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

[Dave Hansen]

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.

> @@ -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.

> +			    !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[].

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.