linux-kernel - Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request

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Date:   Tue, 5 Apr 2022 23:44:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request

On Tue, Apr 05, 2022, Sean Christopherson wrote:
> On Fri, Mar 18, 2022, Chenyi Qiang wrote:
> > @@ -4976,6 +4980,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
> >  		}
> >  	}
> >  
> > +	if (events->flags & KVM_VCPUEVENT_TRIPLE_FAULT)
> > +		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> > +
> >  	kvm_make_request(KVM_REQ_EVENT, vcpu);
> 
> Looks correct, but this really needs a selftest, at least for the SET path since
> the intent is to use that for the NOTIFY handling.  Doesn't need to be super fancy,
> e.g. do port I/O from L2, inject a triple fault, and verify L1 sees the appropriate
> exit.
> 
> Aha!  And for the GET path, abuse KVM_X86_SET_MCE with CR4.MCE=0 to coerce KVM into
> making a KVM_REQ_TRIPLE_FAULT, that way there's no need to try and hit a timing
> window to intercept the request.

Drat, I bet that MCE path means the WARN in nested_vmx_vmexit() can be triggered
by userspace.  If so, this patch makes it really, really easy to hit, e.g. queue the
request while L2 is active, then do KVM_SET_NESTED_STATE to force an "exit" without
bouncing through kvm_check_nested_events().

  WARN_ON_ONCE(kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu))

I don't think SVM has a user-triggerable WARN, but the request should still be
dropped on forced exit from L2, e.g. I believe this is the correct fix:

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 9a6dc2b38fcf..18c5e96b12a5 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1128,6 +1128,7 @@ void svm_leave_nested(struct kvm_vcpu *vcpu)
        if (is_guest_mode(vcpu)) {
                svm->nested.nested_run_pending = 0;
                svm->nested.vmcb12_gpa = INVALID_GPA;
+               kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);
 
                leave_guest_mode(vcpu);
 
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f18744f7ff82..0587ef647553 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6276,6 +6276,7 @@ void vmx_leave_nested(struct kvm_vcpu *vcpu)
 {
        if (is_guest_mode(vcpu)) {
                to_vmx(vcpu)->nested.nested_run_pending = 0;
+               kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);
                nested_vmx_vmexit(vcpu, -1, 0, 0);
        }
        free_nested(vcpu);