[PATCH v5 0/5] ARM: support THREAD_INFO_IN_TASK

[Amit Kachhap]

On 9/18/21 2:14 PM, Ard Biesheuvel wrote:
> 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>

This series boots with both CURRENT_POINTER_IN_TPIDRURO and 
!CURRENT_POINTER_IN_TPIDRURO mode so,

Tested-by: Amit Daniel Kachhap <amit.kachhap at arm.com>

Thanks,
Amit Daniel

> 
> [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
>