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Date:   Tue, 16 Jul 2019 10:59:23 +0800
From:   Dave Young <dyoung@redhat.com>
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,
        Josh Boyer <jwboyer@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Kees Cook <keescook@chromium.org>, linux-acpi@vger.kernel.org
Subject: Re: [PATCH V35 15/29] acpi: Ignore acpi_rsdp kernel param when the
 kernel has been locked down
Message-ID: <20190716025923.GA5793@dhcp-128-65.nay.redhat.com>
References: <20190715195946.223443-1-matthewgarrett@google.com>
 <20190715195946.223443-16-matthewgarrett@google.com>
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Hi,
On 07/15/19 at 12:59pm, Matthew Garrett wrote:
> From: Josh Boyer <jwboyer@redhat.com>
> 
> This option allows userspace to pass the RSDP address to the kernel, which
> makes it possible for a user to modify the workings of hardware .  Reject
> the option when the kernel is locked down.
> 
> Signed-off-by: Josh Boyer <jwboyer@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> cc: Dave Young <dyoung@redhat.com>
> cc: linux-acpi@vger.kernel.org
> ---
>  drivers/acpi/osl.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
> index 9c0edf2fc0dd..06e7cffc4386 100644
> --- a/drivers/acpi/osl.c
> +++ b/drivers/acpi/osl.c
> @@ -26,6 +26,7 @@
>  #include <linux/list.h>
>  #include <linux/jiffies.h>
>  #include <linux/semaphore.h>
> +#include <linux/security.h>
>  
>  #include <asm/io.h>
>  #include <linux/uaccess.h>
> @@ -180,7 +181,7 @@ acpi_physical_address __init acpi_os_get_root_pointer(void)
>  	acpi_physical_address pa;
>  
>  #ifdef CONFIG_KEXEC
> -	if (acpi_rsdp)
> +	if (acpi_rsdp && !security_locked_down(LOCKDOWN_ACPI_TABLES))
>  		return acpi_rsdp;

I'm very sorry I noticed this late, but have to say this will not work for
X86 with latest kernel code.

acpi_physical_address __init acpi_os_get_root_pointer(void)
{
        acpi_physical_address pa;

#ifdef CONFIG_KEXEC
        if (acpi_rsdp)
                return acpi_rsdp;
#endif
        pa = acpi_arch_get_root_pointer();
        if (pa)
                return pa;
[snip]

In above code, the later acpi_arch_get_root_pointer still get acpi_rsdp
from cmdline param if provided.

You can check the arch/x86/boot/compressed/acpi.c, and
arch/x86/kernel/acpi/boot.c

In X86 early code, cmdline provided acpi_rsdp pointer will be saved in boot_params.acpi_rsdp_addr;
and the used in x86_default_get_root_pointer
 
>  #endif
>  	pa = acpi_arch_get_root_pointer();
> -- 
> 2.22.0.510.g264f2c817a-goog
> 

Thanks
Dave