Re: [RFC PATCH 0/9] arm64: use unwind data on GCC for shadow call stack

Re: [RFC PATCH 0/9] arm64: use unwind data on GCC for shadow call stack

[Nick Desaulniers]

On Wed, Oct 13, 2021 at 8:22 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> This series is a proof of concept implementation of using unwind tables
> to locate PACIASP/AUTIASP instructions in the code, and patching them
> into shadow call stack pushes/pops at boot time if the platform in
> question does not support pointer authentication in hardware. This way,
> the overhead of the shadow call stack is only imposed if it actually
> gives any benefit. It also means that the compiler does not need to
> generate the code, so this works with GCC as well.
>
> In fact, it only works with GCC at the moment, as Clang does not seem to
> implement the DW_CFA_negate_ra_state correctly, which is emitted after
> each PACIASP or AUTIASP instruction (Clang only does the former).
> However, GCC does not appear to get it quite right either, as it emits
> the directive in the wrong place in some cases (but in a way that can be
> worked around).

Can we work on getting bug reports to the compiler vendors? Then we
can have something free of workarounds, and more toolchain portable.

>
> Note that this only implements it for the core kernel. Modules should be
> straight-forward, and most of the code can be reused. Also, the
> transformation is applied unconditionally, even if the hardware does
> implement PAC, but this does not really matter for a PoC.
>
> One obvious downside is the size of the unwind tables (3 MiB for
> defconfig), although there are plenty of use cases where this does not
> really matters (and I haven't checked the compressed size). However,
> there may be other reasons why we'd want to have access to these unwind
> tables (reliable stack traces), so this will need to be discussed before
> I intend to take this any further.
>
> Cc: Kees Cook <keescook@google.com>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: Fangrui Song <maskray@google.com>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Dan Li <ashimida@linux.alibaba.com>
>
> Ard Biesheuvel (9):
>   arm64: assembler: enable PAC for non-leaf assembler routines
>   arm64: cache: use ALIAS version of linkage macros for local aliases
>   arm64: crypto: avoid overlapping linkage definitions for AES-CBC
>   arm64: aes-neonbs: move frame pop to end of function
>   arm64: chacha-neon: move frame pop forward
>   arm64: smccc: create proper stack frames for HVC/SMC calls
>   arm64: assembler: add unwind annotations to frame push/pop macros
>   arm64: unwind: add asynchronous unwind tables to the kernel proper
>   arm64: implement dynamic shadow call stack for GCC
>
>  Makefile                              |   4 +-
>  arch/Kconfig                          |   4 +-
>  arch/arm64/Kconfig                    |  11 +-
>  arch/arm64/Makefile                   |   7 +-
>  arch/arm64/crypto/aes-modes.S         |   4 +-
>  arch/arm64/crypto/aes-neonbs-core.S   |   8 +-
>  arch/arm64/crypto/chacha-neon-core.S  |   9 +-
>  arch/arm64/include/asm/assembler.h    |  32 ++-
>  arch/arm64/include/asm/linkage.h      |  16 +-
>  arch/arm64/kernel/Makefile            |   2 +
>  arch/arm64/kernel/head.S              |   3 +
>  arch/arm64/kernel/patch-scs.c         | 223 ++++++++++++++++++++
>  arch/arm64/kernel/smccc-call.S        |  40 ++--
>  arch/arm64/kernel/vmlinux.lds.S       |  20 ++
>  arch/arm64/mm/cache.S                 |   8 +-
>  drivers/firmware/efi/libstub/Makefile |   1 +
>  16 files changed, 347 insertions(+), 45 deletions(-)
>  create mode 100644 arch/arm64/kernel/patch-scs.c
>
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers