linux-kernel - Re: [PATCH v6 23/25] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Date:   Wed, 1 Nov 2023 15:14:58 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dave.hansen@...el.com, peterz@...radead.org, chao.gao@...el.com,
        rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v6 23/25] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On Tue, Oct 31, 2023, Maxim Levitsky wrote:
> On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > @@ -685,6 +686,13 @@ void kvm_set_cpu_caps(void)
> >  		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> >  	if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> >  		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> > +	/*
> > +	 * The feature bit in boot_cpu_data.x86_capability could have been
> > +	 * cleared due to ibt=off cmdline option, then add it back if CPU
> > +	 * supports IBT.
> > +	 */
> > +	if (cpuid_edx(7) & F(IBT))
> > +		kvm_cpu_cap_set(X86_FEATURE_IBT);
> 
> The usual policy is that when the host doesn't support a feature, then the guest
> should not support it either. On the other hand, for this particular feature,
> it is probably safe to use it. Just a point for a discussion.

Agreed, this needs extra justification.  It's "safe" in theory, but if the admin
disabled IBT because of a ucode bug, then all bets are off.

I'm guessing this was added because of the virtualization hole?  I.e. if KVM
allows CR4.CET=1 for shadow stacks, then KVM can't (easily?) prevent the guest
from also using IBT.