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

Stephen Smalley
2020-08-05T19:57:53Z

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.