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Ritesh Harjani (IBM) c2549d7495 powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare()
KUAP means kernel cannot touch user memory unless it explicitly is
enabled. In the kernel it should stay AMR_KUAP_BLOCKED. While returning
to userspace just before RFI, kernel should restore the user AMR value
back.

Looks like GENERIC_ENTRY might be treating arch_exit_to_user_mode_prepare()
as the last architecture step before returning to userspace.
commit bee25f97ad ("powerpc: Enable GENERIC_ENTRY feature")
therefore called kuap_user_restore() from that hook. But on PowerPC that
is too early. After irqentry_exit() / syscall_exit_to_user_mode() we
still run platform specific exit routines.

e.g. code snippets showing both exception handling and system call
handling as the callers of function arch_exit_to_user_mode_prepare()
which does kuap_user_restore(). The below path shows that calling
kuap_user_restore() is too early when called from
arch_exit_to_user_mode_prepare().

Exception handling in exceptions-64s.S
=======================================

bl	CFUNC(do_page_fault)
	  ..DEFINE_INTERRUPT_HANDLER_ASYNC(do_page_fault)
	      arch_interrupt_async_enter_prepare(regs);
	      state = irqentry_enter(regs);
	      instrumentation_begin();
	      irq_enter_rcu();
	      handler(regs);
	      nap_adjust_return(regs);
	      irq_exit_rcu();
	      instrumentation_end();
	      arch_interrupt_async_exit_prepare(regs);
	      irqentry_exit(regs, state); 		   <<< too early
	        irqentry_exit_to_user_mode()
		  __exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_IRQ);
		    arch_exit_to_user_mode_prepare(regs, ti_work);  <<< too early
b	interrupt_return_srr
		.. bl	CFUNC(interrupt_exit_user_prepare) <<< already calls kuap_user_restore

prep_irq_for_enabled_exit() retry can run kernel code with IRQs on. So
only when that routine is fully finished is when the user KUAP should be
fully restored which interrupt_exit_user_prepare() already takes care of
before returning.

Similarly for system call handling in interrupt_64.S
======================================================

	bl	CFUNC(system_call_exception)

.Lsyscall_exit:
	addi	r4,r1,STACK_INT_FRAME_REGS
	li	r5,0 /* !scv */
	bl	CFUNC(syscall_exit_prepare)
		  .. kuap_assert_locked();
		     syscall_exit_to_user_mode(regs); <<< too early
		       syscall_exit_to_user_mode_prepare(regs);  <<< too early
 		     kuap_user_restore(regs);         <<< already calls

syscall_exit_prepare(), which can enable IRQs, replay a pending
interrupt, and only then rfi. Those functions already restore KUAP
immediately before rfi.

Note that if we restore the user AMR too early like in the current code
as shown from the code snippets above, then we get the following warning
when CONFIG_PPC_KUAP_DEBUG is enabled:
  WARNING: arch/powerpc/include/asm/book3s/64/kup.h:293 at interrupt_exit_user_prepare+0x1a0/0x1c0
  Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected)
  TRAP: 0700
  LR: c00000000000d8d4 CTR: c0000000021fe500
  MSR: <SF,EE,ME,IR,DR,RI,LE>  CR: 44000804  XER: 20040000
  interrupt_exit_user_prepare+0x1a0/0x1c0
  interrupt_return_srr_user+0x8/0x12c

Fixes: bee25f97ad ("powerpc: Enable GENERIC_ENTRY feature")
Fixes: 02565a782c ("powerpc: Introduce syscall exit arch functions")
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/52fee44fd23acf8e1c024ace668728e626a783a8.1788101609.git.ritesh.list@gmail.com
2026-09-03 15:10:03 +05:30
arch powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() 2026-09-03 15:10:03 +05:30
block for-7.3/block-20260819 2026-08-20 13:55:16 -07:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto This push fixes a memory allocation overrun in crypto acomp. 2026-08-28 09:28:40 -07:00
Documentation Coccinelle patches for Linux v7.3 2026-08-30 10:42:40 -07:00
drivers i2c-fixes for v7.3-rc1 2026-08-30 10:53:24 -07:00
fs tracing fixes for v7.3: 2026-08-30 09:22:00 -07:00
include - Revert a commit to spinlock cleanup guards that got caught up 2026-08-30 09:57:35 -07:00
init Bootconfig updates for v7.3: 2026-08-25 08:59:09 -07:00
io_uring io_uring-7.3-20260828 2026-08-28 16:51:14 -07:00
ipc mm.git review status for master..mm-nonmm-stable 2026-08-23 08:07:11 -07:00
kernel - Revert a commit to spinlock cleanup guards that got caught up 2026-08-30 09:57:35 -07:00
lib mm.git review status for linus..mm-stable 2026-08-27 09:17:06 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm mm.git review status for linus..mm-stable 2026-08-27 09:17:06 -07:00
net A wide variety of mostly CephFS fixes and cleanups, split between 2026-08-28 11:51:05 -07:00
rust Rust fixes for v7.3 2026-08-30 09:47:39 -07:00
samples tracing fixes for v7.3: 2026-08-30 09:22:00 -07:00
scripts Coccinelle patches for Linux v7.3 2026-08-30 10:42:40 -07:00
security Hi, 2026-08-28 09:36:27 -07:00
sound sound fixes for 7.3-rc1 2026-08-28 10:01:02 -07:00
tools Rust fixes for v7.3 2026-08-30 09:47:39 -07:00
usr usr: Correct a spelling by changing a letter 2026-08-05 14:51:58 +02:00
virt Arm: 2026-08-25 11:48:04 -07:00
.clang-format drm/bridge: rename drm_for_each_bridge_in_chain_scoped() to drm_for_each_bridge_in_chain() 2026-07-14 17:55:51 +02:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add Nathan Chancellor 2026-08-03 21:10:12 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap CREDITS/mailmap: add some info about Darrick J. Wong 2026-08-30 10:29:08 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS CREDITS/mailmap: add some info about Darrick J. Wong 2026-08-30 10:29:08 -07:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS SCSI updates for 7.3 2026-08-29 11:55:36 -07:00
Makefile Linux 7.3-rc1 2026-08-30 13:34:40 -07:00
README README: remove out of place emdashes 2026-07-15 10:51:29 -06:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer: Getting started with kernel development
* Academic Researcher: Studying kernel internals and architecture
* Security Expert: Hardening and vulnerability analysis
* Backport/Maintenance Engineer: Maintaining stable kernels
* System Administrator: Configuring and troubleshooting
* Maintainer: Leading subsystems and reviewing patches
* Hardware Vendor: Writing drivers for new hardware
* Distribution Maintainer: Packaging kernels for distros
* AI Coding Assistant: LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst