Re: [PATCH v2] selinux: measure state and policy capabilities

[Paul Moore]

On Fri, Jan 29, 2021 at 11:49 AM Lakshmi Ramasubramanian
<nramas@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> Suggested-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
> Suggested-by: Paul Moore <paul@xxxxxxxxxxxxxx>
> ---
>  security/selinux/ima.c         | 77 ++++++++++++++++++++++++++++++++--
>  security/selinux/include/ima.h |  6 +++
>  security/selinux/selinuxfs.c   |  6 +++
>  security/selinux/ss/services.c |  2 +-
>  4 files changed, 86 insertions(+), 5 deletions(-)
>
> diff --git a/security/selinux/ima.c b/security/selinux/ima.c
> index 03715893ff97..5c7f73cd1117 100644
> --- a/security/selinux/ima.c
> +++ b/security/selinux/ima.c
> @@ -13,18 +13,73 @@
>  #include "ima.h"
>
>  /*
> - * selinux_ima_measure_state - Measure hash of the SELinux policy
> + * selinux_ima_collect_state - Read selinux configuration settings
>   *
> - * @state: selinux state struct
> + * @state: selinux_state
>   *
> - * NOTE: This function must be called with policy_mutex held.
> + * On success returns the configuration settings string.
> + * On error, returns NULL.
>   */
> -void selinux_ima_measure_state(struct selinux_state *state)
> +static char *selinux_ima_collect_state(struct selinux_state *state)
> +{
> +       const char *on = "=1;", *off = "=0;";
> +       char *buf;
> +       int buf_len, i;
> +
> +       /*
> +        * Size of the following string including the terminating NULL char
> +        *    initialized=0;enforcing=0;checkreqprot=0;
> +        */
> +       buf_len = 42;

It might be safer over the long term, and self-documenting, to do the
following instead:

  buf_len = strlen("initialized=0;enforcing=0;checkreqprot=0;") + 1;

> +       for (i = 0; i < __POLICYDB_CAPABILITY_MAX; i++)
> +               buf_len += strlen(selinux_policycap_names[i]) + 3;

's/3/strlen(on)/' or is that too much?

> +
> +       buf = kzalloc(buf_len, GFP_KERNEL);
> +       if (!buf)
> +               return NULL;
> +
> +       strscpy(buf, "initialized", buf_len);

I wonder if it might be a good idea to add a WARN_ON() to the various
copies, e.g.:

  rc = strXXX(...);
  WARN_ON(rc);

The strscpy/strlcat protections should ensure that nothing terrible
happens with respect to wandering off the end of the string, or
failing to NUL terminate, but they won't catch a logic error where the
string is not allocated correctly (resulting in a truncated buffer).

> +       strlcat(buf, selinux_initialized(state) ? on : off, buf_len);
> +
> +       strlcat(buf, "enforcing", buf_len);
> +       strlcat(buf, enforcing_enabled(state) ? on : off, buf_len);
> +
> +       strlcat(buf, "checkreqprot", buf_len);
> +       strlcat(buf, checkreqprot_get(state) ? on : off, buf_len);
> +
> +       for (i = 0; i < __POLICYDB_CAPABILITY_MAX; i++) {
> +               strlcat(buf, selinux_policycap_names[i], buf_len);
> +               strlcat(buf, state->policycap[i] ? on : off, buf_len);
> +       }
> +
> +       return buf;
> +}

-- 
paul moore
www.paul-moore.com