Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

[H.J. Lu]

On Wed, Feb 26, 2020 at 10:05 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 2/5/20 10:19 AM, Yu-cheng Yu wrote:
> > +# Check assembler Shadow Stack suppot
>
>                                   ^ support
>
> > +ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> > +  ifeq ($(call as-instr, saveprevssp, y),)
> > +      $(error CONFIG_X86_INTEL_SHADOW_STACK_USER not supported by the assembler)
> > +  endif
> > +endif
>
> Is this *just* looking for instruction support in the assembler?
>
> We usually just .byte them, like this for pkeys:
>
>         asm volatile(".byte 0x0f,0x01,0xee\n\t"
>                      : "=a" (pkru), "=d" (edx)
>                      : "c" (ecx));
>
> That way everybody with old toolchains can still build the kernel (and
> run/test code with your config option on, btw...).

CET requires a complete new OS image from kernel, toolchain, run-time.
CET enabled kernel without the rest of updated OS won't give you CET
at all.

-- 
H.J.