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

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Date:   Thu, 27 Jun 2019 18:52:02 -0700 (PDT)
From:   Jaskaran Singh Khurana <jaskarankhurana@...ux.microsoft.com>
To:     Milan Broz <gmazyland@...il.com>
cc:     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,
        ebiggers@...gle.com, mpatocka@...hat.com
Subject: Re: [RFC PATCH v5 1/1] Add dm verity root hash pkcs7 sig
 validation.



On Thu, 27 Jun 2019, Milan Broz wrote:

> Hi,
>
> I tried to test test the patch, two comments below.
>
> On 19/06/2019 21:10, Jaskaran Khurana wrote:
>> The verification is to support cases where the roothash is not secured by
>> Trusted Boot, UEFI Secureboot or similar technologies.
>> One of the use cases for this is for dm-verity volumes mounted after boot,
>> the root hash provided during the creation of the dm-verity volume has to
>> be secure and thus in-kernel validation implemented here will be used
>> before we trust the root hash and allow the block device to be created.
>>
>> The signature being provided for verification must verify the root hash and
>> must be trusted by the builtin keyring for verification to succeed.
>>
>> The hash is added as a key of type "user" and the description is passed to
>> the kernel so it can look it up and use it for verification.
>>
>> Kernel commandline parameter will indicate whether to check (only if
>> specified) or force (for all dm verity volumes) roothash signature
>> verification.
>>
>> Kernel commandline: dm_verity.verify_sig=1 or 2 for check/force root hash
>> signature validation respectively.
>
> 1) I think the switch should be just boolean - enforce signatures for all dm-verity targets
> (with default to false/off).
>
> The rest should be handled by simple logic - if the root_hash_sig_key_desc option
> is specified, the signature MUST be validated in the constructor, all errors should cause
> failure (bad reference in keyring, bad signature, etc).
>
> (Now it ignores for example bad reference to the keyring, this is quite misleading.)
>
> If a user wants to activate a dm-verity device without a signature, just remove
> optional argument referencing the signature.
> (This is not possible with dm_verity.verify_sig set to true/on.)
>
>
> 2) All DM targets must provide the same mapping table status ("dmsetup table"
> command) as initially configured.
> The output of the command should be directly usable as mapping table constructor.
>
> Your patch is missing that part, I tried to fix it, add-on patch is here
> https://git.kernel.org/pub/scm/linux/kernel/git/mbroz/linux.git/commit/?h=dm-cryptsetup&id=a26c10806f5257e255b6a436713127e762935ad3
> (feel free to fold it in your patch)
>
>
> Thanks,
> Milan
>
Hello Milan,

Thanks for testing and reviewing this. I will take care of the above 
comments change it to a boolean and for the second point merge the add-on 
patch that you shared.

Regards,
Jaskaran