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Accordingly, drop custom_method along with its (outdated anyway)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: Hang Zhang <zh.nvgt@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

Author: rafaeljw

Documentation/firmware-guide/acpi/index.rst

@@ -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

@@ -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/Kconfig

@@ -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