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

[Mike Snitzer]

On Fri, Oct 23 2020 at  6:20am -0400,
Mickaël Salaün <mic@xxxxxxxxxxx> wrote:

> It seems that there is no more question. Mike, Alasdair, could you
> please consider to merge this into the tree?
> 
> On 16/10/2020 14:19, Mickaël Salaün wrote:
> > 
> > On 16/10/2020 13:08, Milan Broz wrote:
> >> 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.
> > It doesn't make sense to set DM_VERITY_VERIFY_ROOTHASH_SIG in generic
> > distro because such policy is configured at build time in the kernel
> > with hardcoded CAs. If the new option is not set then nothing change. I
> > don't see why it could be an issue for use cases we previously defined
> > (with DM_VERITY_VERIFY_ROOTHASH_SIG).
> > 
> >> 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 documentation remains true.
> > DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING depends on
> > DM_VERITY_VERIFY_ROOTHASH_SIG.

Yes, while true that doesn't change the fact that documenting
DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING is useful to potential
consumers of baseline DM_VERITY_VERIFY_ROOTHASH_SIG.

Please update Documentation and post v3, I'll get it merged for 5.11.

Thanks,
Mike