Re: [PATCH RFC 00/12] Enroll kernel keys thru MOK

[Mimi Zohar]

On Tue, 2021-07-06 at 22:43 -0400, Eric Snowberg wrote:
> This is a follow up to the "Add additional MOK vars" [1] series I 
> previously sent.  This series incorporates the feedback given 
> both publicly on the mailing list and privately from Mimi. This 
> series just focuses on getting end-user keys into the kernel trust 
> boundary.
> 
> Currently, pre-boot keys are not trusted within the Linux boundary [2].
> Pre-boot keys include UEFI Secure Boot DB keys and MOKList keys. These
> keys are loaded into the platform keyring and can only be used for kexec.
> If an end-user wants to use their own key within the Linux trust
> boundary, they must either compile it into the kernel themselves or use
> the insert-sys-cert script. Both options present a problem. Many
> end-users do not want to compile their own kernels. With the
> insert-sys-cert option, there are missing upstream changes [3].  Also,
> with the insert-sys-cert option, the end-user must re-sign their kernel
> again with their own key, and then insert that key into the MOK db.
> Another problem with insert-sys-cert is that only a single key can be
> inserted into a compressed kernel.
> 
> Having the ability to insert a key into the Linux trust boundary opens
> up various possibilities.  The end-user can use a pre-built kernel and
> sign their own kernel modules.  It also opens up the ability for an
> end-user to more easily use digital signature based IMA-appraisal.  To
> get a key into the ima keyring, it must be signed by a key within the
> Linux trust boundary.
> 
> Downstream Linux distros try to have a single signed kernel for each
> architecture.  Each end-user may use this kernel in entirely different
> ways.  Some downstream kernels have chosen to always trust platform keys
> within the Linux trust boundary for kernel module signing.  These
> kernels have no way of using digital signature base IMA appraisal. 
> 
> This series adds a new MOK variable to shim.  This variable allows the
> end-user to decide if they want to trust keys enrolled in the MOK within
> the Linux trust boundary.  By default, nothing changes; MOK keys are
> not trusted within the Linux kernel.  They are only trusted after the 
> end-user makes the decision themselves.  The end-user would set this
> through mokutil using a new --trust-mok option [4]. This would work
> similar to how the kernel uses MOK variable to enable/disable signature
> validation as well as use/ignore the db.
> 
> When shim boots, it mirrors the new MokTML Boot Services variable to a new
> MokListTrustedRT Runtime Services variable and extends PCR14. 
> MokListTrustedRT is written without EFI_VARIABLE_NON_VOLATILE set,
> preventing an end-user from setting it after booting and doing a kexec.
> 
> When the kernel boots, if MokListTrustedRT is set and
> EFI_VARIABLE_NON_VOLATILE is not set, the MokListRT is loaded into the
> secondary trusted keyring instead of the platform keyring. Mimi has
> suggested that only CA keys or keys that can be vouched for by other
> kernel keys be loaded. All other certs will load into the platform
> keyring instead.

Loading MOK CA keys onto the "secondary" keyring would need to be an
exception.   Once CA keys are loaded onto the "secondary" keyring,  any
certificates signed by those CA keys may be loaded normally, without
needing an exception, onto the "secondary" keyring.  The kernel MAY
load those keys onto the "secondary" keyring, but really doesn't need
to be involved.

Loading ALL of the MOK db keys onto either the "secondary" or
"platform" keyrings makes the code a lot more complicated.  Is it
really necessary?

thanks,

Mimi

> 
> This is done by introducing a new .mok keyring.  This keyring is only
> used during boot.  After booting it is destroyed and not visible to the
> end-user after booting completes.  This keyring contains a new keyring
> permission that only allows CA keys to be loaded. If the permission
> fails, the key is later loaded into the platform keyring.  After keys
> are added into the .mok keyring, they are moved into the secondary
> trusted keyring.
> 
> Secure Boot keys will never be trusted.  They will always be loaded
> into the platform keyring.  If an end-user wanted to trust one, they
> would need to enroll it into the MOK.
> 
> I have included links to both the mokutil [3] and shim [4] changes I
> have made to support this new functionality.
> 
> Thank you and looking forward to hearing your reviews.
> 
> [1] https://lore.kernel.org/linux-integrity/20210517225714.498032-1-eric.snowberg@xxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/1556221605.24945.3.camel@xxxxxxxxxxxxxxxxxxxxx/
> [3] https://lore.kernel.org/patchwork/cover/902768/
> [4] https://github.com/esnowberg/mokutil/tree/0.3.0-mokvars-v2
> [5] https://github.com/esnowberg/shim/tree/mokvars-v2
> 
> Eric Snowberg (12):
>   KEYS: Add KEY_ALLOC_BYPASS_RESTRICTION option to key_move
>   KEYS: Allow unrestricted keys to be moved to the secondary keyring
>   KEYS: CA link restriction
>   integrity: add integrity_destroy_keyring
>   integrity: Introduce mok keyring
>   integrity: Trust mok keys if MokListTrustedRT found
>   integrity: add add_to_mok_keyring
>   integrity: restrict INTEGRITY_KEYRING_MOK to
>     restrict_link_by_secondary_trusted_or_ca
>   integrity: accessor function to get trust_moklist
>   integrity: add new keyring handler
>   integrity: move keys from the mok keyring into the secondary keyring
>   integrity: Suppress error message for keys added to the mok keyring
> 
>  certs/system_keyring.c                        | 43 +++++++++
>  crypto/asymmetric_keys/restrict.c             | 60 +++++++++++++
>  include/crypto/public_key.h                   |  5 ++
>  include/keys/system_keyring.h                 | 21 +++++
>  security/integrity/Makefile                   |  3 +-
>  security/integrity/digsig.c                   | 26 +++++-
>  security/integrity/integrity.h                | 21 ++++-
>  .../platform_certs/keyring_handler.c          | 17 +++-
>  .../platform_certs/keyring_handler.h          |  5 ++
>  security/integrity/platform_certs/load_uefi.c |  5 +-
>  .../integrity/platform_certs/mok_keyring.c    | 90 +++++++++++++++++++
>  security/keys/keyring.c                       | 10 ++-
>  12 files changed, 294 insertions(+), 12 deletions(-)
>  create mode 100644 security/integrity/platform_certs/mok_keyring.c
> 
> 
> base-commit: 13311e74253fe64329390df80bed3f07314ddd61