Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA critical data hook

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

On 1/12/21 8:27 AM, Paul Moore wrote:
> On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi
> <tusharsu@linux.microsoft.com> wrote:
>> From: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
>>
>> 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>
>> Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
>> ---
>>   Documentation/ABI/testing/ima_policy |  3 +-
>>   security/selinux/Makefile            |  2 +
>>   security/selinux/ima.c               | 64 ++++++++++++++++++++++++++++
>>   security/selinux/include/ima.h       | 24 +++++++++++
>>   security/selinux/include/security.h  |  3 +-
>>   security/selinux/ss/services.c       | 64 ++++++++++++++++++++++++----
>>   6 files changed, 149 insertions(+), 11 deletions(-)
>>   create mode 100644 security/selinux/ima.c
>>   create mode 100644 security/selinux/include/ima.h
> 
> I remain concerned about the possibility of bypassing a measurement by
> tampering with the time, but I appear to be the only one who is
> worried about this so I'm not going to block this patch on those
> grounds.
> 
> Acked-by: Paul Moore <paul@paul-moore.com>

Thanks Paul.

  -lakshmi