[PATCH] KVM: nVMX: Unconditionally uninit the MMU on nested vmexit

[PATCH] KVM: nVMX: Unconditionally uninit the MMU on nested vmexit

[Jan Kiszka]

Three reasons for doing this: 1. arch.walk_mmu points to arch.mmu anyway
in case nested EPT wasn't in use. 2. this aligns VMX with SVM. But 3. is
most important: nested_cpu_has_ept(vmcs12) queries the VMCS page, and if
one guest VCPU manipulates the page of another VCPU in L2, we may be
fooled to skip over the nested_ept_uninit_mmu_context, leaving mmu in
nested state. That can crash the host later on if nested_ept_get_cr3 is
invoked while L1 already left vmxon and nested.current_vmcs12 became
NULL therefore.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 arch/x86/kvm/vmx.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 1d9b0ec..48d0b46 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -8366,8 +8366,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	vcpu->arch.cr4_guest_owned_bits = ~vmcs_readl(CR4_GUEST_HOST_MASK);
 	kvm_set_cr4(vcpu, vmcs12->host_cr4);
 
-	if (nested_cpu_has_ept(vmcs12))
-		nested_ept_uninit_mmu_context(vcpu);
+	nested_ept_uninit_mmu_context(vcpu);
 
 	kvm_set_cr3(vcpu, vmcs12->host_cr3);
 	kvm_mmu_reset_context(vcpu);
-- 
1.8.1.1.298.ge7eed54

Re: [PATCH] KVM: nVMX: Unconditionally uninit the MMU on nested vmexit

[Marcelo Tosatti]

On Sat, Dec 28, 2013 at 04:31:52PM +0100, Jan Kiszka wrote:
> Three reasons for doing this: 1. arch.walk_mmu points to arch.mmu anyway
> in case nested EPT wasn't in use. 2. this aligns VMX with SVM. But 3. is
> most important: nested_cpu_has_ept(vmcs12) queries the VMCS page, and if
> one guest VCPU manipulates the page of another VCPU in L2, we may be
> fooled to skip over the nested_ept_uninit_mmu_context, leaving mmu in
> nested state. That can crash the host later on if nested_ept_get_cr3 is
> invoked while L1 already left vmxon and nested.current_vmcs12 became
> NULL therefore.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>  arch/x86/kvm/vmx.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 1d9b0ec..48d0b46 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -8366,8 +8366,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
>  	vcpu->arch.cr4_guest_owned_bits = ~vmcs_readl(CR4_GUEST_HOST_MASK);
>  	kvm_set_cr4(vcpu, vmcs12->host_cr4);
>  
> -	if (nested_cpu_has_ept(vmcs12))
> -		nested_ept_uninit_mmu_context(vcpu);
> +	nested_ept_uninit_mmu_context(vcpu);
>  
>  	kvm_set_cr3(vcpu, vmcs12->host_cr3);
>  	kvm_mmu_reset_context(vcpu);
> -- 

Applied to master branch, thanks.