Re: [PATCH v3 1/3] powerpc/powernv: Add OPAL API interface to get secureboot state

On 06/12/2019 02:17 AM, Daniel Axtens wrote:

Nayna Jain [off-list ref] writes:

quoted

From: Claudio Carvalho <redacted>

The X.509 certificates trusted by the platform and other information
required to secure boot the OS kernel are wrapped in secure variables,
which are controlled by OPAL.

This patch adds support to read OPAL secure variables through
OPAL_SECVAR_GET call. It returns the metadata and data for a given secure
variable based on the unique key.

Since OPAL can support different types of backend which can vary in the
variable interpretation, a new OPAL API call named OPAL_SECVAR_BACKEND, is
added to retrieve the supported backend version. This helps the consumer
to know how to interpret the variable.

(Firstly, apologies that I haven't got around to asking about this yet!)

Are pluggable/versioned backend a good idea?

There are a few things that worry me about the idea:

  - It adds complexity in crypto (or crypto-adjacent) code, and that
    increases the likelihood that we'll accidentally add a bug with bad
    consequences.

Sorry, I think I am not clear on what exactly you mean here.Can you 
please elaborate or give specifics ?

  - Under what circumstances would would we change the kernel-visible
    behaviour of skiboot? Are we expecting to change the behaviour,
    content or names of the variables in future? Otherwise the only
    relevant change I can think of is a change to hardware platforms, and
    I'm not sure how a change in hardware would lead to change in
    behaviour in the kernel. Wouldn't Skiboot hide h/w differences?

Backends are intended to be an agreement for firmware, kernel and 
userspace on what the format of variables are, what variables should be 
expected, how they should be signed, etc. Though we don't expect it to 
happen very often, we want to anticipate possible changes in the 
firmware which may affect the kernel such as new features, support of 
new authentication mechanisms, addition of new variables. Corresponding 
skiboot patches are on - 
https://lists.ozlabs.org/pipermail/skiboot/2019-June/014641.html

  - If we are worried about a long-term-future change to how secure-boot
    works, would it be better to just add more get/set calls to opal at
    the point at which we actually implement the new system?

The intention is to avoid to re-implement the key/value interface for 
each scheme. Do you mean to deprecate the old APIs and add new APIs with 
every scheme ?

    
  - UEFI added EFI_VARIABLE_AUTHENTICATION_3 in a way that - as far
    as I know - didn't break backwards compatibility. Is there a reason
    we cannot add features that way instead? (It also dropped v1 of the
    authentication header.)
    
  - What is the correct fallback behaviour if a kernel receives a result
    that it does not expect? If a kernel expecting BackendV1 is instead
    informed that it is running on BackendV2, then the cannot access the
    secure variable at all, so it cannot load keys that are potentially
    required to successfully boot (e.g. to validate the module for
    network card or graphics!)

The backend is declaredby the firmware, and is set at compile-time. The 
kernel queriesfirmware on whichbackend is in use, and the backend will 
not change at runtime.If the backend in use by the firmware is not 
supported by the kernel (e.g. kernel is too old), the kernel does not 
attempt to read any secure variables, as it won't understand what the 
format is. This is a secure boot failure condition, as we cannot verify 
the next kernel. With addition of new backends in the skiboot, the 
support will be added to the kernel. Note: skiboot and skiroot should 
always be in sync with backend support.


Thanks & Regards,
     - Nayna