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

Stephen Smalley
2020-08-05T19:51:33Z

On 8/5/20 11:07 AM, Tyler Hicks wrote:

> On 2020-08-05 10:27:43, Stephen Smalley wrote:
>> On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> On Wed, 2020-08-05 at 09:03 -0400, Stephen Smalley wrote:
>>>> On Wed, Aug 5, 2020 at 8:57 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>> On Wed, 2020-08-05 at 08:46 -0400, Stephen Smalley wrote:
>>>>>> 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.
>>>>> Thank you for the clarification. Could the policy hash be included
>>>>> with the other critical data? Does it really need to be measured
>>>>> independently?
>>>> They were split into two separate functions because we wanted to be
>>>> able to support using different templates for them (ima-buf for the
>>>> state variables so that the measurement includes the original buffer,
>>>> which is small and relatively fixed-size, and ima-ng for the policy
>>>> because it is large and we just want to capture the hash for later
>>>> comparison against known-good). Also, the state variables are
>>>> available for measurement always from early initialization, whereas
>>>> the policy is only available for measurement once we have loaded an
>>>> initial policy.
>>> Ok, measuring the policy separately from other critical data makes
>>> sense. Instead of measuring the policy, which is large, measure the
>>> policy hash.
>> I think that was the original approach. However, I had concerns with
>> adding code to SELinux to compute a hash over the policy versus
>> leaving that to IMA's existing policy and mechanism. If that's
>> preferred I guess we can do it that way but seems less flexible and
>> duplicative.
> In AppArmor, we store the sha1 of the raw policy as the policy is
> loaded. The hash is exposed to userspace in apparmorfs. See commit
> 5ac8c355ae00 ("apparmor: allow introspecting the loaded policy pre
> internal transform").
>
> It has proved useful as a mechanism for debugging as sometimes the
> on-disk policy doesn't match the loaded policy and this can be a good
> way to check that while providing support to users. John also mentions
> checkpoint/restore in the commit message and I could certainly see how
> the policy hashes would be useful in that scenario.
>
> When thinking through how Lakshmi's series could be extended for
> AppArmor support, I was thinking that the AppArmor policy measurement
> would be a measurement of these hashes that we already have in place.
>
> Perhaps there's some general usefulness in storing/exposing an SELinux
> policy hash rather than only seeing it as duplicative property required
> this measurement series?

That would be a hash of the policy file that was last loaded via the
selinuxfs interface for loading policy, not a hash of the in-memory
policy data structures at the time of measurement (which is what this
patch series is implementing).  The duplicative part is with respect to
selecting a hash algorithm and hashing the in-memory policy as part of
the SELinux code rather than just passing the policy buffer to IMA for
measurement like any other buffer.  Userspace can already hash the
in-memory policy data itself by running sha256sum or whatever on
/sys/fs/selinux/policy, so we don't need to save or expose that separately.