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 <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
---
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
> On 19 Jul 2019, at 19:42, Paolo Bonzini <[email protected]> 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 <[email protected]>
> Signed-off-by: Paolo Bonzini <[email protected]>
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.
To assist catching these kind of errors more easily in the future.
Besides that, the fix seems correct to me.
Reviewed-by: Liran Alon <[email protected]>
-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
>
On 19/07/19 23:01, Liran Alon wrote:
>
>
>> On 19 Jul 2019, at 19:42, Paolo Bonzini <[email protected]> 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 <[email protected]>
>> Signed-off-by: Paolo Bonzini <[email protected]>
>
> 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 <[email protected]>
>
> -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
>>
>