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From:   Stephen Smalley <stephen.smalley.work@gmail.com>
Date:   Mon, 24 Aug 2020 10:00:07 -0400
Message-ID: <CAEjxPJ5Kok-TBfS=XQ+NUC5tuaZRkyLBOawG4UDky51_bsMnGw@mail.gmail.com>
Subject: Re: [PATCH] SELinux: Measure state and hash of policy using IMA
To:     Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Cc:     Mimi Zohar <zohar@linux.ibm.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        tusharsu@linux.microsoft.com, sashal@kernel.org,
        James Morris <jmorris@namei.org>,
        linux-integrity@vger.kernel.org,
        SElinux list <selinux@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, Aug 21, 2020 at 9:00 PM Lakshmi Ramasubramanian
<nramas@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 policie=
s
> 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 tampere=
d
> 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 polic=
y
> 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-1595389364:287899=
386 696e697469616c697a65643d313b656e61626c65643d313b656e666f7263696e673d303=
b636865636b72657170726f743d313b6e6574776f726b5f706565725f636f6e74726f6c733d=
313b6f70656e5f7065726d733d313b657874656e6465645f736f636b65745f636c6173733d3=
13b616c776179735f636865636b5f6e6574776f726b3d303b6367726f75705f7365636c6162=
656c3d313b6e6e705f6e6f737569645f7472616e736974696f6e3d313b67656e66735f73656=
36c6162656c5f73796d6c696e6b733d303
> 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_runt=
ime_measurements | cut -d' ' -f 6 | xxd -r -p
>
> The output should be the list of key-value pairs. For example,
>  initialized=3D1;enabled=3D1;enforcing=3D0;checkreqprot=3D1;network_peer_=
controls=3D1;open_perms=3D1;extended_socket_class=3D1;always_check_network=
=3D0;cgroup_seclabel=3D1;nnp_nosuid_transition=3D1;genfs_seclabel_symlinks=
=3D0;
>
> To verify the measured data with the current SELinux state:
>
>  =3D> enabled should be set to 1 if /sys/fs/selinux folder exists,
>     0 otherwise
>
> For other entries, compare the integer value in the files
>  =3D> /sys/fs/selinux/enforce
>  =3D> /sys/fs/selinux/checkreqprot
> And, each of the policy capabilities files under
>  =3D> /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/asci=
i_runtime_measurements | cut -d' ' -f 6
>
> This patch is dependent on the following patch series:
>         https://patchwork.kernel.org/patch/11709527/
>         https://patchwork.kernel.org/patch/11730193/
>         https://patchwork.kernel.org/patch/11730757/
>
> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
> Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com> # error: implicit declarat=
ion of function 'vfree'
> Reported-by: kernel test robot <lkp@intel.com> # error: implicit declarat=
ion of function 'crypto_alloc_shash'
> Reported-by: kernel test robot <lkp@intel.com> # sparse: symbol 'security=
_read_selinux_policy' was not declared. Should it be static?
> ---

> +int security_read_policy_kernel(struct selinux_state *state,
> +                               void **data, size_t *len)
> +{
> +       int rc;
> +
> +       rc =3D security_read_policy_len(state, len);
> +       if (rc)
> +               return rc;
> +
> +       *data =3D vmalloc(*len);
> +       if (!*data)
> +               return -ENOMEM;
>
> +       return security_read_selinux_policy(state, data, len);
>  }

See the discussion here:
https://lore.kernel.org/selinux/20200824113015.1375857-1-omosnace@redhat.co=
m/T/#t

In order for this to be safe, you need to ensure that all callers of
security_read_policy_kernel() have taken fsi->mutex in selinuxfs and
any use of security_read_policy_len() occurs while holding the mutex.
Otherwise, the length can change between security_read_policy_len()
and security_read_selinux_policy() if policy is reloaded.