linux-kernel - Re: [PATCH 5/6] KVM: nSVM: always leave the nested state first on KVM_SET_NESTED

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Message-ID: <747878595173b72dfa95f73f4e73c6cabb199bd8.camel@redhat.com>
Date:   Thu, 07 Jan 2021 01:55:48 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 5/6] KVM: nSVM: always leave the nested state first on
 KVM_SET_NESTED_STATE

On Wed, 2021-01-06 at 09:39 -0800, Sean Christopherson wrote:
> On Wed, Jan 06, 2021, Maxim Levitsky wrote:
> > This should prevent bad things from happening if the user calls the
> > KVM_SET_NESTED_STATE twice.
> 
> This doesn't exactly inspire confidence, nor does it provide much help to
> readers that don't already know why KVM should "leave nested" before processing
> the rest of kvm_state.
> 
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> >  arch/x86/kvm/svm/nested.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index c1a3d0e996add..3aa18016832d0 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -1154,8 +1154,9 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
> >  	if (is_smm(vcpu) && (kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE))
> >  		return -EINVAL;
> >  
> > +	svm_leave_nested(svm);
> 
> nVMX sets a really bad example in that it does vmx_leave_nested(), and many
> other things, long before it has vetted the incoming state.  That's not the end
> of the word as the caller is likely going to exit if this ioctl() fails, but it
> would be nice to avoid such behavior with nSVM, especially since it appears to
> be trivially easy to do svm_leave_nested() iff the ioctl() will succeed.

I agree with you. So if I understand correctly I should move the unconditional 
svm_leave_nested(svm) after all the checks are done? I 

Best regards,
	Maxim Levitsky

> 
> > +
> >  	if (!(kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE)) {
> > -		svm_leave_nested(svm);
> >  		svm_set_gif(svm, !!(kvm_state->flags & KVM_STATE_NESTED_GIF_SET));
> >  		return 0;
> >  	}
> > -- 
> > 2.26.2
> >