A process can access a file if the set of MCS categories
for the file is equal-to *or* a subset-of, the set of
MCS categories for the process.
If there are two VMs:
a) svirt_t:s0:c117
b) svirt_t:s0:c117,c720
Then VM (b) is able to access files labelled for VM (a).
IOW, we must discard case where the categories are equal
because that is a subset of many other valid category pairs.
Fixes: https://gitlab.com/libvirt/libvirt/-/issues/153
CVE-2021-3631
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
