On Wed, Jan 13, 2021 at 2:13 PM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: > On Tue, 2021-01-12 at 11:27 -0500, Paul Moore wrote: > > On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi > > <tusharsu@xxxxxxxxxxxxxxxxxxx> 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. > > > > > > 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 label=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> > > > Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxxxxxxxx> > > > --- > > > Documentation/ABI/testing/ima_policy | 3 +- > > > security/selinux/Makefile | 2 + > > > security/selinux/ima.c | 64 ++++++++++++++++++++++++++++ > > > security/selinux/include/ima.h | 24 +++++++++++ > > > security/selinux/include/security.h | 3 +- > > > security/selinux/ss/services.c | 64 ++++++++++++++++++++++++---- > > > 6 files changed, 149 insertions(+), 11 deletions(-) > > > create mode 100644 security/selinux/ima.c > > > create mode 100644 security/selinux/include/ima.h > > > > I remain concerned about the possibility of bypassing a measurement by > > tampering with the time, but I appear to be the only one who is > > worried about this so I'm not going to block this patch on those > > grounds. > > > > Acked-by: Paul Moore <paul@xxxxxxxxxxxxxx> > > Thanks, Paul. > > Including any unique string would cause the buffer hash to change, > forcing a new measurement. Perhaps they were concerned with > overflowing a counter. My understanding is that Lakshmi wanted to force a new measurement each time and felt using a timestamp would be the best way to do that. A counter, even if it wraps, would have a different value each time whereas a timestamp is vulnerable to time adjustments. While a properly controlled and audited system could be configured and monitored to detect such an event (I *think*), why rely on that if it isn't necessary? -- paul moore www.paul-moore.com
