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Date: Tue, 24 Mar 2020 13:32:26 -0700
From: Kees Cook <>
To: Thomas Gleixner <>
Cc: Kees Cook <>,
	Elena Reshetova <>,,
	Andy Lutomirski <>,
	Peter Zijlstra <>,
	Catalin Marinas <>,
	Will Deacon <>,
	Mark Rutland <>,
	Alexander Potapenko <>,
	Ard Biesheuvel <>,
	Jann Horn <>,
	"Perla, Enrico" <>,,,,
Subject: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall


This is a continuation and refactoring of Elena's earlier effort to add
kernel stack base offset randomization. In the time since the previous
discussions, two attacks[1][2] were made public that depended on stack
determinism, so we're no longer in the position of "this is a good idea
but we have no examples of attacks". :)

Earlier discussions also devolved into debates on entropy sources, which
is mostly a red herring, given the already low entropy available due
to stack size. Regardless, entropy can be changed/improved separately
from this series as needed.

Earlier discussions also got stuck debating how much syscall overhead
was too much, but this is also a red herring since the feature itself
needs to be selectable at boot with no cost for those that don't want it:
this is solved here with static branches.

So, here is an improved version, made as arch-agnostic as possible,
with usage added for x86 and arm64. It also includes some small static
branch clean ups.



- move to per-cpu rdtsc() saved on syscall exit
- add static branches for zero-cost dynamic enabling
- Kconfig just selects the default state of static branch
- __builtin_alloca() produces ugly asm without -fno-stack-clash-protection
- made arch agnostic

Kees Cook (5):
  jump_label: Provide CONFIG-driven build state defaults
  init_on_alloc: Unpessimize default-on builds
  stack: Optionally randomize kernel stack offset each syscall
  x86/entry: Enable random_kstack_offset support
  arm64: entry: Enable random_kstack_offset support

 Makefile                         |  4 ++++
 arch/Kconfig                     | 19 +++++++++++++++
 arch/arm64/Kconfig               |  1 +
 arch/arm64/kernel/syscall.c      | 10 ++++++++
 arch/x86/Kconfig                 |  1 +
 arch/x86/entry/common.c          | 12 +++++++++-
 include/linux/jump_label.h       | 19 +++++++++++++++
 include/linux/mm.h               | 18 +++++---------
 include/linux/randomize_kstack.h | 40 ++++++++++++++++++++++++++++++++
 init/main.c                      | 23 ++++++++++++++++++
 mm/page_alloc.c                  | 12 ++--------
 11 files changed, 136 insertions(+), 23 deletions(-)
 create mode 100644 include/linux/randomize_kstack.h


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