Re: [PATCH V34 16/29] acpi: Disable ACPI table override if the kernel is locked down

Kees Cook
2019-06-23T00:00:18Z

On Fri, Jun 21, 2019 at 05:03:45PM -0700, Matthew Garrett wrote:
> From: Linn Crosetto <linn@hpe.com>
>
> 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 <linn@hpe.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> cc: linux-acpi@vger.kernel.org
> ---
> drivers/acpi/tables.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
> index 8fccbe49612a..41d9ccd0e075 100644
> --- a/drivers/acpi/tables.c
> +++ b/drivers/acpi/tables.c
> @@ -34,6 +34,7 @@
> #include <linux/memblock.h>
> #include <linux/earlycpio.h>
> #include <linux/initrd.h>
> +#include <linux/security.h>
> #include "internal.h"
>
> #ifdef CONFIG_ACPI_CUSTOM_DSDT
> @@ -539,6 +540,11 @@ void __init acpi_table_upgrade(void)
> 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,
> all_tables_size, PAGE_SIZE);
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook