On 06/12/2019 02:17 AM, Daniel Axtens wrote:
Nayna Jain <nayna@linux.ibm.com> writes:


From: Claudio Carvalho <cclaudio@linux.ibm.com>

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 declared by the firmware, and is set at compile-time. The kernel queries firmware on which backend 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