Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying shstk

[Andrew Cooper]

On 29/09/2022 23:29, Rick Edgecombe wrote:
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 35f709f619fb..f096f52bd059 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -223,6 +223,19 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_SHADOW_STACK
> +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
> +{
> +	asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: [addr] "r" (addr), [val] "r" (val)
> +			  :: fail);

"1: wrssq %[val], %[addr]\n"
_ASM_EXTABLE(1b, %l[fail])
: [addr] "+m" (*addr)
: [val] "r" (val)
:: fail

Otherwise you've failed to tell the compiler that you wrote to *addr.

With that fixed, it's not volatile because there are no unexpressed side
effects.

~Andrew