[PATCH v3 4/5] LSM: Define SELinux function to measure security state

[Stephen Smalley]

On Fri, Jul 17, 2020 at 6:28 PM Lakshmi Ramasubramanian
<nramas at linux.microsoft.com> wrote:
>
> SELinux configuration and policy are some of the critical data for this
> security module that needs to be measured. To enable this measurement
> SELinux needs to implement the interface function,
> security_measure_data(), that the LSM can call.
>
> Define the security_measure_data() function in SELinux to measure SELinux
> configuration and policy. Call this function to measure SELinux data
> when there is a change in the security module's state.
>
> Sample measurement of SELinux state and hash of the policy:
>
> 10 e32e...5ac3 ima-buf sha256:86e8...4594 selinux-state 656e61626c65643d313b656e666f7263696e673d303b636865636b72657170726f743d313b6e6574706565723d313b6f70656e7065726d3d313b657874736f636b636c6173733d313b616c776179736e6574776f726b3d303b6367726f75707365636c6162656c3d313b6e6e706e6f737569647472616e736974696f6e3d313b67656e66737365636c6162656c73796d6c696e6b3d303b
> 10 f4a7...9408 ima-buf sha256:4941...68fc selinux-policy-hash 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).
>
>   cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements | grep -m 1 "selinux-state" | cut -d' ' -f 6 | xxd -r -p
>
> The output should be the list of key-value pairs. For example,
>  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
>
> The data for selinux-policy-hash is the SHA256 hash of SELinux policy.
>
> 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
>
>   cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements | grep -m 1 "selinux-policy-hash" | cut -d' ' -f 6
>
> Signed-off-by: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
> Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> ---

> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> new file mode 100644
> index 000000000000..659011637ae7
> --- /dev/null
> +++ b/security/selinux/measure.c
> @@ -0,0 +1,155 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Measure SELinux state using IMA subsystem.
> + */
> +#include <linux/ima.h>
> +#include "security.h"
> +
> +/* Pre-allocated buffer used for measuring state */
> +static char *selinux_state_string;
> +static size_t selinux_state_string_len;
> +static char *str_format = "%s=%d;";
> +static int selinux_state_count;
> +
> +void __init selinux_init_measurement(void)
> +{
> +       int i;
> +
> +       /*
> +        * enabled
> +        * enforcing
> +        * checkreqport

checkreqprot (spelling)

What about initialized?  Or do you consider that to be implicitly
true/1 else we wouldn't be taking a measurement?  Only caveat there is
that it provides one more means of disabling measurements (at the same
time as disabling enforcement) by setting it to false/0 via kernel
write flaw.

> +        * All policy capability flags
> +        */
> +       selinux_state_count = 3 + __POLICYDB_CAPABILITY_MAX;
> +
> +       selinux_state_string_len = snprintf(NULL, 0, str_format,
> +                                           "enabled", 0);
> +       selinux_state_string_len += snprintf(NULL, 0, str_format,
> +                                            "enforcing", 0);
> +       selinux_state_string_len += snprintf(NULL, 0, str_format,
> +                                            "checkreqprot", 0);
> +       for (i = 3; i < selinux_state_count; i++) {
> +               selinux_state_string_len +=
> +                       snprintf(NULL, 0, str_format,
> +                                selinux_policycap_names[i-3], 0);
> +       }

What's the benefit of this pattern versus just making the loop go from
0 to __POLICYDB_CAPABILITY_MAX and using selinux_policycap_names[i]?

> +void selinux_measure_state(struct selinux_state *selinux_state)
> +{
> +       void *policy = NULL;
> +       void *policy_hash = NULL;
> +       size_t curr, buflen;
> +       int i, policy_hash_len, rc = 0;
> +
> +       if (!selinux_initialized(selinux_state)) {
> +               pr_warn("%s: SELinux not yet initialized.\n", __func__);
> +               return;
> +       }

We could measure the global state variables before full SELinux
initialization (i.e. policy load).
Only the policy hash depends on having loaded the policy.

> +
> +       if (!selinux_state_string) {
> +               pr_warn("%s: Buffer for state not allocated.\n", __func__);
> +               return;
> +       }
> +
> +       curr = snprintf(selinux_state_string, selinux_state_string_len,
> +                       str_format, "enabled",
> +                       !selinux_disabled(selinux_state));
> +       curr += snprintf((selinux_state_string + curr),
> +                        (selinux_state_string_len - curr),
> +                        str_format, "enforcing",
> +                        enforcing_enabled(selinux_state));
> +       curr += snprintf((selinux_state_string + curr),
> +                        (selinux_state_string_len - curr),
> +                        str_format, "checkreqprot",
> +                        selinux_checkreqprot(selinux_state));
> +
> +       for (i = 3; i < selinux_state_count; i++) {
> +               curr += snprintf((selinux_state_string + curr),
> +                                (selinux_state_string_len - curr),
> +                                str_format,
> +                                selinux_policycap_names[i - 3],
> +                                selinux_state->policycap[i - 3]);
> +       }

Same question here as for the previous loop; seems cleaner to go from
0 to __POLICYDB_CAPABILITY_MAX and use [i].

What public git tree / branch would you recommend trying to use your
patches against?  Didn't seem to apply to any of the obvious ones.