Re: [PATCH V34 14/29] ACPI: Limit access to custom_method when the kernel is locked down

Kees Cook
2019-06-22T23:59:24Z

On Fri, Jun 21, 2019 at 05:03:43PM -0700, Matthew Garrett wrote:
> From: Matthew Garrett <mjg59@srcf.ucam.org>
>
> 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@google.com>

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

-Kees

> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-acpi@vger.kernel.org
> ---
> 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 aa972dc5cb7e..6e56f9f43492 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -8,6 +8,7 @@
> #include <linux/uaccess.h>
> #include <linux/debugfs.h>
> #include <linux/acpi.h>
> +#include <linux/security.h>
>
> #include "internal.h"
>
> @@ -28,6 +29,11 @@ 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 30bc6f058926..cc2b5ee4cadd 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -89,6 +89,7 @@ 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 297a065e6261..1725224f0024 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] = "modified ACPI tables",
> [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook