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Accordingly, drop custom_method along with its (outdated anyway)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>

3 files changed, 0 insertions, 118 deletions

diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
index 3c3f8037ebedd..c645bb453f3b7 100644
--- a/drivers/acpi/Kconfig
+++ b/drivers/acpi/Kconfig
@@ -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)
diff --git a/drivers/acpi/Makefile b/drivers/acpi/Makefile
index 12ef8180d272c..8cc8c0d9c8732 100644
--- a/drivers/acpi/Makefile
+++ b/drivers/acpi/Makefile
@@ -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
diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
deleted file mode 100644
index d39a9b474727d..0000000000000
--- a/drivers/acpi/custom_method.c
+++ /dev/null
@@ -1,103 +0,0 @@
-// 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);