[v5,0/7] Optionally randomize kernel stack offset each syscall

Message

Kees Cook March 9, 2021, 9:42 p.m. UTC

v5:
- rebase to v5.12-rc2
- clean up static branch issues introduced since v4 series
- adjust comments (Mark Rutland)
- update kernel-parameters.txt (Randy Dunlap)
v4: https://lore.kernel.org/lkml/[e-mail]/
v3: https://lore.kernel.org/lkml/[e-mail]/
v2: https://lore.kernel.org/lkml/[e-mail]/
rfc: https://lore.kernel.org/kernel-hardening/[e-mail]/

Hi,

This is a continuation and refactoring of Elena's earlier effort to add
kernel stack base offset randomization. In the time since the earlier
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 the latest improved version, made as arch-agnostic as
possible, with usage added for x86 and arm64. It also includes some small
static branch clean ups, and addresses some surprise performance issues
due to the stack canary[3].

At the very least, the first three patches should land ASAP, the first
is a minor bug fix for v5.11. The next two are optimizations for static
branch usage that Peter already Acked.

If I can get an Ack from an arm64 maintainer, I think this could all
land via -tip to make merging easiest.

Thanks!

-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
[3] https://lore.kernel.org/lkml/202003281520.A9BFF461@keescook/

Kees Cook (7):
  mm: Restore init_on_* static branch defaults
  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
  lkdtm: Add REPORT_STACK for checking stack offsets

 .../admin-guide/kernel-parameters.txt         | 11 +++++
 Makefile                                      |  4 ++
 arch/Kconfig                                  | 23 ++++++++++
 arch/arm64/Kconfig                            |  1 +
 arch/arm64/kernel/Makefile                    |  5 +++
 arch/arm64/kernel/syscall.c                   | 10 +++++
 arch/x86/Kconfig                              |  1 +
 arch/x86/entry/common.c                       |  3 ++
 arch/x86/include/asm/entry-common.h           |  8 ++++
 drivers/misc/lkdtm/bugs.c                     | 17 ++++++++
 drivers/misc/lkdtm/core.c                     |  1 +
 drivers/misc/lkdtm/lkdtm.h                    |  1 +
 include/linux/jump_label.h                    | 19 +++++++++
 include/linux/mm.h                            | 10 +++--
 include/linux/randomize_kstack.h              | 42 +++++++++++++++++++
 init/main.c                                   | 23 ++++++++++
 mm/page_alloc.c                               |  4 +-
 17 files changed, 177 insertions(+), 6 deletions(-)
 create mode 100644 include/linux/randomize_kstack.h