[PATCH v7 00/17] Enroll kernel keys thru MOK

Eric Snowberg
2021-11-16T00:19:17Z

Back in 2013 Linus requested a feature to allow end-users to have the
ability "to add their own keys and sign modules they trust". This was
his *second* order outlined here [1]. There have been many attempts
over the years to solve this problem, all have been rejected. Many
of the failed attempts loaded all preboot firmware keys into the kernel,
including the Secure Boot keys. Many distributions carry one of these
rejected attempts [2], [3], [4]. This series tries to solve this problem
with a solution that takes into account all the problems brought up in
the previous attempts.

On UEFI based systems, this series introduces a new Linux kernel keyring
containing the Machine Owner Keys (MOK) called machine. It also defines
a new MOK variable in shim. This variable allows the end-user to decide
if they want to load MOK keys into the machine keyring. Mimi has suggested
that only CA keys contained within the MOK be loaded into the machine
keyring. All other certs will load into the platform keyring instead.

By default, nothing changes; MOK keys are not loaded into the machine
keyring. They are only loaded after the end-user makes the decision
themselves. The end-user would set this through mokutil using a new
--trust-mok option [5]. This would work similar to how the kernel uses
MOK variables to enable/disable signature validation as well as use/ignore
the db. Any kernel operation that uses either the builtin or secondary
trusted keys as a trust source shall also reference the new machine
keyring as a trust source.

Secure Boot keys will never be loaded into the machine keyring. They
will always be loaded into the platform keyring. If an end-user wanted
to load one, they would need to enroll it into the MOK.

Steps required by the end user:

Sign kernel module with user created key:
$ /usr/src/kernels/$(uname -r)/scripts/sign-file sha512 \
machine_signing_key.priv machine_signing_key.x509 my_module.ko

Import the key into the MOK
$ mokutil --import machine_signing_key.x509

Setup the kernel to load MOK keys into the .machine keyring
$ mokutil --trust-mok

Then reboot, the MokManager will load and ask if you want to trust the
MOK key and enroll the MOK into the MOKList. Afterwards the signed kernel
module will load.

I have included a link to the mokutil [5] changes I have made to support
this new functionality. The shim changes have now been accepted
upstream [6].

[1] https://marc.info/?l=linux-kernel&m=136185386310140&w=2
[2] https://lore.kernel.org/lkml/1479737095.2487.34.camel@linux.vnet.ibm.com/
[3] https://lore.kernel.org/lkml/1556221605.24945.3.camel@HansenPartnership.com/
[4] https://lore.kernel.org/linux-integrity/1e41f22b1f11784f1e943f32bf62034d4e054cdb.camel@HansenPartnership.com/
[5] https://github.com/esnowberg/mokutil/tree/mokvars-v3
[6] https://github.com/rhboot/shim/commit/4e513405b4f1641710115780d19dcec130c5208f

Eric Snowberg (17):
integrity: Introduce a Linux keyring called machine
integrity: Do not allow machine keyring updates following init
KEYS: Create static version of public_key_verify_signature
X.509: Parse Basic Constraints for CA
KEYS: CA link restriction
integrity: restrict INTEGRITY_KEYRING_MACHINE to restrict_link_by_ca
integrity: Fix warning about missing prototypes
integrity: add new keyring handler for mok keys
KEYS: Rename get_builtin_and_secondary_restriction
KEYS: add a reference to machine keyring
KEYS: Introduce link restriction for machine keys
KEYS: integrity: change link restriction to trust the machine keyring
KEYS: link secondary_trusted_keys to machine trusted keys
integrity: store reference to machine keyring
efi/mokvar: move up init order
integrity: Trust MOK keys if MokListTrustedRT found
integrity: Only use machine keyring when uefi_check_trust_mok_keys is
true

certs/system_keyring.c | 44 ++++++++++-
crypto/asymmetric_keys/restrict.c | 43 +++++++++++
crypto/asymmetric_keys/x509_cert_parser.c | 9 +++
drivers/firmware/efi/mokvar-table.c | 2 +-
include/crypto/public_key.h | 15 ++++
include/keys/system_keyring.h | 14 ++++
security/integrity/Kconfig | 12 +++
security/integrity/Makefile | 1 +
security/integrity/digsig.c | 23 +++++-
security/integrity/integrity.h | 17 +++-
.../platform_certs/keyring_handler.c | 18 ++++-
.../platform_certs/keyring_handler.h | 5 ++
security/integrity/platform_certs/load_uefi.c | 4 +-
.../platform_certs/machine_keyring.c | 77 +++++++++++++++++++
14 files changed, 273 insertions(+), 11 deletions(-)
create mode 100644 security/integrity/platform_certs/machine_keyring.c

base-commit: fa55b7dcdc43c1aa1ba12bca9d2dd4318c2a0dbf
--
2.18.4