Re: [PATCH V40 14/29] ACPI: Limit access to custom_method when the kernel is locked down

[Rafael J. Wysocki]

On Tuesday, August 20, 2019 2:17:50 AM CEST Matthew Garrett wrote:
> From: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
> 
> custom_method effectively allows arbitrary access to system memory, making
> it possible for an attacker to circumvent restrictions on module loading.
> Disable it if the kernel is locked down.
> 
> Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> cc: linux-acpi@xxxxxxxxxxxxxxx
> Signed-off-by: James Morris <jmorris@xxxxxxxxx>

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

> ---
>  drivers/acpi/custom_method.c | 6 ++++++
>  include/linux/security.h     | 1 +
>  security/lockdown/lockdown.c | 1 +
>  3 files changed, 8 insertions(+)
> 
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index b2ef4c2ec955..7031307becd7 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -9,6 +9,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/debugfs.h>
>  #include <linux/acpi.h>
> +#include <linux/security.h>
>  
>  #include "internal.h"
>  
> @@ -29,6 +30,11 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
>  
>  	struct acpi_table_header table;
>  	acpi_status status;
> +	int ret;
> +
> +	ret = security_locked_down(LOCKDOWN_ACPI_TABLES);
> +	if (ret)
> +		return ret;
>  
>  	if (!(*ppos)) {
>  		/* parse the table header to get the table length */
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 010637a79eac..390e39395112 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -110,6 +110,7 @@ enum lockdown_reason {
>  	LOCKDOWN_PCI_ACCESS,
>  	LOCKDOWN_IOPORT,
>  	LOCKDOWN_MSR,
> +	LOCKDOWN_ACPI_TABLES,
>  	LOCKDOWN_INTEGRITY_MAX,
>  	LOCKDOWN_CONFIDENTIALITY_MAX,
>  };
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index b1c1c72440d5..6d44db0ddffa 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -25,6 +25,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>  	[LOCKDOWN_PCI_ACCESS] = "direct PCI access",
>  	[LOCKDOWN_IOPORT] = "raw io port access",
>  	[LOCKDOWN_MSR] = "raw MSR access",
> +	[LOCKDOWN_ACPI_TABLES] = "modifying ACPI tables",
>  	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
>  	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>  };
>