Re: [PATCH v2] IMA: support for duplicate data measurement

[Tushar Sugandhi]

On 2021-02-17 12:39 p.m., Mimi Zohar wrote:
> On Wed, 2021-02-17 at 10:53 -0800, Tushar Sugandhi wrote:
> > Thanks for the feedback Mimi.
> > Appreciate it.
> >
> > On 2021-02-17 7:03 a.m., Mimi Zohar wrote:
> > > Hi Tushar,
> > >
> > > The Subject line could be improved.  Perhaps something like - "IMA:
> > > support for duplicate measurement records"
> > Will do.
> >
> > > On Tue, 2021-02-16 at 18:46 -0800, Tushar Sugandhi wrote:
> > > > IMA does not measure duplicate data since TPM extend is a very expensive
> > > > operation.  However, in some cases, the measurement of duplicate data
> > > > is necessary to accurately determine the current state of the system.
> > > > Eg, SELinux state changing from 'audit', to 'enforcing', and back to
> > > > 'audit' again.  In this example, currently, IMA will not measure the
> > > > last state change to 'audit'.  This limits the ability of attestation
> > > > services to accurately determine the current state of the measurements
> > > > on the system.
> > >
> > > This patch description is written from your specific usecase
> > > perspective, but it impacts file and buffer data measurements as well,
> > > not only critical data measurements.  In all of these situations, with
> > > this patch a new measurement record is added/appended to the
> > > measurement list.  Please re-write the patch description making it more
> > > generic.
> > >
> > > For example, I would start with something like, "IMA does not include
> > > duplicate file, buffer or critical data measurement records ..."
> > Agreed.
> > I will generalize the description further and send the v3 for review.
>
> It would be good to boot with the ima_policy=tcb policy with/without
> your patch and account for the different number of measurements.   Are
> all the differences related to duplicate measurements - original file
> hash -> new file hash -> original file hash - similar to what you
> described.
Thanks for the ima_policy=tcb pointer.

I tested my patch with:
 - duplicate buffer content for "measure func=CRITICAL_DATA"
 - and reading the same file twice with "measure func=FILE_CHECK 
mask=MAY_READ"

In both the above use cases, IMA is measuring the duplicate entries with 
the patch, and not measuring the duplicate entries w/o the patch.

I will test the "ima_policy=tcb" boot-scenario as you suggested, before 
posting the next version.

Thanks,
Tushar

> thanks,
>
> Mimi