On 12/11/20 4:32 PM, Tyler Hicks wrote:
On 2020-12-11 15:58:07, Tushar Sugandhi wrote:From: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx> SELinux stores the active policy in memory, so the changes to this data at runtime would have an impact on the security guarantees provided by SELinux. Measuring in-memory SELinux policy through IMA subsystem provides a secure way for the attestation service to remotely validate the policy contents at runtime. Measure the hash of the loaded policy by calling the IMA hook ima_measure_critical_data(). Since the size of the loaded policy can be large (several MB), measure the hash of the policy instead of the entire policy to avoid bloating the IMA log entry. Add "selinux" to the list of supported data sources maintained by IMA to enable measuring SELinux 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-5.10.0-rc1+ 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 data_source=selinux Sample measurement of the 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 grep "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6 Note that the actual verification of SELinux policy would require loading the expected policy into an identical kernel on a pristine/known-safe system and run the sha256sum /sys/kernel/selinux/policy there to get the expected hash. Signed-off-by: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx> Suggested-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>This looks good but I've got one small suggestion below if you roll a v9. Feel free to add: Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx>diff --git a/security/selinux/measure.c b/security/selinux/measure.c new file mode 100644 index 000000000000..a070d8dae403 --- /dev/null +++ b/security/selinux/measure.c @@ -0,0 +1,81 @@ +// SPDX-License-Identifier: GPL-2.0-or-later +/* + * Measure SELinux state using IMA subsystem. + */ +#include <linux/vmalloc.h> +#include <linux/ktime.h> +#include <linux/ima.h> +#include "security.h" + +/* + * This function creates a unique name by appending the timestamp to + * the given string. This string is passed as "event_name" to the IMA + * hook to measure the given SELinux data. + * + * The data provided by SELinux to the IMA subsystem for measuring may have + * already been measured (for instance the same state existed earlier). + * But for SELinux the current data represents a state change and hence + * needs to be measured again. To enable this, pass a unique "event_name" + * to the IMA hook so that IMA subsystem will always measure the given data. + * + * For example, + * At time T0 SELinux data to be measured is "foo". IMA measures it. + * At time T1 the data is changed to "bar". IMA measures it. + * At time T2 the data is changed to "foo" again. IMA will not measure it + * (since it was already measured) unless the event_name, for instance, + * is different in this call. + */ +static char *selinux_event_name(const char *name_prefix) +{ + char *event_name = NULL; + struct timespec64 cur_time; + + ktime_get_real_ts64(&cur_time); + event_name = kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix, + cur_time.tv_sec, cur_time.tv_nsec); + return event_name;There's no longer a need to store the return of kasprintf() in a variable. Just 'return kasprint(...);' and get rid of the event_name variable.
Sure - I'll make the change. -lakshmi
