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Subject: Re: [PATCH] KVM: nVMX: do not use dangling shadow VMCS after guest
 reset
To:     Liran Alon <liran.alon@oracle.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
References: <1563554534-46556-3-git-send-email-pbonzini@redhat.com>
 <6D1C57BE-1A1B-4714-B4E5-E0569A60FD1F@oracle.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <2a84f9d3-f58f-bcf3-56ef-a40fcbbaf8af@redhat.com>
Date:   Fri, 19 Jul 2019 23:39:04 +0200
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On 19/07/19 23:01, Liran Alon wrote:
> 
> 
>> On 19 Jul 2019, at 19:42, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> If a KVM guest is reset while running a nested guest, free_nested will
>> disable the shadow VMCS execution control in the vmcs01.  However,
>> on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
>> the VMCS12 to the shadow VMCS which has since been freed.
>>
>> This causes a vmptrld of a NULL pointer on my machime, but Jan reports
>> the host to hang altogether.  Let's see how much this trivial patch fixes.
>>
>> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> First, nested_release_vmcs12() also sets need_vmcs12_to_shadow_sync
> to false explicitly. This can now be removed.
> 
> Second, I suggest putting a WARN_ON_ONCE() on copy_vmcs12_to_shadow()
> in case shadow_vmcs==NULL.
Both good ideas.  Thanks for the quick review!

Paolo

> To assist catching these kind of errors more easily in the future.
> 
> Besides that, the fix seems correct to me.
> Reviewed-by: Liran Alon <liran.alon@oracle.com>
> 
> -Liran
> 
>> ---
>> arch/x86/kvm/vmx/nested.c | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 6e88f459b323..6119b30347c6 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -194,6 +194,7 @@ static void vmx_disable_shadow_vmcs(struct vcpu_vmx *vmx)
>> {
>> 	secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_SHADOW_VMCS);
>> 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
>> +	vmx->nested.need_vmcs12_to_shadow_sync = false;
>> }
>>
>> static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
>> -- 
>> 1.8.3.1
>>
>