grub-mkconfig: Restore umask for the grub.cfg

The commit ab2e53c8a (grub-mkconfig: Honor a symlink when generating
configuration by grub-mkconfig) has inadvertently discarded umask for
creating grub.cfg in the process of running grub-mkconfig. The resulting
wrong permission (0644) would allow unprivileged users to read GRUB
configuration file content. This presents a low confidentiality risk
as grub.cfg may contain non-secured plain-text passwords.

This patch restores the missing umask and sets the creation file mode
to 0600 preventing unprivileged access.

Fixes: CVE-2021-3981

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub-mkconfig.in

@@ -301,7 +301,10 @@ and /etc/grub.d/* files or please file a bug report with
     exit 1
   else
     # none of the children aborted with error, install the new grub.cfg
+    oldumask=$(umask)
+    umask 077
     cat ${grub_cfg}.new > ${grub_cfg}
+    umask $oldumask
     rm -f ${grub_cfg}.new
   fi
 fi