Re: [PATCH] KVM: nVMX: do not use dangling shadow VMCS after guest reset

[Liran Alon]

> On 19 Jul 2019, at 19:42, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

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 <liran.alon@xxxxxxxxxx>

-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
>