[RFC PATCH 0/4] Allow tasks to have their user stack pointer sanity checked

[Will Deacon]

Hi all,

I attended an interesting talk at LCA last month that described some of the
security features deployed in OpenBSD [1]. One hardening feature that piqued
my interest was, on syscall entry and page faults from userspace, checking
that the user stack pointer for a task points at pages that were either
allocated by the kernel for the initial process stack of mapped with mmap()
using the MAP_STACK flag. This acts as a basic defense against stack
pivoting attacks.

The problem with this checking is that it is a retrospective tightening
of the ABI, but that hasn't stopped me hacking it together behind a couple
of prctl() options.

Anyway, it was fun to implement so I figured I'd post it as an RFC.

Will

[1] https://2019.linux.conf.au/schedule/presentation/164/

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

--->8

Will Deacon (4):
  mm: Check user stack pointer is mapped with MAP_STACK
  mm: Expose user stack pointer checking via prctl()
  mm: Add kconfig entries for user stack pointer checking
  arm64: Check user stack pointer on syscall entry

 arch/arm64/Kconfig          |  1 +
 arch/arm64/kernel/syscall.c |  4 +++
 include/linux/mm.h          | 15 +++++++++-
 include/linux/mman.h        |  3 +-
 include/linux/sched.h       |  4 +++
 include/uapi/linux/prctl.h  |  5 ++++
 kernel/sys.c                |  5 ++++
 mm/Kconfig                  | 17 ++++++++++++
 mm/memory.c                 | 67 +++++++++++++++++++++++++++++++++++++++++++++
 9 files changed, 119 insertions(+), 2 deletions(-)

-- 
2.11.0