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

On Thu, 2025-06-05 at 15:43 +0200, Vitaly Kuznetsov wrote:

James Bottomley [off-list ref] writes:

quoted

On Thu, 2025-06-05 at 09:54 +0200, Vitaly Kuznetsov wrote:

quoted

One additional consideration is the fact that we already trust
'db' for dm-verity (since 6fce1f40e951) and kexec (since
278311e417be) and especially the later gives someone who is able
to control 'db' access to CPL0; a 'db'-signed module (IMO)
wouldn't change much.

Well, the kexec case is because kexec has to verify the new kernel
as shim would and shim would use the UEFI keys.  The dm-verity one
was added for a cloud use case by pressuring the maintainers in
spite of the objection to using the platform keyring (it went to
dm-devel only so not many integrity people saw it):

https://lore.kernel.org/all/20240617220037.594792-1-luca.boccassi@gmail.com/ (local)

The point here is I do think the cloud use case is legitimate, but
it can't be supported simply by ignoring the bare metal security
domain separation concerns of the integrity community.  The
argument that distros have done it so it must be safe isn't really
a winning one (especially as there's no clear explanation of why
they did it).  So either you need a better argument or we need a
way to support both sets of communities ... which is why I was
wondering about a runtime differentiator.

So far, I got two 'runtime' ideas:
- Observe MokListTrustedRT and distrust .platform when it is
non-empty. This can, of course, be combine with a Kconfig for those,
who do not want it at all.

Well, not sure about that specific variable.  It seems to be set but
not used by shim (however it is used in the kernel to decide whether to
import the MoK list), so how would someone with a current distrusted db
get it set?  But there's also MokIgnoreDB (which is actually a RT
import of MokDBState) which is used to prevent importing the db certs
into the platform keyring in the first place.

I think the reason this is so fragmented is because we didn't really
co-ordinate with shim when all the variables and switches were added. 
Perhaps we should document all the variables and expectations before
deciding on a mechanism? 

The one thing we can guarantee is if the cloud use case is booting
without shim (is it?) then none of the RT variables will get created,
so checking any (or a set) of them would work.

and/or
- Sysctl toggle. Keep things as they are by default but make
.platform trusted (either for modules or for everything) when
switched 'on'. This can (optionally) by combined with a previous idea
and have e.g. an 'auto' state for the toggle which follows
MokListTrustedRT.

I'm less keen on user specifiable runtime because the security policy
of the system using a lockdown to make root less privileged than ring 0
can't allow a malicious root to weaken it.  However, let's see if we
can get a proposal that would mitigate that concern.

Ideally, if we can get to something that works for everyone at runtime,
we can remove the current Kconfig explosion which is definitely adding
to the confusion (as shown in the Debian bug reports).

Regards,

James