On 7/29/20 8:23 PM, Mimi Zohar wrote:
On Wed, 2020-07-29 at 10:17 -0700, Lakshmi Ramasubramanian wrote:
Hi Mimi,
I have a query related to measuring data (by IMA subsystem) when that
data has been already been measured.
Consider the following sequence of events:
=> At time T0 IMA hook is called by another subsystem to measure data
"foo". IMA measures it.
=> At time T1 data is "bar". IMA measures it.
=> At time T2 data is "foo" again. But IMA doesn't measure it since it
is already in the measured list.
But for the subsystem making the call to IMA, the state has changed and
"foo" has to be measured again.
One way to address the above is to use unique "event name" in each call
so that IMA measures the given data every time.
Is there a better way to address the above?
Most likely the file is being re-measured, but the new value already exists in
the hash table so it isn't being added to the IMA measurement list or extending
the TPM. When IMA was upstreamed, there was concern about TPM performance and
the number of measurements being extended. We've improved TPM performance quite
a bit. If you're not concerned about TPM performance, I would define a new
template data field based on i_version.
In the use case I am considering the entity being measured is not a
file, but a memory buffer - it is for measuring an LSM's data
constructs. So i_version is not available in this case.
When LSM's data changes from A to B and then back to A, hash(A) already
exists in IMA's hash table. So A is not measured again.
Since LSM state change is not expected to be frequent, TPM performance
shouldn't be a concern.
thanks,
-lakshmi
