Re: [PATCH 0/3] support for duplicate measurement of integrity critical data

[Tushar Sugandhi]

On 2021-02-09 10:53 a.m., Mimi Zohar wrote:
> On Tue, 2021-02-09 at 10:23 -0800, Tushar Sugandhi wrote:
> > > On Mon, 2021-02-08 at 15:22 -0500, Mimi Zohar wrote:
> > > > On Fri, 2021-01-29 at 16:45 -0800, Tushar Sugandhi wrote:
> > > > > IMA does not measure duplicate buffer data since TPM extend is a very
> > > > > expensive operation.  However, in some cases for integrity critical
> > > > > data, 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 integrity critical data on the
> > > > > system.
> > > > >
> > > > > This series addresses this gap by providing the ability to measure
> > > > > duplicate entries for integrity critical data, driven by policy.
> > > >
> > > > The same reason for re-measuring buffer data is equally applicable to
> > > > files.  In both cases, the file or the buffer isn't re-measured if it
> > > > already exists in the htable.   Please don't limit this patch set to
> > > > just buffer data.
> > Agreed.  I wasn't sure if you wanted the support for files, or other
> > buffer measurement scenarios, except critical data.  So I started the
> > implementation with supporting just critical data.  Happy to extend it
> > to files and other buffer measurement scenarios as you suggested.
> >
> > > Instead of making the change on a per measurement rule basis, disabling
> > > "htable" would be the simplest way of forcing re-measurements.  All
> > > that would be needed is a new Kconfig (e.g. CONFIG_IMA_DISABLE_HTABLE)
> > > and the associated test in ima_add_template_entry().
> > Agreed.  Earlier I wasn't sure if you wanted allow_dup support for all
> > the scenarios.  Now that it is clear,  I will implement it as you
> > suggested.  Thank you so much for the pointers.  Appreciate it.
>
> There are two different solutions - per measurement rule, disabling
> htable - being discussed.   Disabling htable requires miminumal
> changes.  Which version are you thinking of implementing?
I am thinking of implementing "disabling 'htable' using a new Kconfig 
(e.g. CONFIG_IMA_DISABLE_HTABLE)".  That is, not using the var 
ima_htable or ima_lookup_digest_entry() if that CONFIG is set.
So the duplicate measurements are allowed when the CONFIG is set.
This would cover all the measurement scenarios through a single CONFIG 
setting.

I am not planning to implement it as a "per measurement rule".

Sorry it wasn't clear in my earlier response.

Thanks,
Tushar

> thanks,
>
> Mimi