[PATCH v5 0/5] ARM: support THREAD_INFO_IN_TASK

[Ard Biesheuvel]

Placing thread_info in the kernel stack leaves it vulnerable to stack
overflow attacks. This short series addresses that by using the existing
THREAD_INFO_IN_TASK infrastructure.

Changes since v4:

- Pass -mtp=cp15 to the compiler to force the use of the TLS register
  when __builtin_thread_pointer() is used - this allows us to enable its
  use on Clang as well.

- Tweak the __switch_to() changes not to affect builds that have the
  feature disabled (as requested by Russell), and to defer update of the
  TLS register to the point where the stack pointer is updated as well.

- Tweak the #ifdef's so we avoid touching the TLS registers on builds
  that may target v6 systems without HWCAP_TLS.

- Use task_cpu() in the final patch to assign thread_info->cpu, so that
  this series can be carried independently of the series I proposed for
  moving the CPU field back into thread_info [1].

Changes since v3:

- Leave the CPU field in thread_info, and keep it in sync at context
  switch time. This is by far the easiest and cleanest way to work
  around the fact that it is infeasible to implement
  raw_smp_processor_id() in terms of task_struct::cpu (for reasons of
  header soup).

- Drop the VFP changes, they are no longer necessary given the previous
  point.

- Drop the change to pass the CPU number to secondary_start_kernel().
  Given that we also need to pass the idle task pointer, which carries
  the CPU number, passing the CPU number directly is redundant.

- Use the TPIDRURO register to carry 'current' while running in the
  kernel, and keep using TPIDRPRW for the per-CPU offset as before. This
  way, there is no need to make any changes to the way the per-CPU offsets
  are programmed. It also avoids the concurrency issues that would
  result from carrying the 'current' pointer in a per-CPU variable.

- Update the per-task stack protector plugin to pull the stack canary
  value directly from the task struct.

Cc: Keith Packard <keithpac at amazon.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Kees Cook <keescook at chromium.org>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Linus Walleij <linus.walleij at linaro.org>

[0] https://lore.kernel.org/all/20210907220038.91021-1-keithpac@amazon.com/
[1] https://lore.kernel.org/all/20210914121036.3975026-1-ardb@kernel.org/

Ard Biesheuvel (4):
  gcc-plugins: arm-ssp: Prepare for THREAD_INFO_IN_TASK support
  ARM: smp: Free up the TLS register while running in the kernel
  ARM: smp: Store current pointer in TPIDRURO register if available
  ARM: smp: Enable THREAD_INFO_IN_TASK

Keith Packard (1):
  ARM: smp: Pass task to secondary_start_kernel

 arch/arm/Kconfig                              |  8 +++-
 arch/arm/Makefile                             |  9 ++--
 arch/arm/include/asm/assembler.h              | 29 ++++++++++++
 arch/arm/include/asm/current.h                | 50 ++++++++++++++++++++
 arch/arm/include/asm/smp.h                    |  3 +-
 arch/arm/include/asm/stackprotector.h         |  2 -
 arch/arm/include/asm/switch_to.h              | 16 +++++++
 arch/arm/include/asm/thread_info.h            | 15 ++++--
 arch/arm/include/asm/tls.h                    | 10 ++--
 arch/arm/kernel/asm-offsets.c                 |  6 +--
 arch/arm/kernel/entry-armv.S                  |  5 ++
 arch/arm/kernel/entry-common.S                |  1 +
 arch/arm/kernel/entry-header.S                |  8 ++++
 arch/arm/kernel/head-common.S                 |  5 ++
 arch/arm/kernel/head-nommu.S                  |  1 +
 arch/arm/kernel/head.S                        |  5 +-
 arch/arm/kernel/process.c                     |  8 ++--
 arch/arm/kernel/smp.c                         | 13 ++++-
 arch/arm/mm/proc-macros.S                     |  3 +-
 scripts/gcc-plugins/arm_ssp_per_task_plugin.c | 27 +++--------
 20 files changed, 174 insertions(+), 50 deletions(-)
 create mode 100644 arch/arm/include/asm/current.h

-- 
2.30.2