linux-kernel - Re: [PATCH v3 4/5] LSM: Define SELinux function to measure security state

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Message-ID: <CAEjxPJ6Rt7u3shLbxoPRHgr-D=CD9d_eXRB07A9qN7RmJwZAwA@mail.gmail.com>
Date:   Mon, 20 Jul 2020 13:40:54 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <jmorris@...ei.org>,
        linux-integrity@...r.kernel.org,
        SElinux list <selinux@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/5] LSM: Define SELinux function to measure security state

On Mon, Jul 20, 2020 at 1:34 PM Lakshmi Ramasubramanian
<nramas@...ux.microsoft.com> wrote:
>
> On 7/20/20 10:06 AM, Stephen Smalley wrote:
>
> >> The above will ensure the following sequence will be measured:
> >>    #1 State A - Measured
> >>    #2 Change from State A to State B - Measured
> >>    #3 Change from State B back to State A - Since the measured data is
> >> same as in #1, the change will be measured only if the event name is
> >> different between #1 and #3
> >
> > Perhaps the timestamp / sequence number should be part of the hashed
> > data instead of the event name?
>
> If the timestamp/seqno is part of the hashed data, on every call to
> measure IMA will add a new entry in the IMA log. This would fill up the
> IMA log - even when there is no change in the measured data.
>
> To avoid that I keep the last measurement in SELinux and measure only
> when there is a change with the timestamp in the event name.
>
> > I can see the appraiser wanting to know two things:
> > 1) The current state of the system (e.g. is it enforcing, is the
> > currently loaded policy the expected one?).
> > 2) Has the system ever been in an unexpected state (e.g. was it
> > temporarily switched to permissive or had an unexpected policy
> > loaded?)
>
> Yes - you are right.
> The appraiser will have to look at the entire IMA log (and the
> corresponding TPM PCR data) to know the above.
>
> Time t0 => State of the system measured
> Time tn => State changed and the new state measured
> Time tm => State changed again and the new state measured.
>
> Say, the measurement at "Time tn" was an illegal change, the appraiser
> would know.
>
> >
> > I applied the patch series on top of the next-integrity branch, added
> > measure func=LSM_STATE to ima-policy, and booted that kernel.  I get
> > the following entries in ascii_runtime_measurements, but seemingly
> > missing the final field:
> >
> > 10 8a09c48af4f8a817f59b495bd82971e096e2e367 ima-ng
> > sha256:21c3d7b09b62b4d0b3ed15ba990f816b94808f90b76787bfae755c4b3a44cd24
> > selinux-state
> > 10 e610908931d70990a2855ddb33c16af2d82ce56a ima-ng
> > sha256:c8898652afd5527ef4eaf8d85f5fee1d91fcccee34bc97f6e55b96746bedb318
> > selinux-policy-hash
> >
> > Thus, I cannot verify. What am I missing?
> >
>
> Looks like the template used is ima-ng which doesn't include the
> measured buffer. Please set template to "ima-buf" in the policy.
>
> For example,
> measure func=LSM_STATE template=ima-buf

It seems like one shouldn't need to manually specify it if it is the
only template that yields a useful result for the LSM_STATE function?