[RFC PATCH v2 0/3] arm64: dynamic shadow call stack support

[Ard Biesheuvel]

Generic kernel images such as Android's GKI typically enable all
security features, which are typically implemented in such a way that
they only take effect if the underlying hardware can support it, but
don't interfere with correct and efficient operation otherwise.

For shadow call stack support, which is always supported by the
hardware, it means it will be enabled even if pointer authentication is
also supported, and enabled for signing return addresses stored on the
stack. The additional security provided by shadow call stack is only
marginal in this case, whereas the performance overhead is not.

Given that return address signing is based on PACIASP/AUTIASP
instructions that implicitly operate on the return address register
(X30) and are not idempotent (i.e., each needs to be emitted exactly
once before the return address is stored on the ordinary stack and after
it has been retrieved from it), we can convert these instruction 1:1
into shadow call stack pushed and pops involving the register X30.
As this is something that can be done at runtime rather than build time,
we can do this conditionally based on whether or not return address
signing is supported on the underlying hardware.

In order to be able to unwind call stacks that involve return address
signing, whether or not the return address is currently signed is
tracked by DWARF CFI directives in the unwinding metadata. This means we
can use this information to locate all PACIASP/AUTIASP instructions in
the binary, instead of having to use brute force and go over all
instructions in the entire program.

This series implements this approach for Clang, which has recently been
fixed to emit all these CFI directives correctly. This series is based
on an older PoC sent out last year [0] that targeted GCC only (due to
this issue). This v2 targets Clang only, as GCC has its own issues with
CFI correctness.

Changes since RFC v1:
- implement boot time check for PAC/BTI support, and only enable dynamic
  SCS if neither are supported;
- implement module patching as well;
- switch to Clang, and drop workaround for GCC bug;

[0] https://lore.kernel.org/linux-arm-kernel/20211013152243.2216899-1-ardb@kernel.org/

Cc: Kees Cook <keescook at google.com>
Cc: Sami Tolvanen <samitolvanen at google.com>
Cc: Fangrui Song <maskray at google.com>
Cc: Nick Desaulniers <ndesaulniers at google.com>
Cc: Dan Li <ashimida at linux.alibaba.com>

Ard Biesheuvel (3):
  arm64: unwind: add asynchronous unwind tables to kernel and modules
  scs: add support for dynamic shadow call stacks
  arm64: implement dynamic shadow call stack for Clang

 Makefile                              |   2 +
 arch/Kconfig                          |   7 +
 arch/arm64/Kconfig                    |  11 +
 arch/arm64/Makefile                   |   5 +
 arch/arm64/include/asm/module.lds.h   |   8 +
 arch/arm64/include/asm/scs.h          |  12 +
 arch/arm64/kernel/Makefile            |   2 +
 arch/arm64/kernel/head.S              |   3 +
 arch/arm64/kernel/irq.c               |   2 +-
 arch/arm64/kernel/module.c            |  10 +
 arch/arm64/kernel/patch-scs.c         | 257 ++++++++++++++++++++
 arch/arm64/kernel/sdei.c              |   2 +-
 arch/arm64/kernel/setup.c             |   5 +
 arch/arm64/kernel/vmlinux.lds.S       |  16 ++
 drivers/firmware/efi/libstub/Makefile |   1 +
 include/linux/scs.h                   |  10 +
 kernel/scs.c                          |  14 +-
 17 files changed, 363 insertions(+), 4 deletions(-)
 create mode 100644 arch/arm64/kernel/patch-scs.c

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2.30.2