acpi: Disable ACPI table override if the kernel is locked down

>From the kernel documentation (initrd_table_override.txt):

If the ACPI_INITRD_TABLE_OVERRIDE compile option is true, it is possible
to override nearly any ACPI table provided by the BIOS with an
instrumented, modified one.

When lockdown is enabled, the kernel should disallow any unauthenticated
changes to kernel space. ACPI tables contain code invoked by the kernel,
so do not allow ACPI tables to be overridden if the kernel is locked down.

Signed-off-by: Linn Crosetto <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Matthew Garrett <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
cc: [e-mail]
Signed-off-by: James Morris <[e-mail]>

drivers/acpi/tables.c

···
 #include <linux/memblock.h>
 #include <linux/earlycpio.h>
 #include <linux/initrd.h>
+#include <linux/security.h>
 #include "internal.h"
 
 #ifdef CONFIG_ACPI_CUSTOM_DSDT
···
 	}
 	if (table_nr == 0)
 		return;
+
+	if (security_locked_down(LOCKDOWN_ACPI_TABLES)) {
+		pr_notice("kernel is locked down, ignoring table override\n");
+		return;
+	}
 
 	acpi_tables_addr =
 		memblock_find_in_range(0, ACPI_TABLE_UPGRADE_MAX_PHYS,