[PATCH V38 14/29] ACPI: Limit access to custom_method when the kernel is locked down

[Matthew Garrett]

From: Matthew Garrett <mj...@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 <mj...@google.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
Reviewed-by: Kees Cook <keesc...@chromium.org>
cc: linux-a...@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 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 155ff026eca4..1c32522b3c5a 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 d99c0bee739d..ecb51b1a5c03 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",
 };
-- 
2.22.0.770.g0f2c4a37fd-goog