linux-kernel - Re: [PATCH V34 28/29] efi: Restrict efivar_ssdt

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Message-ID: <201906221714.CDECCAEDA6@keescook>
Date:   Sat, 22 Jun 2019 17:14:45 -0700
From:   Kees Cook <keescook@...omium.org>
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,
        Matthew Garrett <mjg59@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org
Subject: Re: [PATCH V34 28/29] efi: Restrict efivar_ssdt_load when the kernel
 is locked down

On Fri, Jun 21, 2019 at 05:03:57PM -0700, Matthew Garrett wrote:
> efivar_ssdt_load allows the kernel to import arbitrary ACPI code from an
> EFI variable, which gives arbitrary code execution in ring 0. Prevent
> that when the kernel is locked down.
> 
> Signed-off-by: Matthew Garrett <mjg59@...gle.com>

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

-Kees

> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: linux-efi@...r.kernel.org
> ---
>  drivers/firmware/efi/efi.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 55b77c576c42..9f92a013ab27 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -31,6 +31,7 @@
>  #include <linux/acpi.h>
>  #include <linux/ucs2_string.h>
>  #include <linux/memblock.h>
> +#include <linux/security.h>
>  
>  #include <asm/early_ioremap.h>
>  
> @@ -242,6 +243,11 @@ static void generic_ops_unregister(void)
>  static char efivar_ssdt[EFIVAR_SSDT_NAME_MAX] __initdata;
>  static int __init efivar_ssdt_setup(char *str)
>  {
> +	int ret = security_locked_down(LOCKDOWN_ACPI_TABLES);
> +
> +	if (ret)
> +		return ret;
> +
>  	if (strlen(str) < sizeof(efivar_ssdt))
>  		memcpy(efivar_ssdt, str, strlen(str));
>  	else
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

-- 
Kees Cook