Re: [PATCH v6 0/4] LSM: Measure security module data

From: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
To: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>,
	zohar@linux.ibm.com, stephen.smalley.work@gmail.com,
	sashal@kernel.org, jmorris@namei.org,
	linux-integrity@vger.kernel.org, selinux@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 0/4] LSM: Measure security module data
Date: Wed, 5 Aug 2020 09:21:24 -0700	[thread overview]
Message-ID: <ae80581d-a34c-51f4-d4f9-94c1e341fd15@linux.microsoft.com> (raw)
In-Reply-To: <20200805161449.GC4365@sequoia>

On 8/5/20 9:14 AM, Tyler Hicks wrote:
> On 2020-08-05 09:07:48, Lakshmi Ramasubramanian wrote:
>> On 8/5/20 8:45 AM, Tyler Hicks wrote:
>>> On 2020-08-05 08:36:40, Casey Schaufler wrote:
>>>> On 8/4/2020 6:14 PM, Lakshmi Ramasubramanian wrote:
>>>>> On 8/4/20 6:04 PM, Casey Schaufler wrote:
>>>>>> On 8/4/2020 5:43 PM, Lakshmi Ramasubramanian wrote:
>>>>>>> Critical data structures of security modules are currently not measured.
>>>>>>> Therefore an attestation service, for instance, would not be able to
>>>>>>> attest whether the security modules are always operating with the policies
>>>>>>> and configuration that the system administrator had setup. The policies
>>>>>>> and configuration for the security modules could be tampered with by
>>>>>>> malware by exploiting kernel vulnerabilities or modified through some
>>>>>>> inadvertent actions on the system. Measuring such critical data would
>>>>>>> enable an attestation service to better assess the state of the system.
>>>>>>
>>>>>> I still wonder why you're calling this an LSM change/feature when
>>>>>> all the change is in IMA and SELinux. You're not putting anything
>>>>>> into the LSM infrastructure, not are you using the LSM infrastructure
>>>>>> to achieve your ends. Sure, you *could* support other security modules
>>>>>> using this scheme, but you have a configuration dependency on
>>>>>> SELinux, so that's at best going to be messy. If you want this to
>>>>>> be an LSM "feature" you need to use the LSM hooking mechanism.
>>>>>
>>>>>>
>>>>>> I'm not objecting to the feature. It adds value. But as you've
>>>>>> implemented it it is either an IMA extension to SELinux, or an
>>>>>> SELiux extension to IMA. Could AppArmor add hooks for this without
>>>>>> changing the IMA code? It doesn't look like it to me.
>>>>>
>>>>> The check in IMA to allow the new IMA hook func LSM_STATE and LSM_POLICY when SELinux is enabled is just because SELinux is the only security module using these hooks now.
>>>>>
>>>>> To enable AppArmor, for instance, to use the new IMA hooks to measure state and policy would just require adding the check for CONFIG_SECURITY_APPARMOR. Other than that, there are no IMA changes needed to support AppArmor or other such security modules.
>>>>
>>>> This is exactly what I'm objecting to. What if a system has both SELinux
>>>> and AppArmor compiled in? What if it has both enabled?
>>>
>>> The SELinux state and policy would be measured but the AppArmor
>>> state/policy would be silently ignored. This isn't ideal as the IMA
>>> policy author would need to read the kernel code to figure out which
>>> LSMs are going to be measured.
>>
>> Tyler - I am not sure why AppArmor state\policy would be ignored when both
>> SELinux and AppArmor are enabled. Could you please clarify?
> 
> I think Casey is talking about now (when AppArmor is not supported by
> this feature) and you're talking about the future (when AppArmor may be
> supported by this feature).

Got it - thanks for clarifying.

But with the current code if SELinux is enabled on the system, but 
AppArmor is not and the IMA policy contains "measure func=LSM_STATE" 
then the policy will be rejected as "-EINVAL".
So the policy author would get a feedback even now.
Correct me if I am wrong.

  -lakshmi