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. > >> >> The current userspace configuration through veritysetup does not need >> any patches for your patch, correct? > > Right, it's only different from the kernel point of view. > >> >> Thanks, >> Milan >>
