On Thu, Jul 30, 2020 at 9:12 AM Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx> wrote: > > On 7/30/20 5:05 AM, Mimi Zohar wrote: > > On Wed, 2020-07-29 at 20:41 -0700, Lakshmi Ramasubramanian wrote: > >> 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. > > > > Wouldn't a unique event name result in a new measurement every time? > > > > Adding Stephen. > > Yes - but the LSM would call the IMA hook only when there is a change in > the state. > > I have posted a patch set for this last night - > it defines IMA hooks for measuring LSM data and uses that to measure > SELinux data constructs. It can be used by other security modules as > well. Please take a look. To provide more context, the patch set is enabling the measurement of LSM state variables (e.g. SELinux enabled and enforcing among others) and LSM policy (e.g. the SELinux policy). An appraiser may want to know either or both of: 1) What is the current state/policy of the system, e.g. is it currently enforcing an expected policy? 2) Has the system ever been in a state/policy that was unexpected/unauthorized, e.g. was it switched to permissive mode at some point earlier or did it load an unexpected/unauthorized policy earlier?
