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.
