Re: [PATCH] KVM: arm/arm64: Handle hva aging while destroying the vm

[Christoffer Dall]

Hi Alex,

On Fri, Jun 23, 2017 at 05:21:59PM +0200, Alexander Graf wrote:
> If we want to age an HVA while the VM is getting destroyed, we have a
> tiny race window during which we may end up dereferencing an invalid
> kvm->arch.pgd value.
> 
>    CPU0               CPU1
> 
>    kvm_age_hva()
>                       kvm_mmu_notifier_release()
>                       kvm_arch_flush_shadow_all()
>                       kvm_free_stage2_pgd()
>                       <grab mmu_lock>
>    stage2_get_pmd()
>    <wait for mmu_lock>
>                       set kvm->arch.pgd = 0
>                       <free mmu_lock>
>    <grab mmu_lock>
>    stage2_get_pud()
>    <access kvm->arch.pgd>
>    <use incorrect value>

I don't think this sequence, can happen, but I think kvm_age_hva() can
be called with the mmu_lock held and kvm->pgd already being NULL.

Is that possible for the mmu notifiers to be calling clear(_flush)_young
while also calling notifier_release?

If so, the patch below looks good to me.

Thanks,
-Christoffer


> 
> This patch adds a check for that case.
> 
> Signed-off-by: Alexander Graf <agraf@xxxxxxx>
> ---
>  virt/kvm/arm/mmu.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index f2d5b6c..227931f 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -861,6 +861,10 @@ static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache
>  	pgd_t *pgd;
>  	pud_t *pud;
>  
> +	/* Do we clash with kvm_free_stage2_pgd()? */
> +	if (!kvm->arch.pgd)
> +		return NULL;
> +
>  	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
>  	if (WARN_ON(stage2_pgd_none(*pgd))) {
>  		if (!cache)
> -- 
> 1.8.5.6
> 
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