In Fedora/RHEL's selinux-policy package we ship a pre-built SELinux
policy store in the RPMs. When updating the main policy RPM, care must
be taken to rebuild the policy using `semodule -B` if there are any
other SELinux modules installed (whether shipped via another RPM or
manually installed locally).
However, this way of shipping/managing the policy creates complications
on systems, where system files are managed by rpm-ostree (such as Fedora
CoreOS or Red Hat CoreOS), where the "package update" process is more
sophisticated.
(Disclaimer: The following is written according to my current limited
understanding of rpm-ostree and may not be entirely accurate, but the
gist of it should match the reality.)
Basically, one can think of rpm-ostree as a kind of Git for system
files. The package content is provided on a "branch", where each
"commit" represents a set of package updates layered on top of the
previous commit (i.e. it is a rolling release with some defined
package content snapshots). The user can then maintain their own branch
with additional package updates/installations/... and "rebase" it on top
of the main branch as needed. On top of that, the user can also have
additional configuration files (or modifications to existing files) in
/etc, which represent an additional layer on top of the package content.
When updating the system (i.e. rebasing on a new "commit" of the "main
branch"), the files on the running system are not touched and the new
system state is prepared under a new root directory, which is chrooted
into on the next reboot.
When an rpm-ostree system is updated, there are three moments when the
SELinux module store needs to be rebuilt to ensure that all modules are
included in the binary policy:
1. When the local RPM installations are applied on top of the base
system snapshot.
2. When local user configuartion is applied on top of that.
3. On system shutdown, to ensure that any changes in local configuration
performed since (2.) are reflected in the final new system image.
Forcing a full rebuild at each step is not optimal and in many cases is
not necessary, as the user may not have any custom modules installed.
Thus, this patch extends libsemanage to compute a checksum of the
content of all enabled modules, which is stored in the store, and adds a
flag to the libsemanage handle that instructs it to check the module
content checksum against the one from the last successful transaction
and force a full policy rebuild if they don't match.
This will allow rpm-ostree systems to potentially reduce delays when
reconciling the module store when applying updates.
I wasn't able to measure any noticeable overhead of the hash
computation, which is now added for every transaction (both before and
after this change a full policy rebuild took about 7 seconds on my test
x86 VM). With the new option check_ext_changes enabled, rebuilding a
policy store with unchanged modules took only about 0.96 seconds.
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
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