Messages in this thread | | | Subject | Re: [PATCH v6 1/4] IMA: Add func to measure LSM state and policy | From | Stephen Smalley <> | Date | Wed, 5 Aug 2020 08:46:00 -0400 |
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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.
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