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Subject: Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA
 critical data hook
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Paul Moore <paul@paul-moore.com>
Cc:     Tushar Sugandhi <tusharsu@linux.microsoft.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        casey@schaufler-ca.com, agk@redhat.com, snitzer@redhat.com,
        gmazyland@gmail.com, tyhicks@linux.microsoft.com,
        sashal@kernel.org, James Morris <jmorris@namei.org>,
        nramas@linux.microsoft.com, linux-integrity@vger.kernel.org,
        selinux@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, dm-devel@redhat.com
Date:   Wed, 13 Jan 2021 16:11:09 -0500
In-Reply-To: <CAHC9VhTzaQ_q8gJ0oeok_yJ54XLETNvOuhhKnyRwgqsqvpBLCw@mail.gmail.com>
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On Wed, 2021-01-13 at 14:19 -0500, Paul Moore wrote:
> 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?

Why are you saying that even if the counter wraps a new measurement is
guaranteed.   I agree with the rest of what you said.

Mimi