linux-kernel - Re: [RFC PATCH v5 1/1] Add dm verity root hash pkcs7 sig validation.

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Date:   Fri, 28 Jun 2019 10:03:15 -0700 (PDT)
From:   Jaskaran Singh Khurana <jaskarankhurana@...ux.microsoft.com>
To:     Milan Broz <gmazyland@...il.com>
cc:     Eric Biggers <ebiggers@...nel.org>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, agk@...hat.com, snitzer@...hat.com,
        dm-devel@...hat.com, jmorris@...ei.org, scottsh@...rosoft.com,
        mpatocka@...hat.com
Subject: Re: [RFC PATCH v5 1/1] Add dm verity root hash pkcs7 sig
 validation.


Hello Eric/Milan,

On Fri, 28 Jun 2019, Milan Broz wrote:

> On 28/06/2019 05:00, Eric Biggers wrote:
>>> Hello Eric,
>>>
>>> This started with a config (see V4). We didnot want scripts that pass this
>>> parameter to suddenly stop working if for some reason the verification is
>>> turned off so the optional parameter was just parsed and no validation
>>> happened if the CONFIG was turned off. This was changed to a commandline
>>> parameter after feedback from the community, so I would prefer to keep it
>>> *now* as commandline parameter. Let me know if you are OK with this.
>>>
>>> Regards,
>>> JK
>>
>> Sorry, I haven't been following the whole discussion.  (BTW, you sent out
>> multiple versions both called "v4", and using a cover letter for a single patch
>> makes it unnecessarily difficult to review.)  However, it appears Milan were
>> complaining about the DM_VERITY_VERIFY_ROOTHASH_SIG_FORCE option which set the
>> policy for signature verification, *not* the DM_VERITY_VERIFY_ROOTHASH_SIG
>> option which enabled support for signature verification.  Am I missing
>> something?  You can have a module parameter which controls the "signatures
>> required" setting, while also allowing people to compile out kernel support for
>> the signature verification feature.
>
> Yes, this was exactly my point.
>
> I think I even mention in some reply to use exactly the same config Makefile logic
> as for FEC - to allow completely compile it out of the source:
>
> ifeq ($(CONFIG_DM_VERITY_FEC),y)
> dm-verity-objs                  += dm-verity-fec.o
> endif
>
>> Sure, it means that the signature verification support won't be guaranteed to be
>> present when dm-verity is.  But the same is true of the hash algorithm (e.g.
>> sha512), and of the forward error correction feature.  Since the signature
>> verification is nontrivial and pulls in a lot of other kernel code which might
>> not be otherwise needed (via SYSTEM_DATA_VERIFICATION), it seems a natural
>> candidate for putting the support behind a Kconfig option.
>
> On the other side, dm-verity is meant for a system verification, so if it depends
> on SYSTEM_DATA_VERIFICATION is ... not so surprising :)
>
> But the change above is quite easy and while we already have FEC as config option,
> perhaps let's do it the same here.
>
> Milan
>
Yes, I will make this change. Please consider this discussion as resolved. 
Thanks.

Regards,
Jaskaran.