[PATCH] SELinux: Measure state and hash of policy using IMA

[Stephen Smalley]

On Tue, Sep 8, 2020 at 8:28 AM Stephen Smalley
<stephen.smalley.work at gmail.com> wrote:
>
> On Mon, Sep 7, 2020 at 5:39 PM Lakshmi Ramasubramanian
> <nramas at linux.microsoft.com> wrote:
> >
> > Critical data structures of security modules are currently not measured.
> > Therefore an attestation service, for instance, would not be able to
> > attest whether the security modules are always operating with the policies
> > and configuration that the system administrator had setup. The policies
> > and configuration for the security modules could be tampered with 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.
> >
> > SELinux configuration and policy are some of the critical data for this
> > security module that needs to be measured. This measurement can be used
> > by an attestation service, for instance, to verify if the configuration
> > and policies have been setup correctly and that they haven't been tampered
> > with at runtime.
> >
> > Measure SELinux configuration, 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 quite large, hash of the policy
> > is measured instead of the entire policy to avoid bloating the IMA log.
> >
> > Enable early boot measurement for SELinux in IMA since SELinux
> > initializes its state and policy before custom IMA policy is loaded.
> >
> > 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 -m 1 "selinux-state" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | 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
> >
> > 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 -m 1 "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | cut -d' ' -f 6
> >
> > This patch is based on commit 66ccd2560aff ("selinux: simplify away security_policydb_len()")
> > in "next" branch in https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/selinux.git
> >
> > This patch is dependent on the following patch series and must be
> > applied in the given order:
> >         https://patchwork.kernel.org/patch/11709527/
> >         https://patchwork.kernel.org/patch/11730193/
> >         https://patchwork.kernel.org/patch/11730757/
> >
> > Signed-off-by: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
> > Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> > ---
> >
> > diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
> > index 953314d145bb..9bf0f65d720b 100644
> > --- a/security/integrity/ima/Kconfig
> > +++ b/security/integrity/ima/Kconfig
> > @@ -324,8 +324,7 @@ config IMA_MEASURE_ASYMMETRIC_KEYS
> >
> >  config IMA_QUEUE_EARLY_BOOT_DATA
> >         bool
> > -       depends on IMA_MEASURE_ASYMMETRIC_KEYS
> > -       depends on SYSTEM_TRUSTED_KEYRING
> > +       depends on (IMA_MEASURE_ASYMMETRIC_KEYS && SYSTEM_TRUSTED_KEYRING) || SECURITY_SELINUX
> >         default y
>
> I don't see why this is necessary or desirable.  We should avoid
> leaking dependencies on a single security module into other
> subsystems.
> It might not yet fully support other security modules but we shouldn't
> preclude adding that in the future.
> Up to the IMA maintainer but I would recommend dropping this part.

Sorry, I misread this part; it doesn't make IMA depend on SELinux it
just allows enabling this early boot data feature if SELinux is
enabled since SELinux is a user of it.  Still, it seems unfortunate to
have to explicitly enumerate each consumer of this facility here
whenever a new one is introduced.  Is there a reason for doing so and
not just allowing it to be enabled as long as the things on which it
depends are enabled (i.e. not based on its consumers)?