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

Christoffer Dall
2017-07-03T08:03:20Z

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@suse.de>
> ---
> 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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