On Thu, 2021-08-19 at 09:10 -0400, Mimi Zohar wrote:

On Thu, 2021-08-19 at 14:38 +0300, Jarkko Sakkinen wrote:

quoted

On Wed, 2021-08-18 at 20:20 -0400, Eric Snowberg wrote:

quoted

Many UEFI Linux distributions boot using shim.  The UEFI shim provides
what is called Machine Owner Keys (MOK).  Shim uses both the UEFI Secure
Boot DB and MOK keys to validate the next step in the boot chain.  The
MOK facility can be used to import user generated keys.  These keys can
be used to sign an end-user development kernel build.  When Linux boots,
pre-boot keys (both UEFI Secure Boot DB and MOK keys) get loaded in the
Linux .platform keyring.  

Currently, pre-boot keys are not trusted within the Linux trust boundary
[1]. These platform keys 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 [2].  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.

As of today, I can use a prebuilt kernel, crate my own MOK key and sign
modules. What will be different?

The UEFI db and MOK keys are being loaded onto the .platform keyring,
which is suppose to be limited to verifying the kexec kernel image
signature.  With a downstream patch, kernel modules are being verified
as well.

Initially Patrick Uiterwijk's "[PATCH 0/3] Load keys from TPM2 NV Index
on IMA keyring" patch set attempted to define a new root of trust based
on a key stored in the TPM.  This patch set is similarly attempting to
define a new root of trust based on CA keys stored in the MOK db.

The purpose of this patch set is to define a new, safe trust source
parallel to the builtin keyring, without relying on a downstream patch.
With the new root of trust, the end user could sign his own kernel
modules, sign third party keys, and load keys onto the IMA keyring,
which can be used for signing the IMA policy and other files.

I can, as of today, generate my own mok key and sign my LKM's, and
kernel will verify my LKM's.

What is different?


/Jarkko