Re: [PATCH v6 1/4] IMA: Add func to measure LSM state and policy

[Stephen Smalley]

On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>
> On Wed, 2020-08-05 at 09:03 -0400, Stephen Smalley wrote:
> > On Wed, Aug 5, 2020 at 8:57 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> > > On Wed, 2020-08-05 at 08:46 -0400, Stephen Smalley wrote:
> > > > On 8/4/20 11:25 PM, Mimi Zohar wrote:
> > > >
> > > > > Hi Lakshmi,
> > > > >
> > > > > There's still  a number of other patch sets needing to be reviewed
> > > > > before my getting to this one.  The comment below is from a high level.
> > > > >
> > > > > On Tue, 2020-08-04 at 17:43 -0700, Lakshmi Ramasubramanian wrote:
> > > > > > Critical data structures of security modules need to be measured to
> > > > > > enable an attestation service to verify if the configuration and
> > > > > > policies for the security modules have been setup correctly and
> > > > > > that they haven't been tampered with at runtime. A new IMA policy is
> > > > > > required for handling this measurement.
> > > > > >
> > > > > > Define two new IMA policy func namely LSM_STATE and LSM_POLICY to
> > > > > > measure the state and the policy provided by the security modules.
> > > > > > Update ima_match_rules() and ima_validate_rule() to check for
> > > > > > the new func and ima_parse_rule() to handle the new func.
> > > > > I can understand wanting to measure the in kernel LSM memory state to
> > > > > make sure it hasn't changed, but policies are stored as files.  Buffer
> > > > > measurements should be limited  to those things that are not files.
> > > > >
> > > > > Changing how data is passed to the kernel has been happening for a
> > > > > while.  For example, instead of passing the kernel module or kernel
> > > > > image in a buffer, the new syscalls - finit_module, kexec_file_load -
> > > > > pass an open file descriptor.  Similarly, instead of loading the IMA
> > > > > policy data, a pathname may be provided.
> > > > >
> > > > > Pre and post security hooks already exist for reading files.   Instead
> > > > > of adding IMA support for measuring the policy file data, update the
> > > > > mechanism for loading the LSM policy.  Then not only will you be able
> > > > > to measure the policy, you'll also be able to require the policy be
> > > > > signed.
> > > >
> > > > To clarify, the policy being measured by this patch series is a
> > > > serialized representation of the in-memory policy data structures being
> > > > enforced by SELinux.  Not the file that was loaded.  Hence, this
> > > > measurement would detect tampering with the in-memory policy data
> > > > structures after the policy has been loaded.  In the case of SELinux,
> > > > one can read this serialized representation via /sys/fs/selinux/policy.
> > > > The result is not byte-for-byte identical to the policy file that was
> > > > loaded but can be semantically compared via sediff and other tools to
> > > > determine whether it is equivalent.
> > >
> > > Thank you for the clarification.   Could the policy hash be included
> > > with the other critical data?  Does it really need to be measured
> > > independently?
> >
> > They were split into two separate functions because we wanted to be
> > able to support using different templates for them (ima-buf for the
> > state variables so that the measurement includes the original buffer,
> > which is small and relatively fixed-size, and ima-ng for the policy
> > because it is large and we just want to capture the hash for later
> > comparison against known-good).  Also, the state variables are
> > available for measurement always from early initialization, whereas
> > the policy is only available for measurement once we have loaded an
> > initial policy.
>
> Ok, measuring the policy separately from other critical data makes
> sense.  Instead of measuring the policy, which is large, measure the
> policy hash.

I think that was the original approach.  However, I had concerns with
adding code to SELinux to compute a hash over the policy versus
leaving that to IMA's existing policy and mechanism.  If that's
preferred I guess we can do it that way but seems less flexible and
duplicative.