[dm-devel] [PATCH 7/7] dm: add documentation for IMA measurement support

[Mimi Zohar]

Hi Tushar,

On Sat, 2021-07-24 at 00:25 -0700, Tushar Sugandhi wrote:
> Hi Mimi,
> 
> 
> > Missing from the document is a way of validating the template data.
> > For example, in the original case of file measurements, the template
> > data contains the file hash, which can be recalculated or verified
> > against an allow list.
> > 
> > Other than re-calculating the template data digest based on the
> > template data, and verifying it against the template data digest in the
> > measurement list, would an attestation server be able to verify the
> > template data itself?
> >
> Yes.
> In the context of device-mapper, EVENT_DATA for 'table_load' would
> contain the key-value pairs for various targets in the table
> (crypt, verity, integrity etc.) which the attestation servers
> should be able to verify against the allowed/expected
> key-value pairs specified in the attestation policy.
> 
> To avoid bloating the IMA log with same data from table_load again,
> we only measure hash of the loaded table in the EVENT_DATA -
> when there is a state change for DM device.
> e.g. when EVENT_NAME is 'device_resume', 'table_clear',
> 'device_remove' etc.
> 
> Since the table clear-text is already present in the EVENT_DATA
> buffer for 'table_load', and is available to attestation servers,
> verifying the corresponding hash values in the
> EVENT_DATA in the subsequent DM events should be possible for
> the attestation servers.
> 
> Please let us know if you need further info.

For regular files with signatures, the file signature is verified
against the file hash, both contained within the template data.  For
the SELinux "critical-data", 
commit 2554a48f4437 ("selinux: measure state and policy capabilities")
contains that information.  Missing from this patch set is information
on how the attestation server could verify the DM critical data.

Does the DM record contain everything needed for the attestion server
to verify the template record?  Are things like the hash algorithm hard
coded?

thanks,

Mimi