[PATCH v3] selinux: measure state and policy capabilities

[Paul Moore]

On Fri, Feb 12, 2021 at 11:37 AM Lakshmi Ramasubramanian
<nramas at linux.microsoft.com> wrote:
>
> SELinux stores the configuration state and the policy capabilities
> in kernel memory.  Changes to this data at runtime would have an impact
> on the security guarantees provided by SELinux.  Measuring this data
> through IMA subsystem provides a tamper-resistant way for
> an attestation service to remotely validate it at runtime.
>
> Measure the configuration state and policy capabilities by calling
> the IMA hook ima_measure_critical_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-5.11.0-rc3+ 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 SELinux state and policy capabilities:
>
> 10 2122...65d8 ima-buf sha256:13c2...1292 selinux-state 696e...303b
>
> Execute the following command to extract the measured data
> from the IMA's runtime measurements list:
>
>   grep "selinux-state" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6 | xxd -r -p
>
> The output should be a list of key-value pairs. For example,
>  initialized=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 measurement is consistent with the current SELinux state
> reported on the system, compare the integer values in the following
> files with those set in the IMA measurement (using the following commands):
>
>  - cat /sys/fs/selinux/enforce
>  - cat /sys/fs/selinux/checkreqprot
>  - cat /sys/fs/selinux/policy_capabilities/[capability_file]
>
> Note that the actual verification would be against an expected state
> and done on a separate system (likely an attestation server) requiring
> "initialized=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).
>
> Signed-off-by: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
> Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> Suggested-by: Paul Moore <paul at paul-moore.com>
> ---
>  security/selinux/ima.c         | 87 ++++++++++++++++++++++++++++++++--
>  security/selinux/include/ima.h |  6 +++
>  security/selinux/selinuxfs.c   |  6 +++
>  security/selinux/ss/services.c |  2 +-
>  4 files changed, 96 insertions(+), 5 deletions(-)

This draft seems fine to me, but there is a small logistical blocker
at the moment which means I can't merge this until -rc2 is released,
which likely means this coming Monday.  The problem is that this patch
relies on code that went upstream via in the last merge window via the
IMA tree, not the SELinux tree; normally that wouldn't be a problem as
I typically rebase the selinux/next to Linus' -rc1 tag once the merge
window is closed, but in this particular case the -rc1 tag is
dangerously broken for some system configurations (the tag has since
been renamed) so I'm not rebasing onto -rc1 this time around.

Assuming that -rc2 fixes the swapfile/fs-corruption problem, early
next week I'll rebase selinux/next to -rc2 and merge this patch.
However, if the swapfile bug continues past -rc2 we can consider
merging this via the IMA tree, but I'd assume not do that if possible
due to merge conflict and testing reasons.

-- 
paul moore
www.paul-moore.com