[PATCH v4 4/6] IMA: add policy to measure critical data from kernel components

[Mimi Zohar]

Hi Tushar,

The above Subject line should be truncated to "IMA: add policy to
measure critical data".    

On Wed, 2020-09-23 at 12:20 -0700, Tushar Sugandhi wrote:
> There would be several candidate kernel components suitable for IMA
> measurement. Not all of them would have support for IMA measurement.

This intro is besides the point.  The patch description should describe
what is meant by "critical data".

> Also, system administrators may not want to measure data for all of
> them, even when they support IMA measurement.
> An IMA policy option 
> specific to various kernel components is needed to measure their
> respective critical data.
> 
> This policy option needs to be constrained to measure data for
> specific kernel components that are specified as input values to the
> policy option.
> 
> Add a new IMA policy option - "data_sources:=" to allow measuring
> various critical kernel components. This policy option would enable the
> system administrators to limit the measurement to the components
> listed in "data_sources:=", if the components support IMA measurement.
> 
> The new policy option "data_sources:=" is different from the existing
> policy option "keyrings:=". 
> 
> In case of "keyrings:=", a policy may measure all keyrings (when
> "keyrings:=" option is not provided for func KEY_CHECK), or may
> constrain which keyrings need to be measured (when "keyrings:=" option
> is provided for func KEY_CHECK).
> 
> But unlike "keyrings:=", the entries in "data_sources:=" would have
> different data format. Further, the components listed in
> "data_sources:=" need to be modified to call IMA to measure their
> data. Therefore, unlike "keyrings:=", IMA shouldn't measure all of the
> components by default, when "data_sources:=" is not specified. Because
> measuring non-vetted components just by specifying them as a policy
> option value may impact the overall reliability of the system.
> 
> To address this, "data_sources:=" should be a mandatory policy option
> for func=CRITICAL_DATA. This func is introduced in the 5th patch in this
> series). And the compile-time vetting functionality described above is
> introduced in the 6th patch in this series.
> 
> Signed-off-by: Tushar Sugandhi <tusharsu at linux.microsoft.com>

I don't understand what you mean by "non-vetted" components, nor how
measuring critical data may impact the overall reliability of the
system.

The system owner or adminstrator defines what they want to measure,
including "critical data", based on the policy rules.  They might
decide that they want to constrain which "critical data" is measured by
specifying "data_sources:=", but that decision is their perogative.  
The default should allow measuring all critical data.

 A simple example of "critical data" could be some in memory structure,
along the lines of __ro_after_init, or hash of the memory structure. 
Once the data structure is initialized, the "critical data" measurement
shouldn't change.    From the attestation server perspective, the IMA
measurement list would contain a single record unless the critical data
changes.  The attestation server doesn't need to know anything about
the initial value, just that it has changed in order to trigger some
sort alert.

thanks,

Mimi