[RFC/RFT PATCH 0/2] arm64: per-task stack canaries

[Ard Biesheuvel]

This is a proof of concept implementation of per-task stack canaries for
arm64. The purpose is to reach agreement between the arm64 kernel and GCC
maintainers on how to implement support for this in the compiler.

What these patches show is that we can support per-task stack canaries
on arm64 using only very minor changes on the kernel side, i.e., all
that is needed is exposing the offset of stack_canary in task_struct
via an ELF symbol. With that in place, the compiler needs to emit the
following sequence when -fstack-protector-guard=tls is enabled

  movz    xN, :abs_g0:__stack_chk_guard_offset
  msr     xM, sp_el0
  ldr     xM, [xM, xN]

Note that this does not involve per-CPU variables, and so there are no
concurrency issues to be addressed. sp_el0 is the current task pointer,
whose value never changes from the POV of the task, even when migrating
to another CPU.

Patch #1 implements a GCC plugin that patches the sequence

  adrp    xN, __stack_chk_guard
  add     xN, Xn, :lo12:__stack_chk_guard

into

  mrs     xN, sp_el0
  add     xN, xN, :lo12:__stack_chk_guard_offset

which is a poor man's version of the movz/msr/ldr sequence above (and only
works for small model code), but is sufficient as a proof of concept.

Patch #2 exposes the __stack_chk_guard_offset symbol and wires up the plugin
(if enabled in Kconfig)

Again, the point is not to use GCC plugin based hacks, but to reach agreement
on how to proceed with this for GCC.

Comments welcome.

Ard Biesheuvel (2):
  gcc-plugins: add support plugin for arm64 per-task stack canaries
  arm64: kernel: use a unique stack canary value for each task

 arch/Kconfig                                    |   4 +
 arch/arm64/Kconfig                              |   7 ++
 arch/arm64/include/asm/stackprotector.h         |   4 +-
 arch/arm64/kernel/asm-offsets.c                 |   3 +
 arch/arm64/kernel/process.c                     |   4 +
 arch/arm64/kernel/vmlinux.lds.S                 |   8 ++
 scripts/Makefile.gcc-plugins                    |   2 +
 scripts/gcc-plugins/arm64_ssp_per_task_plugin.c | 121 ++++++++++++++++++++
 8 files changed, 152 insertions(+), 1 deletion(-)
 create mode 100644 scripts/gcc-plugins/arm64_ssp_per_task_plugin.c

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2.11.0