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SubjectRe: [PATCH] [PATCH] AARCH64: Add gcc Shadow Call Stack support
On Tue, Feb 22, 2022 at 01:57:36AM -0800, Dan Li wrote:
> Shadow call stack is available in GCC > 11.2.0, this patch makes
> the corresponding kernel configuration available when compiling
> the kernel with gcc.
>
> Note that the implementation in GCC is slightly different from Clang.
> With SCS enabled, functions will only pop x30 once in the epilogue,
> like:
>
> str x30, [x18], #8
> stp x29, x30, [sp, #-16]!
> ......
> - ldp x29, x30, [sp], #16 //clang
> + ldr x29, [sp], #16 //GCC
> ldr x30, [x18, #-8]!
>
> Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=ce09ab17ddd21f73ff2caf6eec3b0ee9b0e1a11e
>
> Signed-off-by: Dan Li <ashimida@linux.alibaba.com>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

A few open-ended comments below.

> ---
> FYI:
> This function can be used to test if the shadow call stack works:
> //noinline void __noscs scs_test(void)
> noinline void scs_test(void)
> {
> register unsigned long *sp asm("sp");
> unsigned long * lr = sp + 1;
>
> asm volatile("":::"x30");
> *lr = 0;
> }
>
> ffff800008012704: d503233f paciasp
> ffff800008012708: f800865e str x30, [x18], #8
> ffff80000801270c: a9bf7bfd stp x29, x30, [sp, #-16]!
> ffff800008012710: 910003fd mov x29, sp
> ffff800008012714: 910003e0 mov x0, sp
> ffff800008012718: f900041f str xzr, [x0, #8]
> ffff80000801271c: f85f8e5e ldr x30, [x18, #-8]!
> ffff800008012720: f84107fd ldr x29, [sp], #16
> ffff800008012724: d50323bf autiasp
> ffff800008012728: d65f03c0 ret
>
> If SCS protection is enabled, this function will return normally.
> If the function has __noscs attribute (scs disabled), it will crash due to 0
> address access.
>
> arch/Kconfig | 6 +++---
> arch/arm64/Kconfig | 2 +-
> include/linux/compiler-gcc.h | 4 ++++
> 3 files changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 678a80713b21..35db7b72bdb0 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -604,11 +604,11 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
> switching.
>
> config SHADOW_CALL_STACK
> - bool "Clang Shadow Call Stack"
> - depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
> + bool "Shadow Call Stack"
> + depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
> help
> - This option enables Clang's Shadow Call Stack, which uses a
> + This option enables Clang/GCC's Shadow Call Stack, which uses a

I wonder if we want to just ditch the mention of the compiler if both
support it?

> shadow stack to protect function return addresses from being
> overwritten by an attacker. More information can be found in
> Clang's documentation:
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 09b885cc4db5..a48a604301aa 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1255,7 +1255,7 @@ config HW_PERF_EVENTS
> config ARCH_HAS_FILTER_PGPROT
> def_bool y
>
> -# Supported by clang >= 7.0
> +# Supported by clang >= 7.0 or GCC > 11.2.0

Same thing here, although eventually there may be a minimum GCC version
bump to something newer than 11.2.0, which would allow us to just drop
CONFIG_CC_HAVE_SHADOW_CALL_STACK altogether. No strong opinion.

> config CC_HAVE_SHADOW_CALL_STACK
> def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index ccbbd31b3aae..deff5b308470 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -97,6 +97,10 @@
> #define KASAN_ABI_VERSION 4
> #endif
>
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +#define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
> +#endif
> +
> #if __has_attribute(__no_sanitize_address__)
> #define __no_sanitize_address __attribute__((no_sanitize_address))
> #else
> --
> 2.17.1
>

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