Re: [PATCH v6 1/4] IMA: Add func to measure LSM state and policy

Mimi Zohar
2020-08-05T17:19:41Z

On Wed, 2020-08-05 at 08:46 -0400, Stephen Smalley wrote:
> On 8/4/20 11:25 PM, Mimi Zohar wrote:
>
> > Hi Lakshmi,
> >
> > There's still a number of other patch sets needing to be reviewed
> > before my getting to this one. The comment below is from a high level.
> >
> > On Tue, 2020-08-04 at 17:43 -0700, Lakshmi Ramasubramanian wrote:
> > > Critical data structures of security modules need to be measured to
> > > enable an attestation service to verify if the configuration and
> > > policies for the security modules have been setup correctly and
> > > that they haven't been tampered with at runtime. A new IMA policy is
> > > required for handling this measurement.
> > >
> > > Define two new IMA policy func namely LSM_STATE and LSM_POLICY to
> > > measure the state and the policy provided by the security modules.
> > > Update ima_match_rules() and ima_validate_rule() to check for
> > > the new func and ima_parse_rule() to handle the new func.
> > I can understand wanting to measure the in kernel LSM memory state to
> > make sure it hasn't changed, but policies are stored as files. Buffer
> > measurements should be limited to those things that are not files.
> >
> > Changing how data is passed to the kernel has been happening for a
> > while. For example, instead of passing the kernel module or kernel
> > image in a buffer, the new syscalls - finit_module, kexec_file_load -
> > pass an open file descriptor. Similarly, instead of loading the IMA
> > policy data, a pathname may be provided.
> >
> > Pre and post security hooks already exist for reading files. Instead
> > of adding IMA support for measuring the policy file data, update the
> > mechanism for loading the LSM policy. Then not only will you be able
> > to measure the policy, you'll also be able to require the policy be
> > signed.
>
> To clarify, the policy being measured by this patch series is a
> serialized representation of the in-memory policy data structures being
> enforced by SELinux. Not the file that was loaded. Hence, this
> measurement would detect tampering with the in-memory policy data
> structures after the policy has been loaded. In the case of SELinux,
> one can read this serialized representation via /sys/fs/selinux/policy.
> The result is not byte-for-byte identical to the policy file that was
> loaded but can be semantically compared via sediff and other tools to
> determine whether it is equivalent.

Thank you for the clarification. Could the policy hash be included
with the other critical data? Does it really need to be measured
independently?

Mimi