ACPI: Split out custom_method functionality into an own driver

With /sys/kernel/debug/acpi/custom_method root can write
to arbitrary memory and increase his priveleges, even if
these are restricted.

-> Make this an own debug .config option and warn about the
security issue in the config description.

-> Still keep acpi/debugfs.c which now only creates an empty
   /sys/kernel/debug/acpi directory. There might be other
   users of it later.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: rui.zhang@intel.com
Signed-off-by: Len Brown <len.brown@intel.com>

5 files changed, 122 insertions, 79 deletions

diff --git a/Documentation/acpi/method-customizing.txt b/Documentation/acpi/method-customizing.txt
index 3e1d25aee3fb..5f55373dd53b 100644
--- a/Documentation/acpi/method-customizing.txt
+++ b/Documentation/acpi/method-customizing.txt
@@ -66,3 +66,8 @@ Note: We can use a kernel with multiple custom ACPI method running,
       But each individual write to debugfs can implement a SINGLE
       method override. i.e. if we want to insert/override multiple
       ACPI methods, we need to redo step c) ~ g) for multiple times.
+
+Note: Be aware that root can mis-use this driver to modify arbitrary
+      memory and gain additional rights, if root's privileges got
+      restricted (for example if root is not allowed to load additional
+      modules after boot).
diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
index 3a17ca5fff6f..d918e130bef3 100644
--- a/drivers/acpi/Kconfig
+++ b/drivers/acpi/Kconfig
@@ -380,6 +380,21 @@ config ACPI_HED
           which is used to report some hardware errors notified via
           SCI, mainly the corrected errors.
 
+config ACPI_CUSTOM_METHOD
+        tristate "Allow ACPI methods to be inserted/replaced at run time"
+        depends on DEBUG_FS
+        default n
+        help
+          This debug facility allows ACPI AML methods to me inserted and/or
+          replaced without rebooting the system. For details refer to:
+          Documentation/acpi/method-customizing.txt.
+
+          NOTE: This option is security sensitive, because it allows arbitrary
+          kernel memory to be written to by root (uid=0) users, allowing them
+          to bypass certain security measures (e.g. if root is not allowed to
+          load additional kernel modules after boot, this feature may be used
+          to override that restriction).
+
 source "drivers/acpi/apei/Kconfig"
 
 endif   # ACPI
diff --git a/drivers/acpi/Makefile b/drivers/acpi/Makefile
index d113fa5100b2..cba0b2334b9b 100644
--- a/drivers/acpi/Makefile
+++ b/drivers/acpi/Makefile
@@ -62,6 +62,7 @@ obj-$(CONFIG_ACPI_SBS)		+= sbs.o
 obj-$(CONFIG_ACPI_POWER_METER)  += power_meter.o
 obj-$(CONFIG_ACPI_HED)          += hed.o
 obj-$(CONFIG_ACPI_EC_DEBUGFS)   += ec_sys.o
+obj-$(CONFIG_ACPI_CUSTOM_METHOD)+= custom_method.o
 
