Re: [PATCH] stack: replace "o" output with "r" input constraint

[Nathan Chancellor]

On Mon, Apr 19, 2021 at 04:17:41PM -0700, Kees Cook wrote:
> From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> 
> "o" isn't a common asm() constraint to use; it triggers an assertion in
> assert-enabled builds of LLVM that it's not recognized when targeting
> aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
> 13 now, but there isn't really a good reason to be using "o" in particular
> here. To avoid causing build issues for those using assert-enabled builds
> of earlier LLVM versions, the constraint needs changing.
> 
> Instead, if the point is to retain the __builtin_alloca(), we can make ptr
> appear to "escape" via being an input to an empty inline asm block. This
> is preferable anyways, since otherwise this looks like a dead store.
> 
> While the use of "r" was considered in
> https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
> it was only tested as an output (which looks like a dead store, and
> wasn't sufficient). Use "r" as an input constraint instead, which
> behaves correctly across compilers and architectures:
> https://godbolt.org/z/E9cd411ob
> 
> Link: https://reviews.llvm.org/D100412
> Link: https://bugs.llvm.org/show_bug.cgi?id=49956
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

I built arm64 defconfig with and without
CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT with LLVM 12 (which does not have
Nick's LLVM fix) without any issues and did a quick boot test in QEMU,
nothing exploded.

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
>  include/linux/randomize_kstack.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> index fd80fab663a9..bebc911161b6 100644
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
>  		u32 offset = raw_cpu_read(kstack_offset);		\
>  		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
>  		/* Keep allocation even after "ptr" loses scope. */	\
> -		asm volatile("" : "=o"(*ptr) :: "memory");		\
> +		asm volatile("" :: "r"(ptr) : "memory");		\
>  	}								\
>  } while (0)
>  
> -- 
> 2.25.1
>