all due
>> respect that might be a slight over statement w.r.t. dependency on
>> runtime services and not what I was saying about the trustworthiness of
>> UEFI. If I have a UEFI platform, I trust EFI to boot the system but that
>> does not mean I have to trust it to measure my OS kernel or manage the
>> running system. Secure Launch provides a means to start a measurement
>> trust chain starting with CPU taking the first measurement and then I
>> can do things like disabling runtime services in the kernel or do crazy
>> things like using the dynamic launch to switch to a minimal temporary
>> kernel that can do high trust operations such as interfacing with
>> entities outside your trust boundary, e.g. runtime services.
> 
> I understand. However, it is *necessary* for EFI runtime services to
> be available somehow, and this design needs to make that possible.
> Either EFI runtime services need to be considered part of the TCB, or
> we need a mechanism to re-verify the state of the system after making
> an EFI call (such as Andy's suggestion).
> 

Yes if you are on UEFI you will eventually need to deal with RS during
the system's lifetime, unless you don't want to patch your firmware
which I won't comment on what kind of idea that is. And yes I have been
chatting with a few people in the LinuxBoot community about re-verifying
the RS. The answer seemed to be that it might be possible but it doesn't
look like it will be trivial.

>> Please understand I really do not want my own implementation. I tried to
>> see if we could just #include in the minimal needed parts from the
>> in-tree TPM driver but could not find a clean way to do so. Perhaps
>> there might be a future opportunity to collaborate with the TPM driver
>> maintainers to refactor in a way that we can just reuse instead of
>> reimplement.
> 
> I think it's reasonable to assert that boot services can't be part of
> the TCB in this case, and as a result you're justified in not using
> the firmware's TPM implementation. However, we still need a solution
> for access to runtime services.
>