On Friday, April 01, 2011, Thomas Renninger wrote:
> 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@xxxxxxx>
Acked-by: Rafael J. Wysocki <rjw@xxxxxxx>
> CC: lenb@xxxxxxxxxx
> CC: rui.zhang@xxxxxxxxx
> CC: linux-acpi@xxxxxxxxxxxxxxx
>
> ---
> Documentation/acpi/method-customizing.txt | 5 ++
> drivers/acpi/Kconfig | 15 ++++
> drivers/acpi/Makefile | 1 +
> drivers/acpi/custom_method.c | 100 +++++++++++++++++++++++++++++
> drivers/acpi/debugfs.c | 80 +-----------------------
> 5 files changed, 122 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/custom_method.c b/drivers/acpi/custom_method.c
> new file mode 100644
> index 0000000..dc554c2
> --- /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 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();
> }
>
>
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