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

From: Stephen Smalley
Date: Wed Aug 05 2020 - 15:51:33 EST

On 8/5/20 11:07 AM, Tyler Hicks wrote:

On 2020-08-05 10:27:43, Stephen Smalley wrote:
On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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
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
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
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?

That would be a hash of the policy file that was last loaded via the selinuxfs interface for loading policy, not a hash of the in-memory policy data structures at the time of measurement (which is what this patch series is implementing).  The duplicative part is with respect to selecting a hash algorithm and hashing the in-memory policy as part of the SELinux code rather than just passing the policy buffer to IMA for measurement like any other buffer.  Userspace can already hash the in-memory policy data itself by running sha256sum or whatever on /sys/fs/selinux/policy, so we don't need to save or expose that separately.