linux-kernel - Re: [RFC 08/20] ima: Move measurement list related variables into ima

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Message-ID: <5a8c8801-d8ce-ab9b-3d38-25fbb4a4f6bb@linux.ibm.com>
Date:   Thu, 2 Dec 2021 13:03:03 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     jejb@...ux.ibm.com, linux-integrity@...r.kernel.org
Cc:     zohar@...ux.ibm.com, serge@...lyn.com,
        christian.brauner@...ntu.com, containers@...ts.linux.dev,
        dmitry.kasatkin@...il.com, ebiederm@...ssion.com,
        krzysztof.struczynski@...wei.com, roberto.sassu@...wei.com,
        mpeters@...hat.com, lhinds@...hat.com, lsturman@...hat.com,
        puiterwi@...hat.com, jamjoom@...ibm.com,
        linux-kernel@...r.kernel.org, paul@...l-moore.com, rgb@...hat.com,
        linux-security-module@...r.kernel.org, jmorris@...ei.org
Subject: Re: [RFC 08/20] ima: Move measurement list related variables into
 ima_namespace


On 12/2/21 12:44, James Bottomley wrote:
> On Thu, 2021-12-02 at 11:45 -0500, Stefan Berger wrote:
>> On 12/2/21 11:29, James Bottomley wrote:
>>> On Thu, 2021-12-02 at 08:41 -0500, Stefan Berger wrote:
>>>> On 12/2/21 07:46, James Bottomley wrote:
>>>>> On Tue, 2021-11-30 at 11:06 -0500, Stefan Berger wrote:
>>>>>> Move measurement list related variables into the
>>>>>> ima_namespace.
>>>>>> This
>>>>>> way a
>>>>>> front-end like SecurityFS can show the measurement list
>>>>>> inside an
>>>>>> IMA
>>>>>> namespace.
>>>>>>
>>>>>> Implement ima_free_measurements() to free a list of
>>>>>> measurements
>>>>>> and call it when an IMA namespace is deleted.
>>>>> This one worries me quite a lot.  What seems to be happening in
>>>>> this
>>>>> code:
>>>>>
>>>>>> @@ -107,7 +100,7 @@ static int ima_add_digest_entry(struct
>>>>>> ima_namespace *ns,
>>>>>>            qe->entry = entry;
>>>>>>     
>>>>>>            INIT_LIST_HEAD(&qe->later);
>>>>>> -       list_add_tail_rcu(&qe->later, &ima_measurements);
>>>>>> +       list_add_tail_rcu(&qe->later, &ns->ima_measurements);
>>>>>>     
>>>>>>            atomic_long_inc(&ns->ima_htable.len);
>>>>>>            if (update_htable) {
>>>>>>
>>>>> is that we now only add the measurements to the namespace list,
>>>>> but
>>>>> that list is freed when the namespace dies.  However, the
>>>>> measurement
>>>>> is still extended through the PCRs meaning we have incomplete
>>>>> information for a replay after the namespace dies?
>>>> *Not at all.* The measurement list of the namespace is
>>>> independent
>>>> of
>>>> the host.
>>>>
>>>> The cover letter states:
>>> I get that the host can set up a policy to log everything in the
>>> namespace, but that wasn't my question.  My question is can the
>>> guest
>>> set up a policy to log something that doesn't go into the host log
>>> (because the host hasn't asked for it to be logged) but extends a
>>> PCR
>>> anyway, thus destroying the ability of the host to do log replay.
>> host log goes with host TPM and vice versa
>>
>> guest log goes with (optional) vTPM and vice version
> But that's what doesn't seem to happen ... ima_pcr_extend isn't
> virtualized and it's always called from ima_add_template_entry()
> meaning the physical TPM is always extended even for a namespace only
> entry.

You cannot set a measurement rule in the namespace. That is prevented 
per 9/20: ima: Only accept AUDIT rules for IMA non-init_ima_ns 
namespaces for now.

Also, with the tests that I have done with IMA namespaces I have not 
seen any 'evmctl ima_measurement ...' failures.

Have you been able to cause the IMA namespace to do measurements? It 
would be an easy thing to move the tpm_chip into the ima_namespace as 
well, but per 9/20 this shouldn't be necessary at this point.

>   
>
>> Extending the PCR of the host's TPM would require the data to be
>> logged in the host log as well. So, no, it's not possible.
> Well, exactly: if you don't have or want a vTPM per container the only
> way to attest is via the physical TPM which means all entries in the
> namespace must be in the host log, so the host owner can quote and
> reply and they can split the attested log and give assurance to the
> namespaces that their entries are correct.

Yes, this series allows you to log into the system log and along with 
this extend the TPM PCR.


>
> James
>
>