Re: [PATCH] selinux: include a consumer of the new IMA critical data hook

[Lakshmi Ramasubramanian <nramas@linux.microsoft.com>]

On 1/14/21 11:58 AM, Tyler Hicks wrote:
> On 2021-01-14 14:29:09, Paul Moore wrote:
>> On Thu, Jan 14, 2021 at 2:15 PM Lakshmi Ramasubramanian
>> <nramas@linux.microsoft.com> wrote:
>>>
>>> SELinux stores the active policy in memory, so the changes to this data
>>> at runtime would have an impact on the security guarantees provided
>>> by SELinux.  Measuring in-memory SELinux policy through IMA subsystem
>>> provides a secure way for the attestation service to remotely validate
>>> the policy contents at runtime.
>>>
>>> Measure the hash of the loaded policy by calling the IMA hook
>>> ima_measure_critical_data().  Since the size of the loaded policy
>>> can be large (several MB), measure the hash of the policy instead of
>>> the entire policy to avoid bloating the IMA log entry.
>>>
>>> To enable SELinux data measurement, the following steps are required:
>>>
>>> 1, Add "ima_policy=critical_data" to the kernel command line arguments
>>>     to enable measuring SELinux data at boot time.
>>> For example,
>>>    BOOT_IMAGE=/boot/vmlinuz-5.10.0-rc1+ root=UUID=fd643309-a5d2-4ed3-b10d-3c579a5fab2f ro nomodeset security=selinux ima_policy=critical_data
>>>
>>> 2, Add the following rule to /etc/ima/ima-policy
>>>     measure func=CRITICAL_DATA label=selinux
>>>
>>> Sample measurement of the hash of SELinux policy:
>>>
>>> To verify the measured data with the current SELinux policy run
>>> the following commands and verify the output hash values match.
>>>
>>>    sha256sum /sys/fs/selinux/policy | cut -d' ' -f 1
>>>
>>>    grep "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6
>>>
>>> Note that the actual verification of SELinux policy would require loading
>>> the expected policy into an identical kernel on a pristine/known-safe
>>> system and run the sha256sum /sys/kernel/selinux/policy there to get
>>> the expected hash.
>>>
>>> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
>>> Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
>>> Acked-by: Paul Moore <paul@paul-moore.com>
>>> Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
>>> ---
>>>   Documentation/ABI/testing/ima_policy |  3 +-
>>>   security/selinux/Makefile            |  2 +
>>>   security/selinux/ima.c               | 44 +++++++++++++++++++
>>>   security/selinux/include/ima.h       | 24 +++++++++++
>>>   security/selinux/include/security.h  |  3 +-
>>>   security/selinux/ss/services.c       | 64 ++++++++++++++++++++++++----
>>>   6 files changed, 129 insertions(+), 11 deletions(-)
>>>   create mode 100644 security/selinux/ima.c
>>>   create mode 100644 security/selinux/include/ima.h
>>
>> I think this has changed enough that keeping the "Acked-by" and
>> "Reviewed-by" tags is probably not a good choice.  I took a quick look
>> and this still looks okay from a SELinux perspective, I'll leave Mimi
>> to comment on it from a IMA perspective.

Thanks for reviewing the change Paul.

>>
>> Unless Tyler has reviewed this version prior to your posting, it might
>> be a good idea to remove his "Reviewed-by" unless he has a chance to
>> look this over again before it is merged.
> 
> Thanks for calling this out. I hadn't reviewed it prior to the posting
> but I was keeping an eye on the thread.
> 
> This new revision still looks good to me and I like the idea of
> controlling re-measurements via policy. So,
> 
>   Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>

Thanks for the quick response Tyler.

  -lakshmi