linux-kernel - Re: [PATCH v3 3/4] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

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Message-ID: <aWEUTQeNXugBYAZA@google.com>
Date: Fri, 9 Jan 2026 06:44:29 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chao Gao <chao.gao@...el.com>, Xin Li <xin@...or.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v3 3/4] KVM: nVMX: Disallow access to vmcs12 fields that
 aren't supported by "hardware"

On Fri, Jan 09, 2026, Xiaoyao Li wrote:
> On 1/9/2026 12:15 PM, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > index 61113ead3d7b..ac7a17560c8f 100644
> > --- a/arch/x86/kvm/vmx/nested.c
> > +++ b/arch/x86/kvm/vmx/nested.c
> > @@ -111,6 +111,9 @@ static void init_vmcs_shadow_fields(void)
> >   			  field <= GUEST_TR_AR_BYTES,
> >   			  "Update vmcs12_write_any() to drop reserved bits from AR_BYTES");
> > +		if (get_vmcs12_field_offset(field) < 0)
> > +			continue;
> > +
> 
> why shadow_read_only_fields[] doesn't need such guard?
> 
> IIUC, copy_vmcs12_to_shadow() will VMWRITE shadowed readonly field even if
> it doesn't exist on the hardware?

Because I fixated on the existing checks and didn't look at the first for-loop.

This time around I'll test by hacking in shadowed fields arbitrary shadow fields.