Re: [PATCH v2] dm verity: Add support for signature verification with 2nd keyring

[Milan Broz]

On 16/10/2020 10:49, Mickaël Salaün wrote:
> On 16/10/2020 10:29, Mickaël Salaün wrote:
> > On 15/10/2020 18:52, Mike Snitzer wrote:
> > > Can you please explain why you've decided to make this a Kconfig CONFIG
> > > knob?  Why not either add: a dm-verity table argument? A dm-verity
> > > kernel module parameter? or both (to allow a particular default but then
> > > per-device override)?
> >
> > The purpose of signed dm-verity images is to authenticate files, or said
> > in another way, to enable the kernel to trust disk images in a flexible
> > way (i.e. thanks to certificate's chain of trust). Being able to update
> > such chain at run time requires to use the second trusted keyring. This
> > keyring automatically includes the certificate authorities from the
> > builtin trusted keyring, which are required to dynamically populate the
> > secondary trusted keyring with certificates signed by an already trusted
> > authority. The roots of trust must then be included at build time in the
> > builtin trusted keyring.
> >
> > To be meaningful, using dm-verity signatures implies to have a
> > restricted user space, i.e. even the root user has limited power over
> > the kernel and the rest of the system. Blindly trusting data provided by
> > user space (e.g. dm-verity table argument, kernel module parameter)
> > defeat the purpose of (mandatory) authenticated images.
> >
> > > Otherwise, _all_ DM verity devices will be configured to use secondary
> > > keyring fallback.  Is that really desirable?
> >
> > That is already the current state (on purpose).
>
> I meant that when DM_VERITY_VERIFY_ROOTHASH_SIG is set, dm-verity
> signature becomes mandatory. This new configuration
> DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING extend this trust to the
> secondary trusted keyring, which contains certificates signed (directly
> or indirectly) by CA from the builtin trusted keyring.
>
> So yes, this new (optional) configuration *extends* the source of trust
> for all dm-verity devices, and yes, it is desirable. I think it should
> have been this way from the beginning (as for other authentication
> mechanisms) but it wasn't necessary at that time.

Well, I understand why you need a config option here.
And using the secondary keyring actually makes much more sense to me than
the original approach.

But please do not forget that dm-verity is sometimes used in different
contexts where such strict in-kernel certificate trust is unnecessary.
With your configure options set, you deliberately remove the possibility
to configure such devices.
I understand that it is needed for "trusted" systems, but we should be clear
in the documentation.
Maybe also add note to /Documentation/admin-guide/device-mapper/verity.rst ?
We already mention DM_VERITY_VERIFY_ROOTHASH_SIG there.

The current userspace configuration through veritysetup does not need
any patches for your patch, correct?

Thanks,
Milan