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SubjectLinux DRTM on UEFI platforms
We're still trying to come to a conclusion about the most maintainable 
approach to getting DRTM implementations like Intel TXT working on UEFI
platforms under Linux. I'm going to try to summarise the situation here
- I'm not an expert, so details may be inaccurate, but I think this is
the rough shape of things.

Under normal circumstances on UEFI platforms, the bootloader (or system
firmware directly) load the kernel and execute code in the boot stub.
This boot stub code interacts with the firmware in various ways,
including working around some platform-specific issues and doing things
like copying the TPM event log into memory that can be used by the
kernel proper. While it remains technically possible to boot the kernel
on x86 UEFI platforms by jumping directly to the kernel entry point and
skipping the boot stub, this risks disabling certain functionality and
leaving the kernel in a slightly unexpected state. The contract that
exists between the UEFI stub and the kernel is not meaningfully
documented - portions are carried out by updating the contents of the
bootparams struct on x86 or devicetree on ARM, but other information is
conveyed via UEFI configuration tables, the boot stub may modify the
contents of certain UEFI variables, and some of this is implicit in
the form of hardware state.

The Trenchboot project is attempting to implement DRTM under Linux.
Traditional measured boot takes the form of a static root of trust -
each component in the boot chain is measured by the previous component
before being executed, and each measures what it considers to be
security relevant configuration and behavioural state. Modifying any
component will change the measurements, even if the end result is
equivalent. Dynamic Root of Trust Measurement aims to provide more
useful measurements by performing a measurement at a specific point in
time - as a result, DRTM cares only about what the state *is*, not what
happened before then.

There's a bunch of complexity involved in this that's vendor specific,
but the short version is that a launch environment is set up and a
specific CPU instruction executed that triggers a secure launch event.
The Trenchboot code consists of patches to bootloaders to support
setting up this environment, and patches to the kernel to provide
support for the "Secure Launch" kernel that handles DRTM before handing
control off to the kernel proper, skipping the UEFI boot stub.

Since this skips the UEFI boot stub, it's then the responsibility of the
bootloader to implement the functionality of the boot stub. This is made
difficult due to the absence of an express contract between the boot
stub and the rest of the kernel - our assumption has been that the code
is the contract, and this is free to change arbitrarily since both the
producer and the consumer of the information can be updated
simultaneously and can't get out of sync. That seems to leave us with
two options:

1) Formalise the contract between the boot stub and the kernel, making
it easier for other boot loaders to implement the contract. This has the
downside that even within the scope of the contract we may end up with
divergent behaviour, and also new functionality in the kernel may not be
available without the bootloader also being updated.

2) Modify the boot stub such that it can be provided with a callback,
and instead of jumping to the kernel it can jump to the callback. The
boot flow would then involve the boot loader setting up the launch
environment, calling the boot stub code, and then performing the DRTM
event. This avoids requiring an explicit contract (the boot stub would
remain part of the kernel image and would be updated in lockstep).

One complexity for both approaches is that the secure launch environment
needs to be aware of all security-critical state in the system. The
firmware nominally exposes that information for everything it's aware
of that's security-critical, but the boot stub may have performed
security-critical actions or exposed security-critical state that the
firmware is unaware of. The secure launch code from Trenchboot has to be
aware of this in order to ensure it's measured.

If there's a contract, then in theory this would be embodied in the
contract and when the contract is updated the secure launch code could
also be updated. If there's no contract, every change to the boot stub
would need to be examined and the secure launch code updated if
necessary.

While the secure launch code is currently out of tree, the goal is for
it to be integrated into the kernel tree. My feeling is that it's
going to be easier to manage this if everything is in tree - ie, all the
EFI setup code that does anything security-critical is either in the
firmware (and so the firmware takes responsibility for exposing it) or
is in the Linux tree (and so we take responsibility for updating the
secure launch code whenever something relevant changes in the boot
stub). But this requires support from the EFI maintainers in terms of
not merging things until we've established whether updates are required
to the secure launch code. The alternative is effectively the same,
except it would require updating the contract instead.

But, as mentioned, that's just my feeling - I know that Daniel feels
that the contract approach is preferable. So:

1) From an EFI maintainer perspective, is making the contract between
the boot stub and the kernel explicit viable?

2) If so, is it desirable?

3) If either (1) or (2) is "no", is it reasonable to ensure that all
potentially security-critical state or configuration changes to the
boot stub are reviewed by DRTM people to verify whether the secure
launch code needs to be updated to match?

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