linux-kernel - Re: [PATCH] KVM: nVMX: Use vmx_need_pf_intercept() when deciding if L0 wants a #PF

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Date:   Wed, 11 Aug 2021 22:55:44 -0700
From:   Oliver Upton <oupton@...gle.com>
To:     Sean Christopherson <seanjc@...gle.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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH] KVM: nVMX: Use vmx_need_pf_intercept() when deciding if
 L0 wants a #PF

On Wed, Aug 11, 2021 at 9:56 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Use vmx_need_pf_intercept() when determining if L0 wants to handle a #PF
> in L2 or if the VM-Exit should be forwarded to L1.  The current logic fails
> to account for the case where #PF is intercepted to handle
> guest.MAXPHYADDR < host.MAXPHYADDR and ends up reflecting all #PFs into
> L1.  At best, L1 will complain and inject the #PF back into L2.  At
> worst, L1 will eat the unexpected fault and cause L2 to hang on infinite
> page faults.
>
> Note, while the bug was technically introduced by the commit that added
> support for the MAXPHYADDR madness, the shame is all on commit
> a0c134347baf ("KVM: VMX: introduce vmx_need_pf_intercept").
>
> Fixes: 1dbf5d68af6f ("KVM: VMX: Add guest physical address check in EPT violation and misconfig")
> Cc: stable@...r.kernel.org
> Cc: Peter Shier <pshier@...gle.com>
> Cc: Oliver Upton <oupton@...gle.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Oliver Upton <oupton@...gle.com>

--
Thanks,
Oliver

> ---
>  arch/x86/kvm/vmx/nested.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index bc6327950657..8bcbe57b560f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5830,7 +5830,8 @@ static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu,
>                 if (is_nmi(intr_info))
>                         return true;
>                 else if (is_page_fault(intr_info))
> -                       return vcpu->arch.apf.host_apf_flags || !enable_ept;
> +                       return vcpu->arch.apf.host_apf_flags ||
> +                              vmx_need_pf_intercept(vcpu);
>                 else if (is_debug(intr_info) &&
>                          vcpu->guest_debug &
>                          (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
> --
> 2.33.0.rc1.237.g0d66db33f3-goog
>