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

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2021-01-13 at 21:40 -0500, Paul Moore wrote:
> On Wed, Jan 13, 2021 at 6:11 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > On Wed, 2021-01-13 at 17:10 -0500, Paul Moore wrote:
> > > On Wed, Jan 13, 2021 at 4:11 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > > > 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.
> > >
> > > I was assuming that the IMA code simply compares the passed
> > > "policy_event_name" value to the previous value, if they are different
> > > a new measurement is taken, if they are the same the measurement
> > > request is ignored.  If this is the case the counter value is only
> > > important in as much as that it is different from the previous value,
> > > even simply toggling a single bit back and forth would suffice in this
> > > case.  IMA doesn't keep a record of every previous "policy_event_name"
> > > value does it?  Am I misunderstanding how
> > > ima_measure_critical_data(...) works?
> >
> > Originally, there was quite a bit of discussion as to how much or how
> > little should be measured for a number of reasons.  One reason is that
> > the TPM is relatively slow.  Another reason is to limit the size of the
> > measurement list.  For this reason, duplicate hashes aren't added to
> > the measurement list or extended into the TPM.
> >
> > When a dentry is removed from cache, its also removed from IMA's iint
> > cache.  A subsequent file read would result in adding the measurement
> > and extending the TPM again.  ima_lookup_digest_entry() is called to
> > prevent adding the duplicate entry.
> >
> > Lakshmi is trying to address the situation where an event changes a
> > value, but then is restored to the original value.  The original and
> > subsequent events are measured, but restoring to the original value
> > isn't re-measured.  This isn't any different than when a file is
> > modified and then reverted.
> >
> > Instead of changing the name like this, which doesn't work for files,
> > allowing duplicate measurements should be generic, based on policy.
> 
> Perhaps it is just the end of the day and I'm a bit tired, but I just
> read all of the above and I have no idea what your current thoughts
> are regarding this patch.

Other than appending the timestamp, which is a hack, the patch is fine.
Support for re-measuring an event can be upstreamed independently.

Mimi