linux-kernel - Re: [PATCH v8 4/7] KVM: VMX: Load CET states on vmentry/vmexit

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Date:   Thu, 12 Dec 2019 09:04:24 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        jmattson@...gle.com, yu.c.zhang@...ux.intel.com,
        yu-cheng.yu@...el.com
Subject: Re: [PATCH v8 4/7] KVM: VMX: Load CET states on vmentry/vmexit

On Wed, Dec 11, 2019 at 08:35:10AM -0800, Sean Christopherson wrote:
> On Wed, Dec 11, 2019 at 09:54:23AM +0800, Yang Weijiang wrote:
> > On Tue, Dec 10, 2019 at 01:23:05PM -0800, Sean Christopherson wrote:
> > > On Fri, Nov 01, 2019 at 04:52:19PM +0800, Yang Weijiang wrote:
> > > > @@ -2834,6 +2837,9 @@ void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
> > > >  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > >  	unsigned long hw_cr0;
> > > >  
> > > > +	if (!(cr0 & X86_CR0_WP) && kvm_read_cr4_bits(vcpu, X86_CR4_CET))
> > > > +		cr0 |= X86_CR0_WP;
> > > 
> > > Huh?  What's the interaction between CR4.CET and CR0.WP?  If there really
> > > is some non-standard interaction then it needs to be documented in at least
> > > the changelog and probably with a comment as well.
> > >
> > The processor does not allow CR4.CET to be set if CR0.WP = 0 (similarly, it
> > does not allow CR0.WP to be cleared while CR4.CET = 1).
> 
> Ya, as you surmised below, this needs to be a #GP condition.
>
OK, will do it.

> Have you tested SMM at all?  The interaction between CR0 and CR4 may be
> problematic for em_rsm() and/or rsm_enter_protected_mode().
>
Not yet, what's an easy way to test code in SMM mode?
Thanks!

> > > > +
> > > >  	hw_cr0 = (cr0 & ~KVM_VM_CR0_ALWAYS_OFF);
> > > >  	if (enable_unrestricted_guest)
> > > >  		hw_cr0 |= KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST;
> > > > @@ -2936,6 +2942,22 @@ static bool guest_cet_allowed(struct kvm_vcpu *vcpu, u32 feature, u32 mode)
> > > >  	return false;
> > > >  }
> > > >  
> > > > +bool is_cet_bit_allowed(struct kvm_vcpu *vcpu)
> > > > +{
> > > > +	unsigned long cr0;
> > > > +	bool cet_allowed;
> > > > +
> > > > +	cr0 = kvm_read_cr0(vcpu);
> > > > +	cet_allowed = guest_cet_allowed(vcpu, X86_FEATURE_SHSTK,
> > > > +					XFEATURE_MASK_CET_USER) ||
> > > > +		      guest_cet_allowed(vcpu, X86_FEATURE_IBT,
> > > > +					XFEATURE_MASK_CET_USER);
> > > > +	if ((cr0 & X86_CR0_WP) && cet_allowed)
> > > > +		return true;
> > > 
> > > So, attempting to set CR4.CET if CR0.WP=0 takes a #GP?  But attempting
> > > to clear CR0.WP if CR4.CET=1 is ignored?
> > > 
> > Per above words in spec., inject #GP to guest in either case?
> > 
> > > > +
> > > > +	return false;
> > > > +}
> > > > +