Re: [RFC PATCH v9 18/27] x86/cet/shstk: Introduce WRUSS instruction

[Kees Cook]

On Wed, Feb 05, 2020 at 10:19:26AM -0800, Yu-cheng Yu wrote:
> WRUSS is a new kernel-mode instruction but writes directly to user Shadow
> Stack (SHSTK) memory.  This is used to construct a return address on SHSTK
> for the signal handler.
> 
> This instruction can fault if the user SHSTK is not valid SHSTK memory.
> In that case, the kernel does a fixup.

Since these functions aren't used in this patch, should this get merged
with patch 19?

-Kees

> 
> v4:
> - Change to asm goto.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 6d37b8fcfc77..1b9b2e79c353 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -222,6 +222,38 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_INTEL_CET
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	asm_volatile_goto("1: wrussd %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	WARN_ONCE(1, "%s used but not supported.\n", __func__);
> +	return -EFAULT;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	asm_volatile_goto("1: wrussq %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  
> -- 
> 2.21.0
> 

-- 
Kees Cook