On Thu, 10 Nov 2022 at 01:01, Morten Linderud [off-list ref] wrote:
On Tue, Nov 23, 2021 at 11:41:23PM -0500, Eric Snowberg wrote:
quoted
A new Machine Owner Key (MOK) variable called MokListTrustedRT has been
introduced in shim. When this UEFI variable is set, it indicates the
end-user has made the decision themselves that they wish to trust MOK keys
within the Linux trust boundary. It is not an error if this variable
does not exist. If it does not exist, the MOK keys should not be trusted
within the kernel.
Hi Eric,
I've been milling around on this patch-set for a while and I have a few issues
with the description of the commit and what the code actually does.
efi_mokvar_entry_find doesn't simply read an UEFI variable as the commit message
suggests, it will look for the MOK variable loaded into the EFI configuration
table. This implies we need this table setup in early boot to take usage of this
patch set.
The only bootloader that does setup this table, is the `shim` as described. But
no other bootloader implements support for the MOK EFI configuration table.
Does any other bootloader implement support for the (volatile)
MokListTrustedRT variable?
Note that this variable is intentionally volatile, and should be
rejected by the kernel if it is not. The point of these RT variables
or the config tables is that they can only be set at boot if a signed
and therefore trusted agent created them.
Permitting non-volatile variables here defeats the purpose of secure
boot, which aims to prevent exploits from gaining persistence. It
would be bad if you could corrupt the trusted boot chain forever by
setting a variable once.
This effectively means that there is still no way for Machine Owners to load
keys into the keyring, for things like module signing, without the shim present
in the bootchain. I find this a bit weird.
Is this an intentional design decision, or could other ways be supported as
well?
Yes.
If we are looking for a way to use EFI variables to inject additional
certificates into the keyring without the ability to authenticate
them, we should I'd strongly recommend that we disable that by default
and add a big fat warning that it is incompatible with the guarantees
secure boot aims to provide.