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

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Date:   Fri, 8 Apr 2022 02:48:33 +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 Thu, Apr 07, 2022, Chenyi Qiang wrote:
> 
> 
> On 4/7/2022 5:15 AM, Sean Christopherson wrote:
> > On Tue, Apr 05, 2022, Sean Christopherson wrote:
> > > 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:
> > 
> > Confirmed the WARN can be triggered by abusing this patch, I'll get a patch out
> > once I figure out why kvm/queue is broken.
> > 
> > diff --git a/tools/testing/selftests/kvm/x86_64/state_test.c b/tools/testing/selftests/kvm/x86_64/state_test.c
> > index 2e0a92da8ff5..b7faeae3dcc4 100644
> > --- a/tools/testing/selftests/kvm/x86_64/state_test.c
> > +++ b/tools/testing/selftests/kvm/x86_64/state_test.c
> > @@ -210,6 +210,12 @@ int main(int argc, char *argv[])
> >                  memset(&regs1, 0, sizeof(regs1));
> >                  vcpu_regs_get(vm, VCPU_ID, &regs1);
> > 
> > +               if (stage == 6) {
> > +                       state->events.flags |= 0x20;
> > +                       vcpu_events_set(vm, VCPU_ID, &state->events);
> > +                       vcpu_nested_state_set(vm, VCPU_ID, &state->nested, false);
> > +               }
> > +
> >                  kvm_vm_release(vm);
> > 
> >                  /* Restore state in a new VM.  */
> 
> Also verified the WARN with this. Then, is it still necessary to add an
> individual selftest about the working flow of save/restore triple fault
> event?

Yeah, the above hack fails the test even on a good kernel.  It's not an actual test
of the feature, just a hack to confirm the bug.