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

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Message-ID: <b7ccaa01-0398-f108-a70d-c67753d9fa6d@gmail.com>
Date:   Fri, 16 Oct 2020 13:08:43 +0200
From:   Milan Broz <gmazyland@...il.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        Mike Snitzer <snitzer@...hat.com>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Jaskaran Khurana <jaskarankhurana@...ux.microsoft.com>,
        dm-devel@...hat.com, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v2] dm verity: Add support for signature verification with
 2nd keyring

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