Hi Janne, On Fri, 2020-01-10 at 10:48 +0200, Janne Karhunen wrote: > On Wed, Jan 8, 2020 at 1:18 PM Janne Karhunen <janne.karhunen@xxxxxxxxx> wrote: > > > > Some systems can end up carrying lots of entries in the ima > > measurement list. Since every entry is using a bit of kernel > > memory, allow the sysadmin to export the measurement list to > > the filesystem to free up some memory. > > Hopefully this addressed comments from everyone. The flush event can > now be triggered by the admin anytime and unique file names can be > used for each flush (log.1, log.2, ...) etc, so getting to the correct > item should be easy. > > While it can now be argued that since this is an admin-driven event, > kernel does not need to write the file. However, the intention is to > bring out a second patch a bit later that adds a variable to define > the max number of entries to be kept in the kernel memory and > workqueue based automatic flushing. In those cases the kernel has to > be able to write the file without any help from the admin.. The implications of exporting and removing records from the IMA- measurement list needs to be considered carefully. Verifying a TPM quote will become dependent on knowing where the measurements are stored. The existing measurement list is stored in kernel memory and, barring a kernel memory attack, is protected from modification. Before upstreaming this or a similar patch, there needs to be a discussion as to how the measurement list will be protected once is it exported to userspace. This patch now attempts to address two very different scenarios. The first scenario is where userspace is requesting exporting and removing of the measurement list records. The other scenario is the kernel exporting and removing of the measurement list records. Conflating these two different use cases might not be the right solution, as we originally thought. The kernel already exports the IMA measurement list to userspace via a securityfs file. From a userspace perspective, missing is the ability of removing N number of records. In this scenario, userspace would be responsible for safely storing the measurements (e.g. blockchain). The kernel would only be responsible for limiting permission, perhaps based on a capability, before removing records from the measurement list. In the kernel usecase, somehow the kernel would need to safely export the measurement list, or some portion of the measurement list, to a file and then delete that portion. What protects the exported records stored in a file from modification? Instead of exporting the measurement records, one option as suggested by Amir Goldstein, would be to use a vfs_tmpfile() to get an anonymous file for backing store. The existing securityfs measurement lists would then read from this private copy of the anonymous file. I've Cc'ed fsdevel for additional comments/suggestions. thanks, Mimi
