Re: [RFC 08/20] ima: Move measurement list related variables into ima_namespace

Stefan Berger
2021-12-02T18:03:21Z

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
>
>