[PATCH 3/3] acpi: Split out custom_method functionality into an own driver

Thomas Renninger
2011-03-31T11:33:44Z

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 and empty
/sys/kernel/debug/acpi directory. There might be other
users of it later.

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Rafael J. Wysocki <rjw@sisk.pl>
CC: lenb@kernel.org
CC: rui.zhang@intel.com
CC: linux-acpi@vger.kernel.org
---
Documentation/acpi/method-customizing.txt | 5 ++
drivers/acpi/Kconfig | 15 +++++
drivers/acpi/Makefile | 1 +
drivers/acpi/debugfs.c | 80 +----------------------------
4 files changed, 22 insertions(+), 79 deletions(-)

diff --git a/Documentation/acpi/method-customizing.txt b/Documentation/acpi/method-customizing.txt
index 3e1d25a..5f55373 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 2aa042a..3feeec8 100644
--- a/drivers/acpi/Kconfig
+++ b/drivers/acpi/Kconfig
@@ -381,6 +381,21 @@ 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 d113fa5..cba0b23 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/debugfs.c b/drivers/acpi/debugfs.c
index e7abc6e..182a9fc 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();
}
--
1.7.3.4