[PATCH RFC 0/1] module: Optionally use .platform keyring for signatures verification

[Vitaly Kuznetsov <vkuznets@redhat.com>]

UEFI SecureBoot 'db' keys are currently not trusted for modules signatures
verification. RedHat based downstream distros (RHEL, Fedora, ...) carry a
patch changing that for many years (since 2019 at least). This RFC is an
attempt to upstream it as the functionality seems to be generally useful.

Previously, pre-boot keys (SecureBoot 'db', MOK) were not trusted within
kernel at all. Things have changed since '.machine' keyring got introduced
making MOK keys optionally trusted. Before that, there was a discussion to
make .platform trusted by default:
https://lore.kernel.org/lkml/1556116431-7129-1-git-send-email-robeholmes@gmail.com/
which didn't go very far because the assumption was that this is only useful
when the user has control over 'db'. I believe there's a fairly common
use-case where this is true.

The use-case: virtualized and cloud infrastructure generally provide an
ability to customize SecureBoot variables, in particular, it is possible
to bring your own SecureBoot 'db'. This may come handy when a user wants to
load a third party kernel module (self built or provided by a third party
vendor) while still using a distro provided kernel. Generally, distro
provided kernels sign modules with an ephemeral key and discard the private
part during the build. While MOK can sometimes be used to sign something
out-of-tree, it is a tedious process requiring either a manual intervention
with shim or a 'certmule' 
(see https://blogs.oracle.com/linux/post/the-machine-keyring). In contrast,
the beauty of using SecureBoot 'db' in this scenario is that for public
clouds and virtualized infrastructure it is normally a property of the OS
image (or the whole infrastructure/host) and not an individual instance;
this means that all instances created from the same template will have 'db'
keys in '.platform' by default.

The suggested approach is not to change the default, but to introduce a
Kconfig variable (CONFIG_MODULE_SIG_PLATFORM) doing the job. Note, the 
kernel already trusts '.platform' for kexec (see commit 278311e417be 
("kexec, KEYS: Make use of platform keyring for signature verify"))
and dm-verity (see commit 6fce1f40e951 ("dm verity: add support for
signature verification with platform keyring")) so maybe changing the
default or introducing a generic '.plarform is fully trusted' option
would actually be better.

Vitaly Kuznetsov (1):
  module: Make use of platform keyring for module signature verify

 Documentation/admin-guide/module-signing.rst |  6 ++++++
 kernel/module/Kconfig                        | 11 +++++++++++
 kernel/module/signing.c                      |  9 ++++++++-
 security/integrity/Kconfig                   |  2 +-
 4 files changed, 26 insertions(+), 2 deletions(-)

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2.49.0