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

From: Stephen Smalley
Date: Wed Aug 05 2020 - 15:57:53 EST

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.