linux-kernel - Re: [PATCH v2 21/21] KVM:x86: Support CET supervisor shadow stack MSR access

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Date:   Thu, 4 May 2023 04:17:57 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "john.allen@....com" <john.allen@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 21/21] KVM:x86: Support CET supervisor shadow stack MSR
 access

On Thu, 2023-05-04 at 09:20 +0800, Yang, Weijiang wrote:
> 
> On 5/4/2023 1:07 AM, Edgecombe, Rick P wrote:
> > On Fri, 2023-04-21 at 09:46 -0400, Yang Weijiang wrote:
> > > +
> > > +       incpt = !is_cet_state_supported(vcpu,
> > > XFEATURE_MASK_CET_KERNEL);
> > > +       incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> > > +
> > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> > > MSR_TYPE_RW, incpt);
> > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> > > MSR_TYPE_RW, incpt);
> > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> > > MSR_TYPE_RW, incpt);
> > > +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> > > MSR_TYPE_RW, incpt);
> > >    }
> > Why is this tied to XFEATURE_MASK_CET_KERNEL? I don't know how the
> > SVM
> > side works, but the host kernel doesn't use this xfeature. Just not
> > clear on what the intention is. Why not use
> > kvm_cet_kernel_shstk_supported() again?
> 
> I don't know how SVM supports supervisor SHSTK either, here just
> follows 
> the spec.

What aspect of the spec is this?

> 
> to add the dependency check. Maybe you're right, I need to use 
> kvm_cet_kernel_shstk_supported()
> 
> in my patch set and leave the work to SVM enabling patches. I'll
> change 
> it, thanks!

Oh, I see the the SVM patch [0] is adding XFEATURE_MASK_CET_KERNEL to
kvm_caps.supported_xss as long as kvm_cpu_cap_has(X86_FEATURE_SHSTK).
And it does not look to be checking XSS host support like how 
kvm_caps.supported_xss is set in your patch. It should depend on host
support, right? Is that the intent of kvm_caps.supported_xss?

Separate from all that, the code above is in VMX, so not sure how it
affects SVM in any case.

I might be confused here. The code just looked suspicious.

[0]
https://lore.kernel.org/kvm/20221012203910.204793-8-john.allen@amd.com/