 # processor has its own "processor." module_param namespace
 processor-y                     := processor_driver.o processor_throttling.o
diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
new file mode 100644
index 000000000000..5d42c2414ae5
--- /dev/null
+++ b/drivers/acpi/custom_method.c
@@ -0,0 +1,100 @@
+/*
+ * debugfs.c - ACPI debugfs interface to userspace.
+ */
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/uaccess.h>
+#include <linux/debugfs.h>
+#include <acpi/acpi_drivers.h>
+
+#include "internal.h"
+
+#define _COMPONENT              ACPI_SYSTEM_COMPONENT
+ACPI_MODULE_NAME("custom_method");
+MODULE_LICENSE("GPL");
+
+static struct dentry *cm_dentry;
+
+/* /sys/kernel/debug/acpi/custom_method */
+
+static ssize_t cm_write(struct file *file, const char __user * user_buf,
+                        size_t count, loff_t *ppos)
+{
+        static char *buf;
+        static u32 max_size;
+        static u32 uncopied_bytes;
+
+        struct acpi_table_header table;
+        acpi_status status;
+
+        if (!(*ppos)) {
+                /* parse the table header to get the table length */
+                if (count <= sizeof(struct acpi_table_header))
+                        return -EINVAL;
+                if (copy_from_user(&table, user_buf,
+                                   sizeof(struct acpi_table_header)))
+                        return -EFAULT;
+                uncopied_bytes = max_size = table.length;
+                buf = kzalloc(max_size, GFP_KERNEL);
+                if (!buf)
+                        return -ENOMEM;
+        }
+
+        if (buf == NULL)
+                return -EINVAL;
+
+        if ((*ppos > max_size) ||
+            (*ppos + count > max_size) ||
+            (*ppos + count < count) ||
+            (count > uncopied_bytes))
+                return -EINVAL;
+
+        if (copy_from_user(buf + (*ppos), user_buf, count)) {
+                kfree(buf);
+                buf = NULL;
+                return -EFAULT;
+        }
+
+        uncopied_bytes -= count;
+        *ppos += count;
+
+        if (!uncopied_bytes) {
+                status = acpi_install_method(buf);
+                kfree(buf);
+                buf = NULL;
+                if (ACPI_FAILURE(status))
+                        return -EINVAL;
+                add_taint(TAINT_OVERRIDDEN_ACPI_TABLE);
+        }
+
+        return count;
+}
+
+static const struct file_operations cm_fops = {
+        .write = cm_write,
+        .llseek = default_llseek,
+};
+
+static int __init acpi_custom_method_init(void)
+{
+        if (acpi_debugfs_dir == NULL)
+                return -ENOENT;
+
+        cm_dentry = debugfs_create_file("custom_method", S_IWUSR,
+                                        acpi_debugfs_dir, NULL, &cm_fops);
+        if (cm_dentry == NULL)
+                return -ENODEV;
+
+        return 0;
+}
+
+static void __exit acpi_custom_method_exit(void)
+{
+        if (cm_dentry)
+                debugfs_remove(cm_dentry);
+ }
+
+module_init(acpi_custom_method_init);
+module_exit(acpi_custom_method_exit);
diff --git a/drivers/acpi/debugfs.c b/drivers/acpi/debugfs.c
index e7abc6e3bba0..182a9fc36355 100644
--- a/drivers/acpi/debugfs.c
+++ b/drivers/acpi/debugfs.c
@@ -3,9 +3,6 @@
  */
 
 #include <linux/init.h>
-#include <linux/module.h>
-#include <linux/kernel.h>
-#include <linux/uaccess.h>
 #include <linux/debugfs.h>
 #include <acpi/acpi_drivers.h>
 
@@ -13,84 +10,9 @@
 ACPI_MODULE_NAME("debugfs");
 
 struct dentry *acpi_debugfs_dir;
-static struct dentry *cm_dentry;
-
-/* /sys/kernel/debug/acpi/custom_method */
-
-static ssize_t cm_write(struct file *file, const char __user * user_buf,
-                        size_t count, loff_t *ppos)
-{
-        static char *buf;
-        static u32 max_size;
-        static u32 uncopied_bytes;
-
-        struct acpi_table_header table;
-        acpi_status status;
-
-        if (!(*ppos)) {
-                /* parse the table header to get the table length */
-                if (count <= sizeof(struct acpi_table_header))
-                        return -EINVAL;
-                if (copy_from_user(&table, user_buf,
-                                   sizeof(struct acpi_table_header)))
-                        return -EFAULT;
-                uncopied_bytes = max_size = table.length;
-                buf = kzalloc(max_size, GFP_KERNEL);
-                if (!buf)
-                        return -ENOMEM;
-        }
-
-        if (buf == NULL)
-                return -EINVAL;
-
-        if ((*ppos > max_size) ||
-            (*ppos + count > max_size) ||
-            (*ppos + count < count) ||
-            (count > uncopied_bytes))
-                return -EINVAL;
-
-        if (copy_from_user(buf + (*ppos), user_buf, count)) {
-                kfree(buf);
-                buf = NULL;
-                return -EFAULT;
-        }
-
-        uncopied_bytes -= count;
-        *ppos += count;
-
-        if (!uncopied_bytes) {
-                status = acpi_install_method(buf);
-                kfree(buf);
-                buf = NULL;
-                if (ACPI_FAILURE(status))
-                        return -EINVAL;
-                add_taint(TAINT_OVERRIDDEN_ACPI_TABLE);
-        }
-
-        return count;
-}
-
-static const struct file_operations cm_fops = {
-        .write = cm_write,
-        .llseek = default_llseek,
-};
-
-static int __init acpi_custom_method_init(void)
-{
-        if (!acpi_debugfs_dir)
-                return -ENOENT;
-
-        cm_dentry = debugfs_create_file("custom_method", S_IWUSR,
-                                        acpi_debugfs_dir, NULL, &cm_fops);
-        if (!cm_dentry)
-                return -ENODEV;
-
-        return 0;
-}
+EXPORT_SYMBOL_GPL(acpi_debugfs_dir);
 
 void __init acpi_debugfs_init(void)
 {
         acpi_debugfs_dir = debugfs_create_dir("acpi", NULL);
-
-        acpi_custom_method_init();
 }