Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 1 Jul 2019 10:33:00 -0700 (PDT)
From:   Jaskaran Singh Khurana <jaskarankhurana@linux.microsoft.com>
To:     Milan Broz <gmazyland@gmail.com>
cc:     James Morris <jmorris@namei.org>,
        Eric Biggers <ebiggers@kernel.org>,
        linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-integrity@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, agk@redhat.com, snitzer@redhat.com,
        dm-devel@redhat.com, scottsh@microsoft.com, mpatocka@redhat.com
Subject: Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig
 validation.
In-Reply-To: <749ddf56-3cb6-42c8-9ccc-71e09558400f@gmail.com>
Message-ID: <alpine.LRH.2.21.1907011029100.31396@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.inter>
References: <20190619191048.20365-1-jaskarankhurana@linux.microsoft.com> <20190628040041.GB673@sol.localdomain> <alpine.LRH.2.21.1906282040490.15624@namei.org> <749ddf56-3cb6-42c8-9ccc-71e09558400f@gmail.com>
User-Agent: Alpine 2.21 (LRH 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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.