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Subject: Re: [PATCH v6 0/4] LSM: Measure security module data
To:     Tyler Hicks <tyhicks@linux.microsoft.com>,
        Casey Schaufler <casey@schaufler-ca.com>
Cc:     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
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 <c7c168f2-e30b-d2c5-abcb-1b6919197474@schaufler-ca.com>
 <20200805154504.GB4365@sequoia>
From:   Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Message-ID: <7c7a076b-6ba7-2e8d-409a-b3b4e4738c41@linux.microsoft.com>
Date:   Wed, 5 Aug 2020 09:07:48 -0700
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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?

When both the security modules are enabled, IMA policy validator would 
look like below:

if (IS_ENABLED(CONFIG_SECURITY_SELINUX) ||
     IS_ENABLED(CONFIG_SECURITY_APPARMOR)) &&
     strcmp(args[0].from, "LSM_STATE") == 0)
                 entry->func = LSM_STATE;

Similar one for LSM_POLICY validation.

Both SELinux and AppArmor can call ima_measure_lsm_state() and 
ima_measure_lsm_policy() to measure state and policy respectively.

I don't think we should go the route of defining IMA hooks per security 
module (i.e., SELINUX_STATE, APPARMOR_STATE, SELINUX_POLICY, etc.) 
Instead keep the hook generic that any SM could use - which is what I 
have tried to address in this patch series.

>>
>>>
>>> Please see Patch 1/4
>>>
>>> +            else if (IS_ENABLED(CONFIG_SECURITY_SELINUX) &&
>>> +                 strcmp(args[0].from, "LSM_STATE") == 0)
>>> +                entry->func = LSM_STATE;
>>> +            else if (IS_ENABLED(CONFIG_SECURITY_SELINUX) &&
>>> +                 strcmp(args[0].from, "LSM_POLICY") == 0)
>>> +                entry->func = LSM_POLICY;
>>>
>>> And, if early boot measurement is needed for AppArmor the following change in IMA's Kconfig
>>>
>>> Patch 4/4
>>>
>>> +config IMA_QUEUE_EARLY_BOOT_DATA
>>>       bool
>>> +    depends on SECURITY_SELINUX || (IMA_MEASURE_ASYMMETRIC_KEYS && SYSTEM_TRUSTED_KEYRING)
>>>       default y
>>>
>>> If you think calling this an "LSM feature" is not appropriate, please suggest a better phrase.
>>
>> In the code above you are under CONFIG_SECURITY_SELINUX.
>> I would suggest that it's an SELinux feature, so you should
>> be using SELINUX_STATE and SELINUX_POLICY, as I suggested
>> before. Just because SELinux has state and policy to measure
>> doesn't mean that a different module might not have other data,
>> such as history, that should be covered as well.

Good point - if other SMs have data besides state and policy, we can 
define IMA hooks to measure that as well.

But I still think it is not the right approach to call this 
SELINUX_STATE and SELINUX_POLICY - it will lead to unnecessary code 
duplication in IMA as more SMs are onboarded, in my opinion. Correct me 
if I am wrong.

  -lakshmi

> 
> In addition to SELINUX_STATE and SELINUX_POLICY, we should also consider
> the proposed LSM_STATE and LSM_POLICY func values but require an "lsm"
> rule conditional.
> 
> So the current proposed rules:
> 
>   measure func=LSM_STATE
>   measure func=LSM_POLICY
> 
> Would become:
> 
>   measure func=LSM_STATE lsm=selinux
>   measure func=LSM_POLICY lsm=selinux
> 
> The following rules would be rejected:
> 
>   measure func=LSM_STATE
>   measure func=LSM_POLICY
>   measure func=LSM_STATE lsm=apparmor
>   measure func=LSM_POLICY lsm=smack
> 
> Of course, the apparmor and smack rules could/would be allowed when
> proper support is in place.
> 

> 
>>
>> I realize that IMA already has compile time dependencies to
>> determine which xattrs to measure. There's no reason that
>> the xattr list couldn't be determined at boot time, with
>> each security module providing the XATTR_NAME values it
>> uses.
>>
>>>
>>> But like I said above, with minimal change in IMA other security modules can be supported to measure STATE and POLICY data.
>>>
>>>   -lakshmi
>>>
>>>