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

From: Casey Schaufler
Date: Wed Aug 05 2020 - 12:32:09 EST

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?

> 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
>      bool
>      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.

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

> But like I said above, with minimal change in IMA other security modules can be supported to measure STATE and POLICY data.
>  -lakshmi