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

[Lakshmi Ramasubramanian]

On 1/14/21 11:58 AM, Tyler Hicks wrote:
> On 2021-01-14 14:29:09, Paul Moore wrote:
> > On Thu, Jan 14, 2021 at 2:15 PM Lakshmi Ramasubramanian
> > <nramas@xxxxxxxxxxxxxxxxxxx> wrote:
> > > 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 <nramas@xxxxxxxxxxxxxxxxxxx>
> > > Suggested-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
> > > Acked-by: Paul Moore <paul@xxxxxxxxxxxxxx>
> > > Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >   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
> >
> > I think this has changed enough that keeping the "Acked-by" and
> > "Reviewed-by" tags is probably not a good choice.  I took a quick look
> > and this still looks okay from a SELinux perspective, I'll leave Mimi
> > to comment on it from a IMA perspective.

Thanks for reviewing the change Paul.

> > Unless Tyler has reviewed this version prior to your posting, it might
> > be a good idea to remove his "Reviewed-by" unless he has a chance to
> > look this over again before it is merged.
>
> Thanks for calling this out. I hadn't reviewed it prior to the posting
> but I was keeping an eye on the thread.
>
> This new revision still looks good to me and I like the idea of
> controlling re-measurements via policy. So,
>
>   Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>

Thanks for the quick response Tyler.

 -lakshmi