Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA critical data hook

[Lakshmi Ramasubramanian <nramas@linux.microsoft.com>]

On 1/13/21 6:49 PM, Mimi Zohar wrote:

Hi Mimi,

>>>>>>>> I remain concerned about the possibility of bypassing a measurement by
>>>>>>>> tampering with the time, but I appear to be the only one who is
>>>>>>>> worried about this so I'm not going to block this patch on those
>>>>>>>> grounds.
>>>>>>>>
>>>>>>>> Acked-by: Paul Moore <paul@paul-moore.com>
>>>>>>>
>>>>>>> Thanks, Paul.
>>>>>>>
>>>>>>> Including any unique string would cause the buffer hash to change,
>>>>>>> forcing a new measurement.  Perhaps they were concerned with
>>>>>>> overflowing a counter.
>>>>>>
>>>>>> My understanding is that Lakshmi wanted to force a new measurement
>>>>>> each time and felt using a timestamp would be the best way to do that.
>>>>>> A counter, even if it wraps, would have a different value each time
>>>>>> whereas a timestamp is vulnerable to time adjustments.  While a
>>>>>> properly controlled and audited system could be configured and
>>>>>> monitored to detect such an event (I *think*), why rely on that if it
>>>>>> isn't necessary?
>>>>>
>>>>> Why are you saying that even if the counter wraps a new measurement is
>>>>> guaranteed.   I agree with the rest of what you said.
>>>>
>>>> I was assuming that the IMA code simply compares the passed
>>>> "policy_event_name" value to the previous value, if they are different
>>>> a new measurement is taken, if they are the same the measurement
>>>> request is ignored.  If this is the case the counter value is only
>>>> important in as much as that it is different from the previous value,
>>>> even simply toggling a single bit back and forth would suffice in this
>>>> case.  IMA doesn't keep a record of every previous "policy_event_name"
>>>> value does it?  Am I misunderstanding how
>>>> ima_measure_critical_data(...) works?
>>>
>>> Originally, there was quite a bit of discussion as to how much or how
>>> little should be measured for a number of reasons.  One reason is that
>>> the TPM is relatively slow.  Another reason is to limit the size of the
>>> measurement list.  For this reason, duplicate hashes aren't added to
>>> the measurement list or extended into the TPM.
>>>
>>> When a dentry is removed from cache, its also removed from IMA's iint
>>> cache.  A subsequent file read would result in adding the measurement
>>> and extending the TPM again.  ima_lookup_digest_entry() is called to
>>> prevent adding the duplicate entry.
>>>
>>> Lakshmi is trying to address the situation where an event changes a
>>> value, but then is restored to the original value.  The original and
>>> subsequent events are measured, but restoring to the original value
>>> isn't re-measured.  This isn't any different than when a file is
>>> modified and then reverted.
>>>
>>> Instead of changing the name like this, which doesn't work for files,
>>> allowing duplicate measurements should be generic, based on policy.
>>
>> Perhaps it is just the end of the day and I'm a bit tired, but I just
>> read all of the above and I have no idea what your current thoughts
>> are regarding this patch.
> 
> Other than appending the timestamp, which is a hack, the patch is fine.
> Support for re-measuring an event can be upstreamed independently.
> 

Thanks for clarifying the details related to duplicate measurement 
detection and re-measuring.

I will keep the timestamp for the time being, even though its a hack, as 
it helps with re-measuring state changes in SELinux. We will add support 
for "policy driven" re-measurement as a subsequent patch series.

thanks,
  -lakshmi