acpi: Don't register the acpi command when locked down

The command is not allowed when lockdown is enforced. Otherwise an
attacker can instruct the GRUB to load an SSDT table to overwrite
the kernel lockdown configuration and later load and execute
unsigned code.

Fixes: CVE-2020-14372

Reported-by: Máté Kukri <km@mkukri.xyz>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -4074,6 +4074,11 @@ Normally, this command will replace the Root System Description Pointer
 (RSDP) in the Extended BIOS Data Area to point to the new tables. If the
 @option{--no-ebda} option is used, the new tables will be known only to
 GRUB, but may be used by GRUB's EFI emulation.
+
+Note: The command is not allowed when lockdown is enforced (@pxref{Lockdown}).
+      Otherwise an attacker can instruct the GRUB to load an SSDT table to
+      overwrite the kernel lockdown configuration and later load and execute
+      unsigned code.
 @end deffn
 
 

grub-core/commands/acpi.c

@@ -27,6 +27,7 @@
 #include <grub/mm.h>
 #include <grub/memory.h>
 #include <grub/i18n.h>
+#include <grub/lockdown.h>
 
 #ifdef GRUB_MACHINE_EFI
 #include <grub/efi/efi.h>
@@ -775,7 +776,7 @@ static grub_extcmd_t cmd;
 
 GRUB_MOD_INIT(acpi)
 {
-  cmd = grub_register_extcmd ("acpi", grub_cmd_acpi, 0,
+  cmd = grub_register_extcmd_lockdown ("acpi", grub_cmd_acpi, 0,
                                        N_("[-1|-2] [--exclude=TABLE1,TABLE2|"
                                           "--load-only=TABLE1,TABLE2] FILE1"
                                           " [FILE2] [...]"),