selinux: Don't ignore ENOENT in Permissive mode

In selinux driver there's virSecuritySELinuxSetFileconImpl()
which is responsible for actual setting of SELinux label on given
file and handling possible failures. In fhe failure handling code
we decide whether failure is fatal or not. But there is a bug:
depending on SELinux mode (Permissive vs. Enforcing) the ENOENT
is either ignored or considered fatal. This not correct - ENOENT
must always be fatal for couple of reasons:

- In virSecurityStackTransactionCommit() the seclabels are set
  for individual secdrivers (e.g. SELinux first and then DAC),
  but if one secdriver succeeds and another one fails, then no
  rollback is performed for the successful one leaking remembered
  labels.

- QEMU would fail opening the file anyways (if neither of
  secdrivers reported error and thus cancelled domain startup)

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2004850
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

src/security/security_selinux.c

@@ -1280,9 +1280,11 @@ virSecuritySELinuxSetFileconImpl(const char *path,
         } else {
             /* However, don't claim error if SELinux is in Enforcing mode and
              * we are running as unprivileged user and we really did see EPERM.
-             * Otherwise we want to return error if SELinux is Enforcing. */
-            if (security_getenforce() == 1 &&
-                (setfilecon_errno != EPERM || privileged)) {
+             * Otherwise we want to return error if SELinux is Enforcing, or we
+             * saw ENOENT regardless of SELinux mode. */
+            if (setfilecon_errno == ENOENT ||
+                (security_getenforce() == 1 &&
+                 (setfilecon_errno != EPERM || privileged))) {
                 virReportSystemError(setfilecon_errno,
                                      _("unable to set security context '%s' on '%s'"),
                                      tcon, path);