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.
[...]
> 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[].

These features have different CPUIDs but are complementary parts.  I 
don't know if someday there will be shadow-stack-only CPUs, but an 
IBT-only CPU is weird.  What if the kernel checks that the CPU has both 
features and presents only one feature flag (X86_FEATURE_CET), no 
X86_FEATURE_SHSTK or X86_FEATURE_IBT?