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

Tyler Hicks
2020-08-05T20:01:23Z

On 2020-08-05 10:27:43, Stephen Smalley wrote:
> On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
> >
> > On Wed, 2020-08-05 at 09:03 -0400, Stephen Smalley wrote:
> > > On Wed, Aug 5, 2020 at 8:57 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > > > 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?
> > >
> > > They were split into two separate functions because we wanted to be
> > > able to support using different templates for them (ima-buf for the
> > > state variables so that the measurement includes the original buffer,
> > > which is small and relatively fixed-size, and ima-ng for the policy
> > > because it is large and we just want to capture the hash for later
> > > comparison against known-good). Also, the state variables are
> > > available for measurement always from early initialization, whereas
> > > the policy is only available for measurement once we have loaded an
> > > initial policy.
> >
> > Ok, measuring the policy separately from other critical data makes
> > sense. Instead of measuring the policy, which is large, measure the
> > policy hash.
>
> I think that was the original approach. However, I had concerns with
> adding code to SELinux to compute a hash over the policy versus
> leaving that to IMA's existing policy and mechanism. If that's
> preferred I guess we can do it that way but seems less flexible and
> duplicative.

In AppArmor, we store the sha1 of the raw policy as the policy is
loaded. The hash is exposed to userspace in apparmorfs. See commit
5ac8c355ae00 ("apparmor: allow introspecting the loaded policy pre
internal transform").

It has proved useful as a mechanism for debugging as sometimes the
on-disk policy doesn't match the loaded policy and this can be a good
way to check that while providing support to users. John also mentions
checkpoint/restore in the commit message and I could certainly see how
the policy hashes would be useful in that scenario.

When thinking through how Lakshmi's series could be extended for
AppArmor support, I was thinking that the AppArmor policy measurement
would be a measurement of these hashes that we already have in place.

Perhaps there's some general usefulness in storing/exposing an SELinux
policy hash rather than only seeing it as duplicative property required
this measurement series?

Tyler