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

[Mimi Zohar]

On Tue, 2021-01-12 at 11:27 -0500, Paul Moore wrote:
> On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi
> <tusharsu@xxxxxxxxxxxxxxxxxxx> wrote:
> > From: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>
> >
> > 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>
> > Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  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@xxxxxxxxxxxxxx>

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.

Mimi

> > +/*
> > + * selinux_ima_measure_state - Measure hash of the SELinux policy
> > + *
> > + * @state: selinux state struct
> > + *
> > + * NOTE: This function must be called with policy_mutex held.
> > + */
> > +void selinux_ima_measure_state(struct selinux_state *state)
> > +{
> > +       struct timespec64 cur_time;
> > +       void *policy = NULL;
> > +       char *policy_event_name = NULL;
> > +       size_t policy_len;
> > +       int rc = 0;
> > +
> > +       /*
> > +        * Measure SELinux policy only after initialization is completed.
> > +        */
> > +       if (!selinux_initialized(state))
> > +               return;
> > +
> > +       /*
> > +        * Pass a unique "event_name" to the IMA hook so that IMA subsystem
> > +        * will always measure the given data.
> > +        */
> > +       ktime_get_real_ts64(&cur_time);
> > +       policy_event_name = kasprintf(GFP_KERNEL, "%s-%lld:%09ld",
> > +                                     "selinux-policy-hash",
> > +                                     cur_time.tv_sec, cur_time.tv_nsec);
> > +       if (!policy_event_name) {
> > +               pr_err("SELinux: %s: event name for policy not allocated.\n",
> > +                      __func__);
> > +               goto out;
> > +       }
> > +