Re: [PATCH] selinux: include a consumer of the new IMA critical data hook

On Fri, 2021-01-22 at 15:24 -0500, Paul Moore wrote:

On Thu, Jan 14, 2021 at 2:15 PM Lakshmi Ramasubramanian
[off-list ref] wrote:

quoted

SELinux stores the active policy in memory, so the changes to this data
at runtime would have an impact on the security guarantees provided
by SELinux.  Measuring in-memory SELinux policy through IMA subsystem
provides a secure way for the attestation service to remotely validate
the policy contents at runtime.

Measure the hash of the loaded policy by calling the IMA hook
ima_measure_critical_data().  Since the size of the loaded policy
can be large (several MB), measure the hash of the policy instead of
the entire policy to avoid bloating the IMA log entry.

To enable SELinux data measurement, the following steps are required:

1, Add "ima_policy=critical_data" to the kernel command line arguments
   to enable measuring SELinux data at boot time.
For example,
  BOOT_IMAGE=/boot/vmlinuz-5.10.0-rc1+ root=UUID=fd643309-a5d2-4ed3-b10d-3c579a5fab2f ro nomodeset security=selinux ima_policy=critical_data

2, Add the following rule to /etc/ima/ima-policy
   measure func=CRITICAL_DATA label=selinux

Sample measurement of the hash of SELinux policy:

To verify the measured data with the current SELinux policy run
the following commands and verify the output hash values match.

  sha256sum /sys/fs/selinux/policy | cut -d' ' -f 1

  grep "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6

Note that the actual verification of SELinux policy would require loading
the expected policy into an identical kernel on a pristine/known-safe
system and run the sha256sum /sys/kernel/selinux/policy there to get
the expected hash.

Signed-off-by: Lakshmi Ramasubramanian <redacted>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Tyler Hicks <redacted>
---
 Documentation/ABI/testing/ima_policy |  3 +-
 security/selinux/Makefile            |  2 +
 security/selinux/ima.c               | 44 +++++++++++++++++++
 security/selinux/include/ima.h       | 24 +++++++++++
 security/selinux/include/security.h  |  3 +-
 security/selinux/ss/services.c       | 64 ++++++++++++++++++++++++----
 6 files changed, 129 insertions(+), 11 deletions(-)
 create mode 100644 security/selinux/ima.c
 create mode 100644 security/selinux/include/ima.h

Hi Mimi,

Just checking as I didn't see a reply to this from you in my inbox,
you merged this into the IMA linux-next branch, yes?

The patches are first staged in the linux-integrity #next-integrity-
testing branch, before being staged in the #next-integrity branch,
which is picked up by linux-next.  Sorry, they've been staged in the
next-integrity-testing branch, but are now next-integrity.

Mim