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

[Lakshmi Ramasubramanian]

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@xxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > 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