Messages in this thread | | | Subject | Re: [NEEDS-REVIEW] [PATCH v15 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states | From | "Yu, Yu-cheng" <> | Date | Mon, 30 Nov 2020 10:06:44 -0800 |
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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
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