[PATCH v4 00/36] arm64/gcs: Provide support for GCS in userspace

[Mark Brown]

The arm64 Guarded Control Stack (GCS) feature provides support for
hardware protected stacks of return addresses, intended to provide
hardening against return oriented programming (ROP) attacks and to make
it easier to gather call stacks for applications such as profiling.

When GCS is active a secondary stack called the Guarded Control Stack is
maintained, protected with a memory attribute which means that it can
only be written with specific GCS operations.  The current GCS pointer
can not be directly written to by userspace.  When a BL is executed the
value stored in LR is also pushed onto the GCS, and when a RET is
executed the top of the GCS is popped and compared to LR with a fault
being raised if the values do not match.  GCS operations may only be
performed on GCS pages, a data abort is generated if they are not.

The combination of hardware enforcement and lack of extra instructions
in the function entry and exit paths should result in something which
has less overhead and is more difficult to attack than a purely software
implementation like clang's shadow stacks.

This series implements support for use of GCS by userspace, along with
support for use of GCS within KVM guests.  It does not enable use of GCS
by either EL1 or EL2, this will be implemented separately.  Executables
are started without GCS and must use a prctl() to enable it, it is
expected that this will be done very early in application execution by
the dynamic linker or other startup code.

x86 has an equivalent feature called shadow stacks, this series depends
on the x86 patches for generic memory management support for the new
guarded/shadow stack page type and shares APIs as much as possible.  As
there has been extensive discussion with the wider community around the
ABI for shadow stacks I have as far as practical kept implementation
decisions close to those for x86, anticipating that review would lead to
similar conclusions in the absence of strong reasoning for divergence.

The main divergence I am concious of is that x86 allows shadow stack to
be enabled and disabled repeatedly, freeing the shadow stack for the
thread whenever disabled, while this implementation keeps the GCS
allocated after disable but refuses to reenable it.  This is to avoid
races with things actively walking the GCS during a disable, we do
anticipate that some systems will wish to disable GCS at runtime but are
not aware of any demand for subsequently reenabling it.

x86 uses an arch_prctl() to manage enable and disable, since only x86
and S/390 use arch_prctl() a generic prctl() was proposed[1] as part of a
patch set for the equivalent RISC-V zisslpcfi feature which I initially
adopted fairly directly but following review feedback has been revised
quite a bit.

There is an open issue with support for CRIU, on x86 this required the
ability to set the GCS mode via ptrace.  This series supports
configuring mode bits other than enable/disable via ptrace but it needs
to be confirmed if this is sufficient.

There's a few bits where I'm not convinced with where I've placed
things, in particular the GCS write operation is in the GCS header not
in uaccess.h, I wasn't sure what was clearest there and am probably too
close to the code to have a clear opinion.  The reporting of GCS in
/proc/PID/smaps is also a bit awkward.

The series depends on the x86 shadow stack support:

   https://lore.kernel.org/lkml/20230227222957.24501-1-rick.p.edgecombe@xxxxxxxxx/

I've rebased this onto v6.5-rc4 but not included it in the series in
order to avoid confusion with Rick's work and cut down the size of the
series, you can see the branch at:

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/misc.git arm64-gcs

[1] https://lore.kernel.org/lkml/20230213045351.3945824-1-debug@xxxxxxxxxxxx/

Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
Changes in v4:
- Implement flags for map_shadow_stack() allowing the cap and end of
  stack marker to be enabled independently or not at all.
- Relax size and alignment requirements for map_shadow_stack().
- Add more blurb explaining the advantages of hardware enforcement.
- Link to v3: https://lore.kernel.org/r/20230731-arm64-gcs-v3-0-cddf9f980d98@xxxxxxxxxx

Changes in v3:
- Rebase onto v6.5-rc4.
- Add a GCS barrier on context switch.
- Add a GCS stress test.
- Link to v2: https://lore.kernel.org/r/20230724-arm64-gcs-v2-0-dc2c1d44c2eb@xxxxxxxxxx

