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Date:   Sat, 22 Jun 2019 17:00:12 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Matthew Garrett <matthewgarrett@google.com>
Cc:     jmorris@namei.org, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Linn Crosetto <linn@hpe.com>,
        David Howells <dhowells@redhat.com>,
        Matthew Garrett <mjg59@google.com>, linux-acpi@vger.kernel.org
Subject: Re: [PATCH V34 16/29] acpi: Disable ACPI table override if the
 kernel is locked down
Message-ID: <201906221700.B1CA4EF77E@keescook>
References: <20190622000358.19895-1-matthewgarrett@google.com>
 <20190622000358.19895-17-matthewgarrett@google.com>
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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