Re: [PATCH v5 7/7] selinux: measure state and hash of the policy using IMA

[Mimi Zohar]

Hi Lakshmi, Tushar,

On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
> From: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>
> 
> Critical data structures of security modules are currently not measured.

Right, this patch set adds support for measuring kernel integrity
critical data.  Why is this statement needed?

> Therefore an attestation service, for instance, would not be able to
> attest whether the security modules are always operating with the policies
> and configurations that the system administrator had setup. The policies
> and configurations for the security modules could be tampered by rogue
> user mode agents or modified through some inadvertent actions on
> the system. Measuring such critical data would enable an attestation
> service to reliably assess the security configuration of the system.

Please rewrite this paragraph from the perspective of measuring files
and other buffer data is not sufficient, providing an explanation why
it is isn't sufficient.

This is a rather large patch.   I'm surprised that it isn't broken up
into two patches, one that measures SELinux policy data and another
which measures the state, but that is Stephen's and Paul's call.

Mimi


> SELinux configuration and policy are some of the critical data for this
> security module that need to be measured. This measurement can be used
> by an attestation service, for instance, to verify if the configurations
> and policies have been setup correctly and that they haven't been
> tampered at run-time.
> 
> Measure SELinux configurations, policy capabilities settings, and
> 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.
> 
> Add "selinux" to the list of supported data sources maintained by IMA
> to enable measuring SELinux data.
> 
> 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 data_sources=selinux template=ima-buf
> 
> Sample measurement of SELinux state and hash of the policy:
> 
> 10 e32e...5ac3 ima-buf sha256:86e8...4594 selinux-state-1595389364:287899386 696e697469616c697a65643d313b656e61626c65643d313b656e666f7263696e673d303b636865636b72657170726f743d313b6e6574776f726b5f706565725f636f6e74726f6c733d313b6f70656e5f7065726d733d313b657874656e6465645f736f636b65745f636c6173733d313b616c776179735f636865636b5f6e6574776f726b3d303b6367726f75705f7365636c6162656c3d313b6e6e705f6e6f737569645f7472616e736974696f6e3d313b67656e66735f7365636c6162656c5f73796d6c696e6b733d303
> 10 9e81...0857 ima-buf sha256:4941...68fc selinux-policy-hash-1597335667:462051628 8d1d...1834
> 
> To verify the measurement check the following:
> 
> Execute the following command to extract the measured data
> from the IMA log for SELinux configuration (selinux-state).
> 
>   grep "selinux-state" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6 | xxd -r -p
> 
> The output should be the list of key-value pairs. For example,
>  initialized=1;enabled=1;enforcing=0;checkreqprot=1;network_peer_controls=1;open_perms=1;extended_socket_class=1;always_check_network=0;cgroup_seclabel=1;nnp_nosuid_transition=1;genfs_seclabel_symlinks=0;
> 
> To verify the measured data with the current SELinux state:
> 
>  => enabled should be set to 1 if /sys/fs/selinux folder exists,
>     0 otherwise
> 
> For other entries, compare the integer value in the files
>  => /sys/fs/selinux/enforce
>  => /sys/fs/selinux/checkreqprot
> And, each of the policy capabilities files under
>  => /sys/fs/selinux/policy_capabilities
> 
> Note that the actual verification would be against an expected state
> and done on a system other than the measured system, typically
> requiring "initialized=1; enabled=1;enforcing=1;checkreqprot=0;" for
> a secure state and then whatever policy capabilities are actually set
> in the expected policy (which can be extracted from the policy itself
> via seinfo, for example).
> 
> For selinux-policy-hash, the hash of SELinux policy is included
> in the IMA log entry.
> 
> 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@xxxxxxxxxxxxxxxxxxx>
> Suggested-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>

Please reverse the order of the tags.

thanks,

Mimi