Changes in v2:
- Rebase onto v6.5-rc3.
- Rework prctl() interface to allow each bit to be locked independently.
- map_shadow_stack() now places the cap token based on the size
  requested by the caller not the actual space allocated.
- Mode changes other than enable via ptrace are now supported.
- Expand test coverage.
- Various smaller fixes and adjustments.
- Link to v1: https://lore.kernel.org/r/20230716-arm64-gcs-v1-0-bf567f93bba6@xxxxxxxxxx

---
Mark Brown (36):
      prctl: arch-agnostic prctl for shadow stack
      arm64: Document boot requirements for Guarded Control Stacks
      arm64/gcs: Document the ABI for Guarded Control Stacks
      arm64/sysreg: Add new system registers for GCS
      arm64/sysreg: Add definitions for architected GCS caps
      arm64/gcs: Add manual encodings of GCS instructions
      arm64/gcs: Provide copy_to_user_gcs()
      arm64/cpufeature: Runtime detection of Guarded Control Stack (GCS)
      arm64/mm: Allocate PIE slots for EL0 guarded control stack
      mm: Define VM_SHADOW_STACK for arm64 when we support GCS
      arm64/mm: Map pages for guarded control stack
      KVM: arm64: Manage GCS registers for guests
      arm64/gcs: Allow GCS usage at EL0 and EL1
      arm64/idreg: Add overrride for GCS
      arm64/hwcap: Add hwcap for GCS
      arm64/traps: Handle GCS exceptions
      arm64/mm: Handle GCS data aborts
      arm64/gcs: Context switch GCS state for EL0
      arm64/gcs: Allocate a new GCS for threads with GCS enabled
      arm64/gcs: Implement shadow stack prctl() interface
      arm64/mm: Implement map_shadow_stack()
      arm64/signal: Set up and restore the GCS context for signal handlers
      arm64/signal: Expose GCS state in signal frames
      arm64/ptrace: Expose GCS via ptrace and core files
      arm64: Add Kconfig for Guarded Control Stack (GCS)
      kselftest/arm64: Verify the GCS hwcap
      kselftest/arm64: Add GCS as a detected feature in the signal tests
      kselftest/arm64: Add framework support for GCS to signal handling tests
      kselftest/arm64: Allow signals tests to specify an expected si_code
      kselftest/arm64: Always run signals tests with GCS enabled
      kselftest/arm64: Add very basic GCS test program
      kselftest/arm64: Add a GCS test program built with the system libc
      kselftest/arm64: Add test coverage for GCS mode locking
      selftests/arm64: Add GCS signal tests
      kselftest/arm64: Add a GCS stress test
      kselftest/arm64: Enable GCS for the FP stress tests

