linux-kernel - Re: [PATCH V40 16/29] acpi: Disable ACPI table override if the kernel is locked down

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Date:   Wed, 21 Aug 2019 00:08:48 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Matthew Garrett <matthewgarrett@...gle.com>
Cc:     jmorris@...ei.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Linn Crosetto <lcrosetto@...il.com>,
        David Howells <dhowells@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Kees Cook <keescook@...omium.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH V40 16/29] acpi: Disable ACPI table override if the kernel is locked down

On Tuesday, August 20, 2019 2:17:52 AM CEST Matthew Garrett wrote:
> From: Linn Crosetto <lcrosetto@...il.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 <lcrosetto@...il.com>
> Signed-off-by: David Howells <dhowells@...hat.com>
> Signed-off-by: Matthew Garrett <mjg59@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> cc: linux-acpi@...r.kernel.org
> Signed-off-by: James Morris <jmorris@...ei.org>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
>  drivers/acpi/tables.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
> index de974322a197..b7c29a11c0c1 100644
> --- a/drivers/acpi/tables.c
> +++ b/drivers/acpi/tables.c
> @@ -20,6 +20,7 @@
>  #include <linux/memblock.h>
>  #include <linux/earlycpio.h>
>  #include <linux/initrd.h>
> +#include <linux/security.h>
>  #include "internal.h"
>  
>  #ifdef CONFIG_ACPI_CUSTOM_DSDT
> @@ -577,6 +578,11 @@ 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);
>