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 11:43:42 -0400 |
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On 8/5/20 11:07 AM, Tyler Hicks wrote:
> On 2020-08-05 10:27:43, Stephen Smalley wrote: >> On Wed, Aug 5, 2020 at 9:20 AM Mimi Zohar <zohar@linux.ibm.com> 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@linux.ibm.com> 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. > In AppArmor, we store the sha1 of the raw policy as the policy is > loaded. The hash is exposed to userspace in apparmorfs. See commit > 5ac8c355ae00 ("apparmor: allow introspecting the loaded policy pre > internal transform"). > > It has proved useful as a mechanism for debugging as sometimes the > on-disk policy doesn't match the loaded policy and this can be a good > way to check that while providing support to users. John also mentions > checkpoint/restore in the commit message and I could certainly see how > the policy hashes would be useful in that scenario. > > When thinking through how Lakshmi's series could be extended for > AppArmor support, I was thinking that the AppArmor policy measurement > would be a measurement of these hashes that we already have in place. > > Perhaps there's some general usefulness in storing/exposing an SELinux > policy hash rather than only seeing it as duplicative property required > this measurement series?
That would be a hash of the policy file that was last loaded via the selinuxfs interface for loading policy, not a hash of the in-memory policy data structures at the time of measurement (which is what this patch series is implementing). The duplicative part is with respect to selecting a hash algorithm and hashing the in-memory policy as part of the SELinux code rather than just passing the policy buffer to IMA for measurement like any other buffer. Userspace can already hash the in-memory policy data itself by running sha256sum or whatever on /sys/fs/selinux/policy, so we don't need to save or expose that separately.
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