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

Mimi Zohar
2021-01-13T19:14:36Z

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.

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;
> > + }
> > +