On Tue, Jan 31, 2023 at 5:26 AM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote: > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > 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 an no problems with it have been reported. > > The latter likely means that the number of its users is limited at best > and attempting to fix the issues mentioned above is likely not worth the > effort. Moreover, if it gets broken in the process, the breakage may not > be readily discovered, so deleting it altogheher appeares to be a better > option. > > Accordingly, drop custom_method along with its (outdated anyway) > documentation. > > Link: https://lore.kernel.org/linux-acpi/20221227063335.61474-1-zh.nvgt@xxxxxxxxx/ # [1] > Reported-by: Hang Zhang <zh.nvgt@xxxxxxxxx> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> Sample on iasl "20220331", and thinkpad X200 running coreboot, for easy reference test.dsl created according to existing linux docs (ie. by providing full path to `_Q26`): DefinitionBlock ("", "DSDT", 2, "COREv4", "COREBOOT", 0x20090419) { Method (\_SB.PCI0.LPCB.EC._Q26, 0, NotSerialized) // _Qxx: EC Query, xx=0x00-0xFF { Printf ("_Q26 FUN!") Notify (AC, 0x80) // Status Change PNOT () } } `iasl test.dsl` `iasl -vw 6084 test.dsl` `iasl -vw 6084 -vw 6160 test.dsl` Neither of those combinations make any difference in output `test.aml` binary. With slightly newer iasl "20221020" we have to ignore errors, just like original document describes. Still works fine with `iasl -vw 6084 -vw 6160 test.dsl` `iasl -h` is pretty clear about '-vw' being used to ignore particular error messages, so I really see no substantial change here. modprobe custom_method cat test.aml > /sys/kernel/debug/acpi/custom_method echo 1 > /sys/module/acpi/parameters/aml_debug_output Now upon each connection of AC power brick to lappy, I get following message in dmesg: [ 699.784020] ACPI Debug: "_Q26 FUN!" Latest tested and working linux kernel: 6.1 Sorry for not testing writing acpi stuff on 6.2-rc6 kernel It's sad to see custom_method go, since it lives under debugfs and you have to specifically build this, so I don't see it as a security issue. When I write/troubleshoot DSDT I do it alone, and I do not think that there is a machine for which people developed DSDT concurrently while being connected to same machine, with root access, swapping DSDT table at the same time. It works for me great, and I presume it does so for other embed developers. I guess it's possible to create replacement workflow with kexec and entire table load via CONFIG_ACPI_TABLE_UPGRADE, but note how it would be more difficult and more things could go wrong. Swapping out just single method even when machine has significant uptime feels like a great thing I had with custom_method (like just when some rare condition started occurring). Anyway I get that maintenance burden is too great for this portion of kernel, and obscure use cases of a firmware developer writing DSDT are not a priority. I'm not smart enough to maintain it, so I guess I need to say my goodbyes to this fun part of the kernel. Thanks to everyone that kept it going for this long. Sebastian Grzywna > --- > > -> v2: Update index.rst too. > > --- > Documentation/firmware-guide/acpi/index.rst | 1 > Documentation/firmware-guide/acpi/method-customizing.rst | 89 ------------ > drivers/acpi/Kconfig | 14 -- > drivers/acpi/Makefile | 1 > drivers/acpi/custom_method.c | 103 --------------- > 5 files changed, 208 deletions(-) > > Index: linux-pm/drivers/acpi/Kconfig > =================================================================== > --- linux-pm.orig/drivers/acpi/Kconfig > +++ linux-pm/drivers/acpi/Kconfig > @@ -444,20 +444,6 @@ 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) > Index: linux-pm/drivers/acpi/Makefile > =================================================================== > --- linux-pm.orig/drivers/acpi/Makefile > +++ linux-pm/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 > Index: linux-pm/drivers/acpi/custom_method.c > =================================================================== > --- linux-pm.orig/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); > Index: linux-pm/Documentation/firmware-guide/acpi/method-customizing.rst > =================================================================== > --- linux-pm.orig/Documentation/firmware-guide/acpi/method-customizing.rst > +++ /dev/null > @@ -1,89 +0,0 @@ > -.. SPDX-License-Identifier: GPL-2.0 > - > -======================================= > -Linux ACPI Custom Control Method How To > -======================================= > - > -:Author: Zhang Rui <rui.zhang@xxxxxxxxx> > - > - > -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). > Index: linux-pm/Documentation/firmware-guide/acpi/index.rst > =================================================================== > --- linux-pm.orig/Documentation/firmware-guide/acpi/index.rst > +++ linux-pm/Documentation/firmware-guide/acpi/index.rst > @@ -14,7 +14,6 @@ ACPI Support > dsd/phy > enumeration > osi > - method-customizing > method-tracing > DSD-properties-rules > debug > > >
