[PATCH v2 0/5] Optionally randomize kernel stack offset each syscall

Kees Cook
2020-03-24T20:32:45Z

Hi,

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.

-Kees

[1] https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
[2] https://repositorio-aberto.up.pt/bitstream/10216/125357/2/374717.pdf

v2:
- 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
rfc: https://lore.kernel.org/kernel-hardening/20190329081358.30497-1-elena.reshetova@intel.com/

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

--
2.20.1