Re: [PATCH v24 04/30] x86/cpufeatures: Introduce X86_FEATURE_CET and setup functions

[Yu, Yu-cheng]

On 4/9/2021 10:14 AM, Borislav Petkov wrote:
> On Fri, Apr 09, 2021 at 08:52:52AM -0700, Yu, Yu-cheng wrote:
> > Recall we had complicated code for the XSAVES features detection in
> > xstate.c.  Dave Hansen proposed the solution and then the whole thing
> > becomes simple.  Because of this flag, even when only the shadow stack is
> > available, the code handles it nicely.
>
> Is that what you mean?
>
> @@ -53,6 +55,8 @@ static short xsave_cpuid_features[] __initdata = {
>   	X86_FEATURE_INTEL_PT,
>   	X86_FEATURE_PKU,
>   	X86_FEATURE_ENQCMD,
> +	X86_FEATURE_CET, /* XFEATURE_CET_USER */
> +	X86_FEATURE_CET, /* XFEATURE_CET_KERNEL */
>
> or what is the piece which becomes simpler?

Yes, this is it.

> > Would this equal to only CONFIG_X86_CET (one Kconfig option)?  In fact, when
> > you proposed only CONFIG_X86_CET, things became much simpler.
>
> When you use CONFIG_X86_SHADOW_STACK instead, it should remain same
> simple no?

Signals, arch_prctl, and ELF header are three places that need to depend 
on either shadow stack or IBT is configured.  To remain simple, we can 
make all three depend on CONFIG_X86_SHADOW_STACK, and in Kconfig, make 
CONFIG_X86_IBT depend on CONFIG_X86_SHADOW_STACK.  Without shadow stack, 
IBT itself is not as useful anyway.

> > Practically, IBT is not much in terms of code size.  Since we have already
> > separated the two, why don't we leave it as-is.  When people start using it
> > more, there will be more feedback, and we can decide if one Kconfig is
> > better?
>
> Because when we add stuff to the kernel, we add the simplest and
> cleanest version possible and later, when we determine that additional
> functionality is needed, *then* we add it. Not the other way around.
>
> Our Kconfig symbol space is already an abomination so we can't just add
> some more and decide later.
>
> What happens in such situations usually is stuff gets added, it bitrots
> and some poor soul - very likely a maintainer who has to mop up after
> everybody - comes and cleans it up. I'd like to save myself that
> cleaning up.
>
> Thx.