linux-kernel - Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig validation.

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Date:   Mon, 1 Jul 2019 10:33:00 -0700 (PDT)
From:   Jaskaran Singh Khurana <jaskarankhurana@...ux.microsoft.com>
To:     Milan Broz <gmazyland@...il.com>
cc:     James Morris <jmorris@...ei.org>,
        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, scottsh@...rosoft.com, mpatocka@...hat.com
Subject: Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig
 validation.


Hello Milan,
On Mon, 1 Jul 2019, Milan Broz wrote:

> On 29/06/2019 06:01, James Morris wrote:
>> On Thu, 27 Jun 2019, Eric Biggers wrote:
>>
>>> I don't understand your justification for this feature.
>>>
>>> If userspace has already been pwned severely enough for the attacker to be
>>> executing arbitrary code with CAP_SYS_ADMIN (which is what the device mapper
>>> ioctls need), what good are restrictions on loading more binaries from disk?
>>>
>>> Please explain your security model.
>>
>> Let's say the system has a policy where all code must be signed with a
>> valid key, and that one mechanism for enforcing this is via signed
>> dm-verity volumes. Validating the signature within the kernel provides
>> stronger assurance than userspace validation. The kernel validates and
>> executes the code, using kernel-resident keys, and does not need to rely
>> on validation which has occurred across a trust boundary.
>
> Yes, but as it is implemented in this patch, a certificate is provided as
> a binary blob by the (super)user that activates the dm-verity device.
>
> Actually, I can put there anything that looks like a correct signature (self-signed
> or so), and dm-verity code is happy because the root hash is now signed.
>
> Maybe could this concept be extended to support in-kernel compiled certificates?
>
> I like the idea of signed root hash, but the truth is that if you have access
> to device activation, it brings nothing, you can just put any cert in the keyring
> and use it.
>
> Milan
>

The signature needs to be trusted by the .builtin_trusted_keys which is
a read-only list of keys that were compiled into the kernel. The 
verify_pkcs7_signature verifies trust against the builtin keyring so I 
think what you are suggesting is covered here.

Regards,
Jaskaran.