 Documentation/admin-guide/kernel-parameters.txt    |   3 +
 Documentation/arch/arm64/booting.rst               |  22 +
 Documentation/arch/arm64/elf_hwcaps.rst            |   3 +
 Documentation/arch/arm64/gcs.rst                   | 228 +++++++++
 Documentation/arch/arm64/index.rst                 |   1 +
 Documentation/filesystems/proc.rst                 |   2 +-
 arch/arm64/Kconfig                                 |  19 +
 arch/arm64/include/asm/cpufeature.h                |   6 +
 arch/arm64/include/asm/el2_setup.h                 |  17 +
 arch/arm64/include/asm/esr.h                       |  28 +-
 arch/arm64/include/asm/exception.h                 |   2 +
 arch/arm64/include/asm/gcs.h                       | 106 ++++
 arch/arm64/include/asm/hwcap.h                     |   1 +
 arch/arm64/include/asm/kvm_arm.h                   |   4 +-
 arch/arm64/include/asm/kvm_host.h                  |  12 +
 arch/arm64/include/asm/pgtable-prot.h              |  14 +-
 arch/arm64/include/asm/processor.h                 |   7 +
 arch/arm64/include/asm/sysreg.h                    |  20 +
 arch/arm64/include/asm/uaccess.h                   |  42 ++
 arch/arm64/include/uapi/asm/hwcap.h                |   1 +
 arch/arm64/include/uapi/asm/ptrace.h               |   8 +
 arch/arm64/include/uapi/asm/sigcontext.h           |   9 +
 arch/arm64/kernel/cpufeature.c                     |  19 +
 arch/arm64/kernel/cpuinfo.c                        |   1 +
 arch/arm64/kernel/entry-common.c                   |  23 +
 arch/arm64/kernel/idreg-override.c                 |   2 +
 arch/arm64/kernel/process.c                        |  85 ++++
 arch/arm64/kernel/ptrace.c                         |  59 +++
 arch/arm64/kernel/signal.c                         | 237 ++++++++-
 arch/arm64/kernel/traps.c                          |  11 +
 arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h         |  17 +
 arch/arm64/kvm/sys_regs.c                          |  22 +
 arch/arm64/mm/Makefile                             |   1 +
 arch/arm64/mm/fault.c                              |  78 ++-
 arch/arm64/mm/gcs.c                                | 234 +++++++++
 arch/arm64/mm/mmap.c                               |  12 +-
 arch/arm64/tools/cpucaps                           |   1 +
 arch/arm64/tools/sysreg                            |  55 +++
 fs/proc/task_mmu.c                                 |   3 +
 include/linux/mm.h                                 |  16 +-
 include/linux/syscalls.h                           |   1 +
 include/uapi/asm-generic/unistd.h                  |   5 +-
 include/uapi/linux/elf.h                           |   1 +
 include/uapi/linux/prctl.h                         |  22 +
 kernel/sys.c                                       |  30 ++
 kernel/sys_ni.c                                    |   1 +
 tools/testing/selftests/arm64/Makefile             |   2 +-
 tools/testing/selftests/arm64/abi/hwcap.c          |  19 +
 tools/testing/selftests/arm64/fp/assembler.h       |  15 +
 tools/testing/selftests/arm64/fp/fpsimd-test.S     |   2 +
 tools/testing/selftests/arm64/fp/sve-test.S        |   2 +
 tools/testing/selftests/arm64/fp/za-test.S         |   2 +
 tools/testing/selftests/arm64/fp/zt-test.S         |   2 +
 tools/testing/selftests/arm64/gcs/.gitignore       |   5 +
 tools/testing/selftests/arm64/gcs/Makefile         |  24 +
 tools/testing/selftests/arm64/gcs/asm-offsets.h    |   0
 tools/testing/selftests/arm64/gcs/basic-gcs.c      | 356 ++++++++++++++
 tools/testing/selftests/arm64/gcs/gcs-locking.c    | 200 ++++++++
 .../selftests/arm64/gcs/gcs-stress-thread.S        | 311 ++++++++++++
 tools/testing/selftests/arm64/gcs/gcs-stress.c     | 532 +++++++++++++++++++++
 tools/testing/selftests/arm64/gcs/gcs-util.h       |  87 ++++
 tools/testing/selftests/arm64/gcs/libc-gcs.c       | 500 +++++++++++++++++++
 tools/testing/selftests/arm64/signal/.gitignore    |   1 +
 .../testing/selftests/arm64/signal/test_signals.c  |  17 +-
 .../testing/selftests/arm64/signal/test_signals.h  |   6 +
 .../selftests/arm64/signal/test_signals_utils.c    |  32 +-
 .../selftests/arm64/signal/test_signals_utils.h    |  39 ++
 .../arm64/signal/testcases/gcs_exception_fault.c   |  59 +++
 .../selftests/arm64/signal/testcases/gcs_frame.c   |  78 +++
 .../arm64/signal/testcases/gcs_write_fault.c       |  67 +++
 .../selftests/arm64/signal/testcases/testcases.c   |   7 +
 .../selftests/arm64/signal/testcases/testcases.h   |   1 +
 72 files changed, 3823 insertions(+), 34 deletions(-)
---
base-commit: ed0e1456f04be7a93c9a186e8e13aed78b555617
change-id: 20230303-arm64-gcs-e311ab0d8729

Best regards,
-- 
Mark Brown <broonie@xxxxxxxxxx>