Commit 0cc46f1a authored by Rafael J. Wysocki's avatar Rafael J. Wysocki
Browse files

ACPI: Drop the custom_method debugfs interface 

The ACPI custom_method debugfs interface is security-sensitive and
concurrent access to it is broken [1].

Moreover, the recipe for preparing a customized version of a given
control method has changed at one point due to ACPICA changes, which
has not been reflected in its documentation, so whoever used it before
has had to adapt and it had gone unnoticed for a long time.

This interface was a bad idea to start with and its implementation is
fragile at the design level.  It's been always conceptually questionable,
problematic from the security standpoint and implemented poorly.

Patches fixing its most apparent functional issues (for example, [2]) don't
actually address much of the above.

Granted, at the time it was introduced, there was no alternative, but
there is the AML debugger in the kernel now and there is the configfs
interface allowing custom ACPI tables to be loaded.  The former can be
used for extensive AML debugging and the latter can be use for testing
new AML. [3]

Accordingly, drop custom_method along with its (outdated anyway)
documentation.

Link: https://lore.kernel.org/linux-acpi/20221227063335.61474-1-zh.nvgt@gmail.com/ # [1]
Link: https://lore.kernel.org/linux-acpi/20231111132402.4142-1-shiqiang.deng213@gmail.com/ [2]
Link: https://stackoverflow.com/questions/62849113/how-to-unload-an-overlay-loaded-using-acpi-config-sysfs

 # [3]
Reported-by: default avatarHang Zhang <zh.nvgt@gmail.com>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: default avatarZhang Rui <rui.zhang@intel.com>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
parent c763aefe

Documentation/firmware-guide/acpi/index.rst

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Original line number Diff line number Diff line
@@ -14,7 +14,6 @@ ACPI Support 
   dsd/phy

   enumeration

   osi

   method-customizing

   method-tracing

   DSD-properties-rules

   debug

Documentation/firmware-guide/acpi/method-customizing.rst

deleted100644 → 0
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Original line number Diff line number Diff line 
.. SPDX-License-Identifier: GPL-2.0



=======================================

Linux ACPI Custom Control Method How To

=======================================



:Author: Zhang Rui <rui.zhang@intel.com>





Linux supports customizing ACPI control methods at runtime.



Users can use this to:



1. override an existing method which may not work correctly,

   or just for debugging purposes.

2. insert a completely new method in order to create a missing

   method such as _OFF, _ON, _STA, _INI, etc.



For these cases, it is far simpler to dynamically install a single

control method rather than override the entire DSDT, because kernel

rebuild/reboot is not needed and test result can be got in minutes.



.. note::



  - Only ACPI METHOD can be overridden, any other object types like

    "Device", "OperationRegion", are not recognized. Methods

    declared inside scope operators are also not supported.



  - The same ACPI control method can be overridden for many times,

    and it's always the latest one that used by Linux/kernel.



  - To get the ACPI debug object output (Store (AAAA, Debug)),

    please run::



      echo 1 > /sys/module/acpi/parameters/aml_debug_output





1. override an existing method

==============================

a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT,

   just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat"

b) disassemble the table by running "iasl -d dsdt.dat".

c) rewrite the ASL code of the method and save it in a new file,

d) package the new file (psr.asl) to an ACPI table format.

   Here is an example of a customized \_SB._AC._PSR method::



      DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715)

      {

         Method (\_SB_.AC._PSR, 0, NotSerialized)

         {

            Store ("In AC _PSR", Debug)

            Return (ACON)

         }

      }



   Note that the full pathname of the method in ACPI namespace

   should be used.

e) assemble the file to generate the AML code of the method.

   e.g. "iasl -vw 6084 psr.asl" (psr.aml is generated as a result)

   If parameter "-vw 6084" is not supported by your iASL compiler,

   please try a newer version.

f) mount debugfs by "mount -t debugfs none /sys/kernel/debug"

g) override the old method via the debugfs by running

   "cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method"



2. insert a new method

======================

This is easier than overriding an existing method.

We just need to create the ASL code of the method we want to

insert and then follow the step c) ~ g) in section 1.



3. undo your changes

====================

The "undo" operation is not supported for a new inserted method

right now, i.e. we can not remove a method currently.

For an overridden method, in order to undo your changes, please

save a copy of the method original ASL code in step c) section 1,

and redo step c) ~ g) to override the method with the original one.





.. 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).

drivers/acpi/Kconfig

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Original line number Diff line number Diff line
@@ -449,20 +449,6 @@ 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

	help

	  This debug facility allows ACPI AML methods to be inserted and/or

	  replaced without rebooting the system. For details refer to:

	  Documentation/firmware-guide/acpi/method-customizing.rst.



	  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).



config ACPI_BGRT

	bool "Boottime Graphics Resource Table support"

	depends on EFI && (X86 || ARM64)

drivers/acpi/Makefile

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Original line number Diff line number Diff line
@@ -101,7 +101,6 @@ obj-$(CONFIG_ACPI_SBS) += sbshc.o 
obj-$(CONFIG_ACPI_SBS)		+= sbs.o

obj-$(CONFIG_ACPI_HED)		+= hed.o

obj-$(CONFIG_ACPI_EC_DEBUGFS)	+= ec_sys.o

obj-$(CONFIG_ACPI_CUSTOM_METHOD)+= custom_method.o

obj-$(CONFIG_ACPI_BGRT)		+= bgrt.o

obj-$(CONFIG_ACPI_CPPC_LIB)	+= cppc_acpi.o

obj-$(CONFIG_ACPI_SPCR_TABLE)	+= spcr.o

drivers/acpi/custom_method.c

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Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0-only

/*

 * custom_method.c - debugfs interface for customizing ACPI control method

 */



#include <linux/init.h>

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/uaccess.h>

#include <linux/debugfs.h>

#include <linux/acpi.h>

#include <linux/security.h>



#include "internal.h"



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;

	int ret;



	ret = security_locked_down(LOCKDOWN_ACPI_TABLES);

	if (ret)

		return ret;



	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;

		/* make sure the buf is not allocated */

		kfree(buf);

		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)) {

		kfree(buf);

		buf = NULL;

		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, LOCKDEP_NOW_UNRELIABLE);

	}



	return count;

}



static const struct file_operations cm_fops = {

	.write = cm_write,

	.llseek = default_llseek,

};



static int __init acpi_custom_method_init(void)

{

	cm_dentry = debugfs_create_file("custom_method", S_IWUSR,

					acpi_debugfs_dir, NULL, &cm_fops);

	return 0;

}



static void __exit acpi_custom_method_exit(void)

{

	debugfs_remove(cm_dentry);

}



module_init(acpi_custom_method_init);

module_exit(acpi_custom_method_exit);