On Wed, 2020-08-05 at 10:27 -0400, Stephen Smalley wrote: > On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: > > On Wed, 2020-08-05 at 09:03 -0400, Stephen Smalley wrote: > > > On Wed, Aug 5, 2020 at 8:57 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: > > > > On Wed, 2020-08-05 at 08:46 -0400, Stephen Smalley wrote: > > > > > On 8/4/20 11:25 PM, Mimi Zohar wrote: > > > > > > > > > > > Hi Lakshmi, > > > > > > > > > > > > There's still a number of other patch sets needing to be reviewed > > > > > > before my getting to this one. The comment below is from a high level. > > > > > > > > > > > > On Tue, 2020-08-04 at 17:43 -0700, Lakshmi Ramasubramanian wrote: > > > > > > > Critical data structures of security modules need to be measured to > > > > > > > enable an attestation service to verify if the configuration and > > > > > > > policies for the security modules have been setup correctly and > > > > > > > that they haven't been tampered with at runtime. A new IMA policy is > > > > > > > required for handling this measurement. > > > > > > > > > > > > > > Define two new IMA policy func namely LSM_STATE and LSM_POLICY to > > > > > > > measure the state and the policy provided by the security modules. > > > > > > > Update ima_match_rules() and ima_validate_rule() to check for > > > > > > > the new func and ima_parse_rule() to handle the new func. > > > > > > I can understand wanting to measure the in kernel LSM memory state to > > > > > > make sure it hasn't changed, but policies are stored as files. Buffer > > > > > > measurements should be limited to those things that are not files. > > > > > > > > > > > > Changing how data is passed to the kernel has been happening for a > > > > > > while. For example, instead of passing the kernel module or kernel > > > > > > image in a buffer, the new syscalls - finit_module, kexec_file_load - > > > > > > pass an open file descriptor. Similarly, instead of loading the IMA > > > > > > policy data, a pathname may be provided. > > > > > > > > > > > > Pre and post security hooks already exist for reading files. Instead > > > > > > of adding IMA support for measuring the policy file data, update the > > > > > > mechanism for loading the LSM policy. Then not only will you be able > > > > > > to measure the policy, you'll also be able to require the policy be > > > > > > signed. > > > > > > > > > > To clarify, the policy being measured by this patch series is a > > > > > serialized representation of the in-memory policy data structures being > > > > > enforced by SELinux. Not the file that was loaded. Hence, this > > > > > measurement would detect tampering with the in-memory policy data > > > > > structures after the policy has been loaded. In the case of SELinux, > > > > > one can read this serialized representation via /sys/fs/selinux/policy. > > > > > The result is not byte-for-byte identical to the policy file that was > > > > > loaded but can be semantically compared via sediff and other tools to > > > > > determine whether it is equivalent. > > > > > > > > Thank you for the clarification. Could the policy hash be included > > > > with the other critical data? Does it really need to be measured > > > > independently? > > > > > > They were split into two separate functions because we wanted to be > > > able to support using different templates for them (ima-buf for the > > > state variables so that the measurement includes the original buffer, > > > which is small and relatively fixed-size, and ima-ng for the policy > > > because it is large and we just want to capture the hash for later > > > comparison against known-good). Also, the state variables are > > > available for measurement always from early initialization, whereas > > > the policy is only available for measurement once we have loaded an > > > initial policy. > > > > Ok, measuring the policy separately from other critical data makes > > sense. Instead of measuring the policy, which is large, measure the > > policy hash. > > I think that was the original approach. However, I had concerns with > adding code to SELinux to compute a hash over the policy versus > leaving that to IMA's existing policy and mechanism. If that's > preferred I guess we can do it that way but seems less flexible and > duplicative. Whether IMA or SELinux calculates the in memory policy hash, it should not impact the original purpose of this patch set - measuring critical state. It's unclear whether this patch set needs to be limited to LSM critical state. Measuring the in memory policy, if needed, should be a separate patch set. Mimi
