Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA critical data hook

[Paul Moore <paul@paul-moore.com>]

On Wed, Jan 13, 2021 at 2:13 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> On Tue, 2021-01-12 at 11:27 -0500, Paul Moore wrote:
> > On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi
> > <tusharsu@linux.microsoft.com> wrote:
> > > From: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> > >
> > > 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@linux.microsoft.com>
> > > Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
> > > Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> > > ---
> > >  Documentation/ABI/testing/ima_policy |  3 +-
> > >  security/selinux/Makefile            |  2 +
> > >  security/selinux/ima.c               | 64 ++++++++++++++++++++++++++++
> > >  security/selinux/include/ima.h       | 24 +++++++++++
> > >  security/selinux/include/security.h  |  3 +-
> > >  security/selinux/ss/services.c       | 64 ++++++++++++++++++++++++----
> > >  6 files changed, 149 insertions(+), 11 deletions(-)
> > >  create mode 100644 security/selinux/ima.c
> > >  create mode 100644 security/selinux/include/ima.h
> >
> > I remain concerned about the possibility of bypassing a measurement by
> > tampering with the time, but I appear to be the only one who is
> > worried about this so I'm not going to block this patch on those
> > grounds.
> >
> > Acked-by: Paul Moore <paul@paul-moore.com>
>
> Thanks, Paul.
>
> Including any unique string would cause the buffer hash to change,
> forcing a new measurement.  Perhaps they were concerned with
> overflowing a counter.

My understanding is that Lakshmi wanted to force a new measurement
each time and felt using a timestamp would be the best way to do that.
A counter, even if it wraps, would have a different value each time
whereas a timestamp is vulnerable to time adjustments.  While a
properly controlled and audited system could be configured and
monitored to detect such an event (I *think*), why rely on that if it
isn't necessary?

-- 
paul moore
www.paul-moore.com