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

From: Lakshmi Ramasubramanian
Date: Wed Aug 05 2020 - 12:27:31 EST

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:

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

     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.


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

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