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

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Date:   Mon, 20 Jul 2020 10:34:06 -0700
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Stephen Smalley <stephen.smalley.work@...il.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 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

thanks,
  -lakshmi