Re: [NEEDS-REVIEW] [PATCH v15 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states

[Yu, Yu-cheng]

On 11/30/2020 9:45 AM, Dave Hansen wrote:
> On 11/10/20 8:21 AM, Yu-cheng Yu wrote:
> > Control-flow Enforcement Technology (CET) adds five MSRs.  Introduce
> > them and their XSAVES supervisor states:
> >
> >      MSR_IA32_U_CET (user-mode CET settings),
> >      MSR_IA32_PL3_SSP (user-mode Shadow Stack pointer),
> >      MSR_IA32_PL0_SSP (kernel-mode Shadow Stack pointer),
> >      MSR_IA32_PL1_SSP (Privilege Level 1 Shadow Stack pointer),
> >      MSR_IA32_PL2_SSP (Privilege Level 2 Shadow Stack pointer).
>
> This patch goes into a bunch of XSAVE work that this changelog only
> briefly touches on.  I think it needs to be beefed up a bit.

I will do that.

> > @@ -835,8 +843,19 @@ void __init fpu__init_system_xstate(void)
> >   	 * Clear XSAVE features that are disabled in the normal CPUID.
> >   	 */
> >   	for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> > -		if (!boot_cpu_has(xsave_cpuid_features[i]))
> > -			xfeatures_mask_all &= ~BIT_ULL(i);
> > +		if (xsave_cpuid_features[i] == X86_FEATURE_SHSTK) {
> > +			/*
> > +			 * X86_FEATURE_SHSTK and X86_FEATURE_IBT share
> > +			 * same states, but can be enabled separately.
> > +			 */
> > +			if (!boot_cpu_has(X86_FEATURE_SHSTK) &&
> > +			    !boot_cpu_has(X86_FEATURE_IBT))
> > +				xfeatures_mask_all &= ~BIT_ULL(i);
> > +		} else {
> > +			if ((xsave_cpuid_features[i] == -1) ||
>
> Where did the -1 come from?  Was that introduced earlier in this series?
>   I don't see any way a xsave_cpuid_features[] can be -1 in the current tree.

Yes, we used to have a hole in xsave_cpuid_features[] and put -1 there. 
Do we want to keep this in case we again have holes in the future?

> > +			    !boot_cpu_has(xsave_cpuid_features[i]))
> > +				xfeatures_mask_all &= ~BIT_ULL(i);
> > +		}
> >   	}
>
> Do we have any other spots in the kernel where we care about:
>
> 	boot_cpu_has(X86_FEATURE_SHSTK) ||
> 	boot_cpu_has(X86_FEATURE_IBT)
>
> ?  If so, we could also address this by declaring a software-defined
> X86_FEATURE_CET and then setting it if SHSTK||IBT is supported, then we
> just put that one feature in xsave_cpuid_features[].

That is a better solution.  I will look into that.

> I'm also not crazy about the loop as it is.  I'd much rather see this in
> a helper like:
>
> bool cpu_supports_xsave_deps(int xfeature)
> {
> 	bool ret;
>
> 	ret = boot_cpu_has(xsave_cpuid_features[xfeature])
>
> 	/*
> 	 * X86_FEATURE_SHSTK is checked in xsave_cpuid_features()
> 	 * but the CET states are needed if either SHSTK or IBT are
> 	 * available.
> 	 */
> 	if (xfeature == XFEATURE_CET_USER ||
> 	    xfeature == XFEATURE_CET_KERNEL)
> 		ret |= boot_cpu_has(X86_FEATURE_IBT)
> 		
> 	return ret;
> }
>
> See how that's extensible?  You can add as many special cases as you want.

Yes.

Thanks,
Yu-cheng