linux-kernel - Re: [PATCH] KVM: MMU: Fix guest stuck during boot due to read/write emulation against GVA

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Date:   Fri, 11 Aug 2017 08:13:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH] KVM: MMU: Fix guest stuck during boot due to read/write
 emulation against GVA

On 11/08/2017 07:59, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Commit c016004494b0 (KVM: x86: Avoid guest page table walk when gpa_available 
> is set) avoids the page table walk when cr2 has already contained a valid GPA. 
> However, that is not the truth if ept == 0 and shadow page table is used. In 
> this scenario cr2 can just contains a valid GVA. The commit results in guest 
> stuck during boot due to read/write emulation against GVA instead of GPA of 
> the guest.
> 
> This patch fixes it by not setting the gpa_available flag if the shadow page table 
> is used.

Yup, I got these on my overnight tests too.  A better fix that I was
going to commit today is:

                if (vcpu->arch.mmu.direct_map) {
                        vcpu->arch.gpa_available = true;
                        vcpu->arch.gpa_val = fault_address;
                }

because AMD gets here even if npt=1.

Paolo

> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/mmu.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 849312d..e4ce20b 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3791,8 +3791,6 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
>  
>  		if (need_unprotect && kvm_event_needs_reinjection(vcpu))
>  			kvm_mmu_unprotect_page_virt(vcpu, fault_address);
> -		vcpu->arch.gpa_available = true;
> -		vcpu->arch.gpa_val = fault_address;
>  		r = kvm_mmu_page_fault(vcpu, fault_address, error_code, insn,
>  				insn_len);
>  		break;
>