linux-kernel - Re: [PATCH v6 1/4] IMA: Add func to measure LSM state and policy

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Date:   Wed, 5 Aug 2020 08:46:00 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        casey@...aufler-ca.com
Cc:     tyhicks@...ux.microsoft.com, sashal@...nel.org, jmorris@...ei.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/4] IMA: Add func to measure LSM state and policy

On 8/4/20 11:25 PM, Mimi Zohar wrote:

> Hi Lakshmi,
>
> There's still  a number of other patch sets needing to be reviewed
> before my getting to this one.  The comment below is from a high level.
>
> On Tue, 2020-08-04 at 17:43 -0700, Lakshmi Ramasubramanian wrote:
>> Critical data structures of security modules need to be measured to
>> enable an attestation service to verify if the configuration and
>> policies for the security modules have been setup correctly and
>> that they haven't been tampered with at runtime. A new IMA policy is
>> required for handling this measurement.
>>
>> Define two new IMA policy func namely LSM_STATE and LSM_POLICY to
>> measure the state and the policy provided by the security modules.
>> Update ima_match_rules() and ima_validate_rule() to check for
>> the new func and ima_parse_rule() to handle the new func.
> I can understand wanting to measure the in kernel LSM memory state to
> make sure it hasn't changed, but policies are stored as files.  Buffer
> measurements should be limited  to those things that are not files.
>
> Changing how data is passed to the kernel has been happening for a
> while.  For example, instead of passing the kernel module or kernel
> image in a buffer, the new syscalls - finit_module, kexec_file_load -
> pass an open file descriptor.  Similarly, instead of loading the IMA
> policy data, a pathname may be provided.
>
> Pre and post security hooks already exist for reading files.   Instead
> of adding IMA support for measuring the policy file data, update the
> mechanism for loading the LSM policy.  Then not only will you be able
> to measure the policy, you'll also be able to require the policy be
> signed.

To clarify, the policy being measured by this patch series is a 
serialized representation of the in-memory policy data structures being 
enforced by SELinux.  Not the file that was loaded.  Hence, this 
measurement would detect tampering with the in-memory policy data 
structures after the policy has been loaded.  In the case of SELinux, 
one can read this serialized representation via /sys/fs/selinux/policy.  
The result is not byte-for-byte identical to the policy file that was 
loaded but can be semantically compared via sediff and other tools to 
determine whether it is equivalent.