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Date: Tue, 7 May 2024 08:08:26 -0700
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Message-ID: <ZjpD2BaO5SXPUEj0@google.com>
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace
From: Sean Christopherson <seanjc@google.com>
To: Rick P Edgecombe <rick.p.edgecombe@intel.com>
Cc: Chao Gao <chao.gao@intel.com>, Dave Hansen <dave.hansen@intel.com>, 
	"x86@kernel.org" <x86@kernel.org>, "peterz@infradead.org" <peterz@infradead.org>, 
	"john.allen@amd.com" <john.allen@amd.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	"mlevitsk@redhat.com" <mlevitsk@redhat.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	Weijiang Yang <weijiang.yang@intel.com>
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On Tue, May 07, 2024, Rick P Edgecombe wrote:
> On Tue, 2024-05-07 at 07:21 -0700, Sean Christopherson wrote:
> >=20
> > Keeping X86_FEATURE_IBT set will result in "ibt" being reported in
> > /proc/cpuinfo,
> > i.e. will mislead userspace into thinking IBT is supported and fully en=
abled
> > by
> > the kernel.=C2=A0 For a security feature, that's a pretty big issue.
>=20
> Since the beginning, if you don't configure kernel IBT in Kconfig but the=
 HW
> supports it, "ibt" will appear in /proc/cpuinfo. It never was a reliable
> indicator of kernel IBT enforcement.

Ah, good to know.

> It is just an indicator of if the IBT feature is usable.

Does ibt=3Doff make IBT unusable for userspace?  Huh.  Looking at the #CP h=
andler,
I take it userspace support for IBT hasn't landed yet?

> I think tying kernel IBT enforcement to the CPU feature is wrong. But if =
you
> disable the HW feature, it makes sense that the enforcement would be
> disabled.
>=20
> CET is something that requires a fair amount of SW enablement. SW needs t=
o do
> things in special ways or things will go wrong. So whether IBT is in use =
and
> whether it is supported by the HW are useful to maintain as separate conc=
epts.
>=20
> >=20
> > To fudge around that, we could add a synthetic feature flag to let the
> > kernel tell KVM whether or not it's safe to virtualize IBT, but I don't=
 see
> > what value that adds over KVM checking raw host CPUID.
>=20
> A synthetic feature flag for kernel IBT seems reasonable to me. It's what=
 I
> suggested on that thread I linked earlier. But Peterz was advocating for =
a bool.
> How enforcement would be exposed, would just be dmesg I guess. Having a n=
ew
> feature flag still makes sense to me. Maybe he could be convinced.

If there's a need for IBT and KERNEL_IBT, I agree a synthetic flag makes se=
nse.
But as above, it's not clear to me why both are needed, at least for KVM's =
sake.
Is the need more apparent when userspace IBT support comes along?