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

Yu, Yu-cheng
2020-11-30T18:07:37Z

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