Subject: Re: [RFC PATCH 0/2] powerpc/pseries: add support for local secure storage called Platform Keystore(PKS)

From: Daniel Axtens <[email-masked]>
Date: Mon, 24 Jan 2022 11:25:17 +1100

Hi Greg,

> Ok, this is like the 3rd or 4th different platform-specific proposal for
> this type of functionality.  I think we need to give up on
> platform-specific user/kernel apis on this (random sysfs/securityfs
> files scattered around the tree), and come up with a standard place for
> all of this.

I agree that we do have a number of platforms exposing superficially
similar functionality. Indeed, back in 2019 I had a crack at a unified
approach: [1] [2].

Looking back at it now, I am not sure it ever would have worked because
the semantics of the underlying firmware stores are quite
different. Here are the ones I know about:

 - OpenPower/PowerNV Secure Variables:
 
   * Firmware semantics:
     - flat variable space
     - variables are fixed in firmware, can neither be created nor
        destroyed
     - variable names are ASCII
     - no concept of policy/attributes
     
   * Current kernel interface semantics:
     - names are case sensitive
     - directory per variable     

 - (U)EFI variables:
 
   * Firmware semantics:
     - flat variable space
     - variables can be created/destroyed but the semantics are fiddly
        [3]
     - variable names are UTF-16 + UUID
     - variables have 32-bit attributes

   * efivarfs interface semantics:
     - file per variable
     - attributes are the first 4 bytes of the file
     - names are partially case-insensitive (UUID part) and partially
        case-sensitive ('name' part)

   * sysfs interface semantics (as used by CONFIG_GOOGLE_SMI)
     - directory per variable
     - attributes are a separate sysfs file
     - to create a variable you write a serialised structure to
        `/sys/firmware/efi/vars/new_var`, to delete a var you write
        to `.../del_var`
     - names are case-sensitive including the UUID

 - PowerVM Partition Key Store Variables:
 
   * Firmware semantics:
     - _not_ a flat space, there are 3 domains ("consumers"): firmware,
        bootloader and OS (not yet supported by the patch set)
     - variables can be created and destroyed but the semantics are
        fiddly and fiddly in different ways to UEFI [4]
     - variable names are arbitrary byte strings: the hypervisor permits
        names to contain nul and /.
     - variables have 32-bit attributes ("policy") that don't align with
        UEFI attributes

   * No stable kernel interface yet

Even if we could come up with some stable kernel interface features
(e.g. decide if we want file per variable vs directory per variable), I
don't know how easy it would be to deal with the underlying semantic
differences - I think userspace would still need substantial
per-platform knowledge.

Or have I misunderstood what you're asking for? (If you want them all to
live under /sys/firmware, these ones all already do...)

Kind regards,
Daniel

[1]: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-May/190735.html
[2]: discussion continues at https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-June/191365.html
[3]: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-June/191496.html

[4]: An unsigned variable cannot be updated, it can only be deleted
     (unless it was created with the immutable policy) and then
     re-created. A signed variable, on the other hand, can be updated
     and the only way to delete it is to submit a validly signed empty
     update.