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Date: Thu, 24 Oct 2019 15:51:15 -0700
To: Will Deacon <>, Catalin Marinas <>, 
	Steven Rostedt <>, Masami Hiramatsu <>, 
	Ard Biesheuvel <>
Cc: Dave Martin <>, Kees Cook <>, 
	Laura Abbott <>, Mark Rutland <>, 
	Nick Desaulniers <>, Jann Horn <>, 
	Miguel Ojeda <>, 
	Masahiro Yamada <>,,,,, Sami Tolvanen <>
Subject: [PATCH v2 00/17] add support for Clang's Shadow Call Stack

This patch series adds support for Clang's Shadow Call Stack
(SCS) mitigation, which uses a separately allocated shadow stack
to protect against return address overwrites. More information
can be found here:

SCS provides better protection against traditional buffer
overflows than CONFIG_STACKPROTECTOR_*, but it should be noted
that SCS security guarantees in the kernel differ from the ones
documented for user space. The kernel must store addresses of
shadow stacks used by other tasks and interrupt handlers in
memory, which means an attacker capable reading and writing
arbitrary memory may be able to locate them and hijack control
flow by modifying shadow stacks that are not currently in use.

SCS is currently supported only on arm64, where the compiler
requires the x18 register to be reserved for holding the current
task's shadow stack pointer. Because of this, the series includes
patches from Ard to remove x18 usage from assembly code.

With -fsanitize=shadow-call-stack, the compiler injects
instructions to all non-leaf C functions to store the return
address to the shadow stack, and unconditionally load it again
before returning. As a result, SCS is currently incompatible
with features that rely on modifying function return addresses
to alter control flow, such as function graph tracing and
kretprobes, although it may be possible to later change these
feature to modify the shadow stack instead. A copy of the return
address is still kept in the kernel stack for compatibility with
stack unwinding, for example.

SCS has a minimal performance overhead, but allocating
shadow stacks increases kernel memory usage. The feature is
therefore mostly useful on hardware that lacks support for PAC

Changes in v2:
 - Changed Ard's KVM patch to use x29 instead of x18 for the
   guest context, which makes restore_callee_saved_regs cleaner
 - Updated help text (and commit messages) to point out
   differences in security properties compared to user space SCS
 - Cleaned up config options: removed the ROP protection choice,
   replaced the CC_IS_CLANG dependency with an arch-specific
   cc-option test, and moved disabling of incompatible config
   options to an arch-specific Kconfig
 - Added CC_FLAGS_SCS, which are filtered out where needed
   instead of using DISABLE_SCS
 - Added a __has_feature guard around __noscs for older clang
 - Changed the shadow stack overflow check for vmapped SCS to
   use SCS_SIZE to avoid surprises when changing configs
 - Renamed SCS_GFP to GFP_SCS
 - Dropped the patch to reserve x18 unconditionally, it's now
   only reserved with SCS
 - Added a clarification why restoring x18 in the EFI RT
   wrapper is safe
 - Added a missing change to arch/arm64/include/asm/suspend.h,
   and a comment to arch/arm64/mm/proc.S to remind that struct
   cpu_suspend_ctx must be kept in sync with the code
 - Moved x18 loading/storing during a context switch to
   cpu_switch_to(), renamed scs_thread_switch() to
   scs_overflow_check(), and removed the now unused scs_load()
 - Added compile-time initialization for init_shadow_call_stack
   and removed scs_set_init_magic()

Ard Biesheuvel (2):
  arm64/lib: copy_page: avoid x18 register in assembler code
  arm64: kernel: avoid x18 as an arbitrary temp register

Sami Tolvanen (15):
  arm64: mm: don't use x18 in idmap_kpti_install_ng_mappings
  arm64: kvm: stop treating register x18 as caller save
  add support for Clang's Shadow Call Stack (SCS)
  scs: add accounting
  scs: add support for stack usage debugging
  kprobes: fix compilation without CONFIG_KRETPROBES
  arm64: disable function graph tracing with SCS
  arm64: disable kretprobes with SCS
  arm64: reserve x18 from general allocation with SCS
  arm64: preserve x18 when CPU is suspended
  arm64: efi: restore x18 if it was corrupted
  arm64: vdso: disable Shadow Call Stack
  arm64: kprobes: fix kprobes without CONFIG_KRETPROBES
  arm64: disable SCS for hypervisor code
  arm64: implement Shadow Call Stack

 Makefile                             |   6 +
 arch/Kconfig                         |  33 +++++
 arch/arm64/Kconfig                   |   9 +-
 arch/arm64/Makefile                  |   4 +
 arch/arm64/include/asm/scs.h         |  45 ++++++
 arch/arm64/include/asm/stacktrace.h  |   4 +
 arch/arm64/include/asm/suspend.h     |   2 +-
 arch/arm64/include/asm/thread_info.h |   3 +
 arch/arm64/kernel/Makefile           |   1 +
 arch/arm64/kernel/asm-offsets.c      |   3 +
 arch/arm64/kernel/cpu-reset.S        |   4 +-
 arch/arm64/kernel/efi-rt-wrapper.S   |   7 +-
 arch/arm64/kernel/entry.S            |  28 ++++
 arch/arm64/kernel/head.S             |   9 ++
 arch/arm64/kernel/irq.c              |   2 +
 arch/arm64/kernel/probes/kprobes.c   |   2 +
 arch/arm64/kernel/process.c          |   2 +
 arch/arm64/kernel/scs.c              |  39 +++++
 arch/arm64/kernel/smp.c              |   4 +
 arch/arm64/kernel/vdso/Makefile      |   2 +-
 arch/arm64/kvm/hyp/Makefile          |   3 +
 arch/arm64/kvm/hyp/entry.S           |  41 +++--
 arch/arm64/lib/copy_page.S           |  38 ++---
 arch/arm64/mm/proc.S                 |  72 +++++----
 drivers/base/node.c                  |   6 +
 fs/proc/meminfo.c                    |   4 +
 include/linux/compiler-clang.h       |   6 +
 include/linux/compiler_types.h       |   4 +
 include/linux/mmzone.h               |   3 +
 include/linux/scs.h                  |  78 ++++++++++
 init/init_task.c                     |   8 +
 kernel/Makefile                      |   1 +
 kernel/fork.c                        |   9 ++
 kernel/kprobes.c                     |  38 ++---
 kernel/sched/core.c                  |   2 +
 kernel/sched/sched.h                 |   1 +
 kernel/scs.c                         | 214 +++++++++++++++++++++++++++
 mm/page_alloc.c                      |   6 +
 mm/vmstat.c                          |   3 +
 39 files changed, 649 insertions(+), 97 deletions(-)
 create mode 100644 arch/arm64/include/asm/scs.h
 create mode 100644 arch/arm64/kernel/scs.c
 create mode 100644 include/linux/scs.h
 create mode 100644 kernel/scs.c


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