security: fix SELinux label generation logic

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: #153


CVE-2021-3631
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/security/security_selinux.c

@@ -383,7 +383,15 @@ virSecuritySELinuxMCSFind(virSecurityManager *mgr,
         VIR_DEBUG("Try cat %s:c%d,c%d", sens, c1 + catMin, c2 + catMin);
 
         if (c1 == c2) {
-            mcs = g_strdup_printf("%s:c%d", sens, catMin + c1);
+            /*
+             * 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.
+             *
+             * IOW, we must discard case where the categories are equal
+             * because that is a subset of other category pairs.
+             */
+            continue;
         } else {
             if (c1 > c2) {
                 int t = c1;