ACPI: Limit access to custom_method when the kernel is locked down

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>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
cc: linux-acpi@vger.kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

3 files changed, 8 insertions, 0 deletions

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",
 };